Produced by Robert Rowe, Charles Franks
and the Online Distributed Proofreading Team.




THE STORY OF GERM LIFE

BY H. W. CONN

PROFESSOR OF BIOLOGY AT WESLEYAN UNIVERSITY,

AUTHOR OF EVOLUTION OF TO-DAY,
THE LIVING WORLD, ETC.





PREFACE.


Since the first edition of this book was published the popular
idea of bacteria to which attention was drawn in the original
preface has undergone considerable modification. Experimental
medicine has added constantly to the list of diseases caused by
bacterial organisms, and the general public has been educated to
an adequate conception of the importance of the germ as the chief
agency in the transmission of disease, with corresponding
advantage to the efficiency of personal and public hygiene. At the
same time knowledge of the benign bacteria and the enormous role
they play in the industries and the arts has become much more
widely diffused. Bacteriology is being studied in colleges as one
of the cultural sciences; it is being widely adopted as a subject
of instruction in high schools; and schools of agriculture and
household science turn out each year thousands of graduates
familiar with the functions of bacteria in daily life. Through
these agencies the popular misconception of the nature of micro-
organisms and their relations to man is being gradually displaced
by a general appreciation of their manifold services. It is not
unreasonable to hope that the many thousands of copies of this
little manual which have been circulated and read have contributed
materially to that end. If its popularity is a safe criterion, the
book has amply fulfilled its purpose of placing before the general
reader in a simple and direct style the main facts of
bacteriology. Beginning with a discussion of the nature of
bacteria, it shows their position in the scale of plant and animal
life. The middle chapters describe the functions of bacteria in
the arts, in the dairy, and in agriculture. The final chapters
discuss the relation of bacteria to disease and the methods by
which the new and growing science of preventive medicine combats
and counteracts their dangerous powers.

JULY, 1915.





CONTENTS.


I.--BACTERIA AS PLANTS

Historical.--Form of bacteria.--Multiplication of bacteria.--Spore
formation.--Motion.--Internal structure.--Animals or plants?--
Classification.--Variation.--Where bacteria are found.

II.--MISCELLANEOUS USES OF BACTERIA IN THE ARTS.

Maceration industries.--Linen.--Jute.--Hemp.--Sponges.--Leather.
--Fermentative industries.--Vinegar--Lactic acid.--Butyric acid.--
Bacteria in tobacco curing.--Troublesome fermentations.

III.--BACTERIA IN THE DAIRY.

Sources of bacteria in milk.--Effect of bacteria on milk.--
Bacteria used in butter making.--Bacteria in cheese making.

IV.--BACTERIA IN NATURAL PROCESSES.

Bacteria as scavengers.--Bacteria as agents in Nature's food
cycle.--Relation of bacteria to agriculture.--Sprouting of seeds.
--The silo.--The fertility of the soil.--Bacteria as sources of
trouble to the farmer.--Coal formation.

V.--PARASITIC BACTERIA AND THEIR RELATION TO DISEASE

Method of producing disease.--Pathogenic germs not strictly
parasitic.--Pathogenic germs that are true parasites.--What
diseases are due to bacteria.--Variability of pathogenic powers.--
Susceptibility of the individual.--Recovery from bacteriological
diseases.--Diseases caused by organisms other than bacteria.

VI.--METHODS OF COMBATING PARASITIC BACTERIA

Preventive medicine.--Bacteria in surgery.--Prevention by
inoculation.--Limits of preventive medicine.--Curative medicine.
--Drugs--Vis medicatrix naturae.--Antitoxines and their use.--
Conclusion.





THE STORY OF GERM LIFE.


CHAPTER I.

BACTERIA AS PLANTS.


During the last fifteen years the subject of bacteriology
[Footnote: The term microbe is simply a word which has been coined
to include all of the microscopic plants commonly included under
the terms bacteria and yeasts.] has developed with a marvellous
rapidity. At the beginning of the ninth decade of the century
bacteria were scarcely heard of outside of scientific circles, and
very little was known about them even among scientists. Today they
are almost household words, and everyone who reads is beginning to
recognise that they have important relations to his everyday life.
The organisms called bacteria comprise simply a small class of low
plants, but this small group has proved to be of such vast
importance in its relation to the world in general that its study
has little by little crystallized into a science by itself. It is
a somewhat anomalous fact that a special branch of science,
interesting such a large number of people, should be developed
around a small group of low plants. The importance of bacteriology
is not due to any importance bacteria have as plants or as members
of the vegetable kingdom, but solely to their powers of producing
profound changes in Nature. There is no one family of plants that
begins to compare with them in importance. It is the object of
this work to point out briefly how much both of good and ill we
owe to the life and growth of these microscopic organisms. As we
have learned more and more of them during the last fifty years, it
has become more and more evident that this one little class of
microscopic plants fills a place in Nature's processes which in
some respects balances that filled by the whole of the green
plants. Minute as they are, their importance can hardly be
overrated, for upon their activities is founded the continued life
of the animal and vegetable kingdom. For good and for ill they are
agents of neverceasing and almost unlimited powers.

HISTORICAL.

The study of bacteria practically began with the use of the
microscope. It was toward the close of the seventeenth century
that the Dutch microscopist, Leeuwenhoek, working with his simple
lenses, first saw the organisms which we now know under this name,
with sufficient clearness to describe them. Beyond mentioning
their existence, however, his observations told little or nothing.
Nor can much more be said of the studies which followed during the
next one hundred and fifty years. During this long period many a
microscope was turned to the observation of these minute
organisms, but the majority of observers were contented with
simply seeing them, marvelling at their minuteness, and uttering
many exclamations of astonishment at the wonders of Nature. A few
men of more strictly scientific natures paid some attention to
these little organisms. Among them we should perhaps mention Von
Gleichen, Muller, Spallanzani, and Needham. Each of these, as well
as others, made some contributions to our knowledge of
microscopical life, and among other organisms studied those which
we now call bacteria. Speculations were even made at these early
dates of the possible causal connection of these organisms with
diseases, and for a little the medical profession was interested
in the suggestion. It was impossible then, however, to obtain any
evidence for the truth of this speculation, and it was abandoned
as unfounded, and even forgotten completely, until revived again
about the middle of the 19th century. During this century of
wonder a sufficiency of exactness was, however, introduced into
the study of microscopic organisms to call for the use of names,
and we find Muller using the names of Monas, Proteus, Vibrio,
Bacillus, and Spirillum, names which still continue in use,
although commonly with a different significance from that given
them by Muller. Muller did indeed make a study sufficient to
recognise the several distinct types, and attempted to classsify
these bodies. They were not regarded as of much importance, but
simply as the most minute organisms known.

Nothing of importance came from this work, however, partly because
of the inadequacy of the microscopes of the day, and partly
because of a failure to understand the real problems at issue.
When we remember the minuteness of the bacteria, the impossibility
of studying any one of them for more than a few moments at a time
--only so long, in fact, as it can be followed under a microscope;
when we remember, too, the imperfection of the compound
microscopes which made high powers practical impossibilities; and,
above all, when we appreciate the looseness of the ideas which
pervaded all scientists as to the necessity of accurate
observation in distinction from inference, it is not strange that
the last century gave us no knowledge of bacteria beyond the mere
fact of the existence of some extremely minute organisms in
different decaying materials. Nor did the 19th century add much to
this until toward its middle. It is true that the microscope was
vastly improved early in the century, and since this improvement
served as a decided stimulus to the study of microscopic life,
among other organisms studied, bacteria received some attention.
Ehrenberg, Dujardin, Fuchs, Perty, and others left the impress of
their work upon bacteriology even before the middle of the
century. It is true that Schwann shrewdly drew conclusions as to
the relation of microscopic organisms to various processes of
fermentation and decay--conclusions which, although not accepted
at the time, have subsequently proved to be correct. It is true
that Fuchs made a careful study of the infection of "blue milk,"
reaching the correct conclusion that the infection was caused by a
microscopic organism which he discovered and carefully studied. It
is true that Henle made a general theory as to the relation of
such organisms to diseases, and pointed out the logically
necessary steps in a demonstration of the causal connection
between any organism and a disease. It is true also that a general
theory of the production of ail kinds of fermentation by living
organisms had been advanced. But all these suggestions made little
impression. On the one hand, bacteria were not recognised as a
class of organisms by themselves--were not, indeed, distinguished
from yeasts or other minute animalcuise. Their variety was not
mistrusted and their significance not conceived. As microscopic
organisms, there were no reasons for considering them of any more
importance than any other small animals or plants, and their
extreme minuteness and simplicity made them of little interest to
the microscopist. On the other hand, their causal connection with
fermentative and putrefactive processes was entirely obscured by
the overshadowing weight of the chemist Liebig, who believed that
fermentations and putrefactions were simply chemical processes.
Liebig insisted that all albuminoid bodies were in a state of
chemically unstable equilibrium, and if left to themselves would
fall to pieces without any need of the action of microscopic
organisms. The force of Liebig's authority and the brilliancy of
his expositions led to the wide acceptance of his views and the
temporary obscurity of the relation of microscopic organisms to
fermentative and putrefactive processes. The objections to
Liebig's views were hardly noticed, and the force of the
experiments of Schwann was silently ignored. Until the sixth
decade of the century, therefore, these organisms, which have
since become the basis of a new branch of science, had hardly
emerged from obscurity. A few microscopists recognised their
existence, just as they did any other group of small animals or
plants, but even yet they failed to look upon them as forming a
distinct group. A growing number of observations was accumulating,
pointing toward a probable causal connection between fermentative
and putrefactive processes and the growth of microscopic
organisms; but these observations were known only to a few, and
were ignored by the majority of scientists.

It was Louis Pasteur who brought bacteria to the front, and it was
by his labours that these organisms were rescued from the
obscurity of scientific publications and made objects of general
and crowning interest. It was Pasteur who first successfully
combated the chemical theory of fermentation by showing that
albuminous matter had no inherent tendency to decomposition. It
was Pasteur who first clearly demonstrated that these little
bodies, like all larger animals and plants, come into existence
only by ordinary methods of reproduction, and not by any
spontaneous generation, as had been earlier claimed. It was
Pasteur who first proved that such a common phenomenon as. the
souring of milk was produced by microscopic organisms growing in
the milk. It was Pasteur who first succeeded in demonstrating that
certain species of microscopic organisms are the cause of certain
diseases, and in suggesting successful methods of avoiding them.
All these discoveries were made in rapid succession. Within ten
years of the time that his name began to be heard in this
connection by scientists, the subject had advanced so rapidly that
it had become evident that here was a new subject of importance to
the scientific world, if not to the public at large. The other
important discoveries which Pasteur made it is not our purpose to
mention here. His claim to be considered the founder of
bacteriology will be recognised from what has already been
mentioned. It was not that he first discovered the organisms, or
first studied them; it was not that he first suggested their
causal connection with fermentation and disease, but it was
because he for the first time placed the subject upon a firm
foundation by proving with rigid experiment some of the
suggestions made by others, and in this way turned the attention
of science to the study of micro-organisms.

After the importance of the subject had been demonstrated by
Pasteur, others turned their attention in the same direction,
either for the purpose of verification or refutation of Pasteur's
views. The advance was not very rapid, however, since
bacteriological experimentation proved to be a subject of
extraordinary difficulty. Bacteria were not even yet recognised as
a group of organisms distinct enough to be grouped by themselves,
but were even by Pasteur at first confounded with yeasts. As a
distinct group of organisms they were first distinguished by
Hoffman in 1869, since which date the term bacteria, as applying
to this special group of organisms, has been coming more and more
into use. So difficult were the investigations, that for years
there were hardly any investigators besides Pasteur who could
successfully handle the subject and reach conclusions which could
stand the test of time. For the next thirty years, although
investigators and investigations continued to increase, we can
find little besides dispute and confusion along this line. The
difficulty of obtaining for experiment any one kind of bacteria by
itself, unmixed with others (pure cultures), rendered advance
almost impossible. So conflicting were the results that the whole
subject soon came into almost hopeless confusion, and very few
steps were taken upon any sure basis. So difficult were the
methods, so contradictory and confusing the results, because of
impure cultures, that a student of to-day who wishes to look up
the previous discoveries in almost any line of bacteriology need
hardly go back of 1880, since he can almost rest assured that
anything done earlier than that was more likely to be erroneous
than correct.

The last fifteen years have, however, seen a wonderful change. The
difficulties had been mostly those of methods of work, and with
the ninth decade of the century these methods were simplified by
Robert Koch. This simplification of method for the first time
placed this line of investigation within the reach of scientists
who did not have the genius of Pasteur. It was now possible to get
pure cultures easily, and to obtain with such pure cultures
results which were uniform and simple. It was now possible to take
steps which had the stamp of accuracy upon them, and which further
experiment did not disprove. From the time when these methods were
thus made manageable the study of bacteria increased with a
rapidity which has been fairly startling, and the information
which has accumulated is almost formidable. The very rapidity with
which the investigations have progressed has brought considerable
confusion, from the fact that the new discoveries have not had
time to be properly assimilated into knowledge. Today many facts
are known whose significance is still uncertain, and a clear
logical discussion of the facts of modern bacteriology is not
possible. But sufficient knowledge has been accumulated and
digested to show us at least the direction along which
bacteriological advance is tending, and it is to the pointing out
of these directions that the following pages will be devoted.

WHAT ARE BACTERIA?

The most interesting facts connected with the subject of
bacteriology concern the powers and influence in Nature possessed
by the bacteria. The morphological side of the subject is
interesting enough to the scientist, but to him alone. Still, it
is impossible to attempt to study the powers of bacteria without
knowing something of the organisms themselves. To understand how
they come to play an important part in Nature's processes, we must
know first how they look and where they are found. A short
consideration of certain morphological facts will therefore be
necessary at the start.

FORM OF BACTERIA.

In shape bacteria are the simplest conceivable structures.
Although there are hundreds of different species, they have only
three general forms, which have been aptly compared to billiard
balls, lead pencils, and corkscrews. Spheres, rods, and spirals
represent all shapes. The spheres may be large or small, and may
group themselves in various ways; the rods may be long or short,
thick or slender; the spirals may be loosely or tightly coiled,
and may have only one or two or may have many coils, and they may
be flexible or stiff; but still rods, spheres, and spirals
comprise all types.

In size there is some variation, though not very great. All are
extremely minute, and never visible to the naked eye. The spheres
vary from 0.25 u to 1.5 u (0.000012 to 0.00006 inches). The rods
may be no more than 0.3 u in diameter, or may be as wide as 1.5 u
to 2.5 u, and in length vary all the way from a length scarcely
longer than their diameter to long threads. About the same may be
said of the spiral forms. They are decidedly the smallest living
organisms which our microscopes have revealed.

In their method of growth we find one of the most characteristic
features. They universally have the power of multiplication by
simple division or fission. Each individual elongates and then
divides in the middle into two similar halves, each of which then
repeats the process. This method of multiplication by simple
division is the distinguishing mark which separates the bacteria
from the yeasts, the latter plants multiplying by a process known
as budding. Fig. 2 shows these two methods of multiplication.

While all bacteria thus multiply by division, certain differences
in the details produce rather striking differences in the results.
Considering first the spherical forms, we find that some species
divide, as described, into two, which separate at once, and each
of which in turn divides in the opposite direction, called
Micrococcus, (Fig. 3). Other species divide only in one direction.
Frequently they do not separate after dividing, but remain
attached. Each, however, again elongates and divides again, but
all still remain attached. There are thus formed long chains of
spheres like strings of beads, called Streptococci (Fig. 4). Other
species divide first in one direction, then at right angles to the
first division, and a third division follows at right angles to
the plane of the first two, thus producing solid groups of fours,
eights, or sixteens (Fig 5), called Sarcina. Each different
species of bacteria is uniform in its method of division, and
these differences are therefore indications of differences in
species, or, according to our present method of classification,
the different methods of division represent different genera. All
bacteria producing Streptococcus chains form a single genus
Streptococcus, and all which divide in three division planes form
another genus, Sarcina, etc.

The rod-shaped bacteria also differ somewhat, but to a less
extent. They almost always divide in a plane at right angles to
their longest dimension. But here again we find some species
separating immediately after division, and thus always appearing
as short rods (Fig. 6), while others remain attached after
division and form long chains. Sometimes they appear to continue
to increase in length without showing any signs of division, and
in this way long threads are formed (Fig. 7). These threads are,
however, potentially at least, long chains of short rods, and
under proper conditions they will break up into such short rods,
as shown in Fig. 7a. Occasionally a rod species may divide
lengthwise, but this is rare. Exactly the same may be said of the
spiral forms. Here, too, we find short rods and long chains, or
long spiral filaments in which can be seen no division into
shorter elements, but which, under certain conditions, break up
into short sections.

RAPIDITY OF MULTIPLICATION.

It is this power of multiplication by division that makes bacteria
agents of such significance. Their minute size would make them
harmless enough if it were not for an extraordinary power of
multiplication. This power of growth and division is almost
incredible. Some of the species which have been carefully watched
under the microscope have been found under favourable conditions
to grow so rapidly as to divide every half hour, or even less. The
number of offspring that would result in the course of twenty-four
hours at this rate is of course easily computed. In one day each
bacterium would produce over 16,500,000 descendants, and in two
days about 281,500,000,000. It has been further calculated that
these 281,500,000,000 would form about a solid pint of bacteria
and weigh about a pound. At the end of the third day the total
descendants would amount to 47,000,000,000,000, and would weigh
about 16,000,000 pounds. Of course these numbers have no
significance, for they are never actual or even possible numbers.
Long before the offspring reach even into the millions their rate
of multiplication is checked either by lack of food or by the
accumulation of their own excreted products, which are injurious
to them. But the figures do have interest since they show faintly
what an unlimited power of multiplication these organisms have,
and thus show us that in dealing with bacteria we are dealing with
forces of almost infinite extent.

This wonderful power of growth is chiefly due to the fact that
bacteria feed upon food which is highly organized and already in
condition for absorption. Most plants must manufacture their own
foods out of simpler substances, like carbonic dioxide (Co2) and
water, but bacteria, as a rule, feed upon complex organic material
already prepared by the previous life of plants or animals. For
this reason they can grow faster than other plants. Not being
obliged to make their own foods like most plants, nor to search
for it like animals, but living in its midst, their rapidity of
growth and multiplication is limited only by their power to seize
and assimilate this food. As they grow in such masses of food,
they cause certain chemical changes to take place in it, changes
doubtless directly connected with their use of the material as
food. Recognising that they do cause chemical changes in food
material, and remembering this marvellous power of growth, we are
prepared to believe them capable of producing changes wherever
they get a foothold and begin to grow. Their power of feeding upon
complex organic food and producing chemical changes therein,
together with their marvellous power of assimilating this material
as food, make them agents in Nature of extreme importance.

DIFFERENCES BETWEEN DIFFERENT SPECIES OF BACTERIA.

While bacteria are thus very simple in form, there are a few other
slight variations in detail which assist in distinguishing them.
The rods are sometimes very blunt at the ends, almost as if cut
square across, while in other species they are more rounded and
occasionally slightly tapering. Sometimes they are
surrounded by a thin layer of some gelatinous substance, which
forms what is called a capsule (Fig. 10). This capsule may connect
them and serve as a cement, to prevent the separate elements of a
chain from falling apart.

Sometimes such a gelatinous secretion will unite great masses of
bacteria into clusters, which may float on the surface of the
liquid in which they grow or may sink to the bottom. Such masses
are called zoogloea, and their general appearance serves as one of
the characters for distinguishing different species of bacteria
(Fig. 10, a and b). When growing in solid media, such as a
nutritious liquid made stiff with gelatine, the different species
have different methods of spreading from their central point of
origin. A single bacterium in the midst of such a stiffened mass
will feed upon it and produce descendants rapidly; but these
descendants, not being able to move through the gelatine, will
remain clustered together in a mass, which the bacteriologist
calls a colony. But their method of clustering, due to different
methods of growth, is by no means always alike, and these colonies
show great differences in general appearance. The differences
appear to be constant, however, for the same species of bacteria,
and hence the shape and appearance of the colony enable
bacteriologists to discern different species (Fig. II). All these
points of difference are of practical use to the bacteriologist in
distinguishing species.

SPORE FORMATION.

In addition to their power of reproduction by simple division,
many species of bacteria have a second method by means of spores.
Spores are special rounded or oval bits of bacteria protoplasm
capable of resisting adverse conditions which would destroy the
ordinary bacteria. They arise among bacteria in two different
methods.

Endogenous spores.--These spores arise inside of the rods or the
spiral forms (Fig. 12). They first appear as slight granular
masses, or as dark points which become gradually distinct from the
rest of the rod. Eventually there is thus formed inside the rod a
clear, highly refractive, spherical or oval spore, which may even
be of a greater diameter than the rod producing it, thus causing
it to swell out and become spindle formed [Fig. 12 c]. These
spores may form in the middle or at the ends of the rods (Fig.
12). They may use up all the protoplasm of the rod in their
formation, or they may use only a small part of it, the rod which
forms them continuing its activities in spite of the formation of
the spores within it. They are always clear and highly refractive
from containing little water, and they do not so readily absorb
staining material as the ordinary rods. They appear to be covered
with a layer of some substance which resists the stain, and which
also enables them to resist various external agencies. This
protective covering, together with their small amount of water,
enables them to resist almost any amount of drying, a high degree
of heat, and many other adverse conditions. Commonly the spores
break out of the rod, and the rod producing them dies, although
sometimes the rod may continue its activity even after the spores
have been produced.

Arthrogenous spores (?).--Certain species of bacteria do not
produce spores as just described, but may give rise to bodies that
are sometimes called arthrospores. These bodies are formed as
short segments of rods. A long rod may sometimes break up into
several short rounded elements, which are clear and appear to have
a somewhat increased power of resisting adverse conditions. The
same may happen among the spherical forms, which only in rare
instances form endogenous spores. Among the spheres which form a
chain of streptococci some may occasionally be slightly different
from the rest. They are a little larger, and have been thought to
have an increased resisting power like that of true spores (Fig.
13 b). It is quite doubtful, however, whether it is proper to
regard these bodies as spores. There is no good evidence that they
have any special resisting power to heat like endogenous spores,
and bacteriologists in general are inclined to regard them simply
as resting cells. The term arthrospores has been given to them to
indicate that they are formed as joints or segments, and this term
may be a convenient one to retain although the bodies in question
are not true spores.

Still a different method of spore formation occurs in a few
peculiar bacteria. In this case (Fig. 14) the protoplasm in the
large thread breaks into many minute spherical bodies, which
finally find exit. The spores thus formed may not be all alike,
differences in size being noticed. This method of spore formation
occurs only in a few special forms of bacteria.

The matter of spore formation serves as one of the points for
distinguishing species. Some species do not form spores, at least
under any of the conditions in which they have been studied.
Others form them readily in almost any condition, and others again
only under special conditions which are adverse to their life. The
method of spore formation is always uniform for any single
species. Whatever be the method of the formation of the spore, its
purpose in the life of the bacterium is always the same. It serves
as a means of keeping the species alive under conditions of
adversity. Its power of resisting heat or drying enables it to
live where the ordinary active forms would be speedily killed.
Some of these spores are capable of resisting a heat of 180
degrees C. (360 degrees F.) for a short time, and boiling water
they can resist for a long time. Such spores when subsequently
placed under favourable conditions will germinate and start
bacterial activity anew.

MOTION.

Some species of bacteria have the power of active motion, and may
be seen darting rapidly to and fro in the liquid in which they are
growing. This motion is produced by flagella which protrude from
the body. These flagella (Fig. 15) arise from a membrane
surrounding the bacterium, but have an intimate connection with
the protoplasmic content. Their distribution is different in
different species of bacteria. Some species have a single
flagellum at one end (Fig. 15 a). Others have one at each end
(Fig. 15 b). Others, again, have, at least just before dividing, a
bunch at one or both ends (Fig. 15 c and d), while others, again,
have many flagella distributed all over the body in dense
profusion (Fig. 15 e). These flagella keep up a lashing to and fro
in the liquid, and the lashing serves to propel the bacteria
through the liquid.

INTERNAL STRUCTURE.

It is hardly possible to say much about the structure of the
bacteria beyond the description of their external forms. With all
the variations in detail mentioned, they are extraordinarily
simple, and about all that can be seen is their external shape. Of
course, they have some internal structure, but we know very little
in regard to it. Some microscopists have described certain
appearances which they think indicate internal structure. Fig. 16
shows some of these appearances. The matter is as yet very
obscure, however. The bacteria appear to have a membranous
covering which sometimes is of a cellulose nature. Within it is
protoplasm which shows various uncertain appearances. Some
microscopists have thought they could find a nucleus, and have
regarded bacteria as cells with inclosed nucleii (Figs. 10 a and
15 f). Others have regarded the whole bacterium as a nucleus
without any protoplasm, while others, again, have concluded that
the discerned internal structure is nothing except an appearance
presented by the physical arrangement of the protoplasm. While we
may believe that they have some internal structure, we must
recognise that as yet microscopists have not been able to make it
out. In short, the bacteria after two centuries of study appear to
us about as they did at first. They must still be described as
minute spheres, rods, or spirals, with no further discernible
structure, sometimes motile and sometimes stationary, sometimes
producing spores and sometimes not, and multiplying universally by
binary fission. With all the development of the modern microscope
we can hardly say more than this. Our advance in knowledge of
bacteria is connected almost wholly with their methods of growth
and the effects they produce in Nature.

ANIMALS OR PLANTS?

There has been in the past not a little question as to whether
bacteria should be rightly classed with plants or with animals.
They certainly have characters which ally them with both. Their
very common power of active independent motion and their common
habit of living upon complex bodies for foods are animal
characters, and have lent force to the suggestion that they are
true animals. But their general form, their method of growth and
formation of threads, and their method of spore formation are
quite plantlike. Their general form is very similar to a group of
low green plants known as Oscillaria. Fig. 17 shows a group of
these Oscillariae, and the similarity of this to some of the
thread-like bacteria is decided. The Oscillariae are, however,
true plants, and are of a green colour. Bacteria are therefore to-
day looked upon as a low type of plant which has no chlorophyll,
[Footnote: Chlorophyll is the green colouring matter of plants.]
but is related to Oscillariae. The absence of the chlorophyll has
forced them to adopt new relations to food, and compels them to
feed upon complex foods instead of the simple ones, which form the
food of green plants. We may have no hesitation, then, in calling
them plants. It is interesting to notice that with this idea their
place in the organic world is reduced to a small one
systematically. They do not form a class by themselves, but are
simply a subclass, or even a family, and a family closely related
to several other common plants. But the absence of chlorophyll and
the resulting peculiar life has brought about a curious anomaly.
Whereas their closest allies are known only to botanists, and are
of no interest outside of their systematic relations, the bacteria
are familiar to every one, and are demanding the life attention of
hundreds of investigators. It is their absence of chlorophyll and
their consequent dependence upon complex foods which has produced
this anomaly.

CLASSIFICATION OF BACTERIA.

While it has generally been recognised that bacteria are plants,
any further classification has proved a matter of great
difficulty, and bacteriologists find it extremely difficult to
devise means of distinguishing species. Their extreme simplicity
makes it no easy matter to find points by which any species can be
recognised. But in spite of their similarity, there is no doubt
that many different species exist. Bacteria which appear to be
almost identical, under the microscope prove to have entirely
different properties, and must therefore be regarded as distinct
species. But how to distinguish them has been a puzzle.
Microscopists have come to look upon the differences in shape,
multiplication, and formation of spores as furnishing data
sufficient to enable them to divide the bacteria into genera. The
genus Bacillus, for instance, is the name given to all rod-shaped
bacteria which form endogenous spores, etc. But to distinguish
smaller subdivisions it has been found necessary to fall back upon
other characters, such as the shape of the colony produced in
solid gelatine, the power to produce disease, or to oxidize
nitrites, etc. Thus at present the different species are
distinguished rather by their physiological than their
morphological characters. This is an unsatisfactory basis of
classification, and has produced much confusion in the attempts to
classify bacteria. The problem of determining the species of
bacteria is to-day a very difficult one, and with our best methods
is still unsatisfactorily solved. A few species of marked
character are well known, and their powers of action so well
understood that they can be readily recognised; but of the great
host of bacteria studied, the large majority have been so slightly
experimented upon that their characters are not known, and it is
impossible, therefore, to distinguish many of them apart. We find
that each bacteriologist working in any special line commonly
keeps a list of the bacteria which he finds, with such data in
regard to them as he has collected. Such a list is of value to
him, but commonly of little value to other bacteriologists from
the insufficiency of the data. Thus it happens that a large part
of the different species of bacteria described in literature to-
day have been found and studied by one investigator alone. By him
they have been described and perhaps named. Quite likely the same
species may have been found by two or three other bacteriologists,
but owing to the difficulty of comparing results and the
incompleteness of the descriptions the identity of the species is
not discovered, and they are probably described again under
different names. The same process may be repeated over and over
again, until the same species of bacterium will come to be known
by several different names, as it has been studied by different
observers.

VARIATION OF BACTERIA.

This matter is made even more confusing by the fact that any
species of bacterium may show more or less variation. At one time
in the history of bacteriology, a period lasting for many years,
it was the prevalent opinion that there was no constancy among
bacteria, but that the same species might assume almost any of the
various forms and shapes, and possess various properties. Bacteria
were regarded by some as stages in the life history of higher
plants. This question as to whether bacteria remain constant in
character for any considerable length of time has ever been a
prominent one with bacteriologists, and even to-day we hardly know
what the final answer will be. It has been demonstrated beyond
peradventure that some species may change their physiological
characters. Disease bacteria, for instance, under certain
conditions lose their powers of developing disease. Species which
sour milk, or others which turn gelatine green, may lose their
characters. Now, since it is upon just such physiological
characters as these that we must depend in order to separate
different species of bacteria from each other, it will be seen
that great confusion and uncertainty will result in our attempts
to define species. Further, it has been proved that there is
sometimes more or less of a metamorphosis in the life history of
certain species of bacteria. The same species may form a short
rod, or a long thread, or break up into spherical spores, and thus
either a short rod, or a thread, or a spherical form may belong to
the same species. Other species may be motile at one time and
stationary at another, while at a third period it is a simple mass
of spherical spores. A spherical form, when it lengthens before
dividing, appears as a short rod, and a short rod form after
dividing may be so short as to appear like a spherical organism.

With all these reasons for confusion, it is not to be wondered at
that no satisfactory classification of bacteria has been reached,
or that different bacteriologists do not agree as to what
constitutes a species, or whether two forms are identical or not.
But with all the confusion there is slowly being obtained
something like system. In spite of the fact that species may vary
and show different properties under different conditions, the
fundamental constancy of species is everywhere recognised to-day
as a fact. The members of the same species may show different
properties under different conditions, but it is believed that
under identical conditions the properties will be constant. It is
no more possible to convert one species into another than it is
among the higher orders of plants. It is believed that bacteria do
form a group of plants by themselves, and are not to be regarded
as stages in the history of higher plants. It is believed that,
together with a considerable amount of variability and an
occasional somewhat long life history with successive stages,
there is also an essential constancy. A systematic classification
has been made which is becoming more or less satisfactory. We are
constantly learning more and more of the characters, so that they
can be recognised in different places by different observers. It
is the conviction of all who work with bacteria that, in spite of
the difficulties, it is only a matter of time when we shall have a
classification and description of bacteria so complete as to
characterize the different species accurately.

Even with our present incomplete knowledge of what characterizes a
species, it is necessary to use some names. Bacteria are commonly
given a generic name based upon their microscopic appearance.
There are only a few of these names. Micrococcus, Streptococcus,
Staphylococcus, Sarcina, Bacterium, Bacillus, Spirillum, are all
the names in common use applying to the ordinary bacteria. There
are a few others less commonly used. To this generic name a
specific name is commonly added, based upon some physiological
character. For example, Bacillus typhosus is the name given to the
bacillus which causes typhoid fever. Such names are of great use
when the species is a common and well-known one, but of doubtful
value for less-known species It frequently happens that a
bacteriologist makes a study of the bacteria found in a certain
locality, and obtains thus a long list of species hitherto
unknown. In these cases it is common simply to number these
species rather than name them. This method is frequently
advisable, since the bacteriologist can seldom hunt up all
bacteriological literature with sufficient accuracy to determine
whether some other bacteriologist may not have found the same
species in an entirely different locality. One bacteriologist, for
example, finds some seventy different species of bacteria in
different cheeses. He studies them enough for his own purposes,
but not sufficiently to determine whether some other person may
not have found the same species perhaps in milk or water. He
therefore simply numbers them--a method which conveys no suggestion
as to whether they may be new species or not. This method avoids
the giving of separate names to the same species found by
different observers, and it is hoped that gradually accumulating
knowledge will in time group together the forms which are really
identical, but which have been described by different observers.

WHERE BACTERIA ARE FOUND.

There are no other plants or animals so universally found in
Nature as the bacteria. It is this universal presence, together
with their great powers of multiplication, which renders them of
so much importance in Nature. They exist almost everywhere on the
surface of the earth. They are in the soil, especially at its
surface. They do not extend to very great depths of soil, however,
few existing below four feet of soil. At the surface they are very
abundant, especially if the soil is moist and full of organic
material. The number may range from a few hundred to one hundred
millions per gramme. [Footnote: One gramme is fifteen grains.] The
soil bacteria vary also in species, some two-score different
species having been described as common in soil. They are in all
bodies of water, both at the surface and below it. They are found
at considerable depths in the ocean. All bodies of fresh water
contain them, and all sediments in such bodies of water are filled
with bacteria. They are in streams of running water in even
greater quantity than in standing water. This is simply because
running streams are being constantly supplied with water which has
been washing the surface of the country and thus carrying off all
surface accumulations. Lakes or reservoirs, however, by standing
quiet allow the bacteria to settle to the bottom, and the water
thus gets somewhat purified. They are in the air, especially in
regions of habitation. Their numbers are greatest near the surface
of the ground, and decrease in the upper strata of air. Anything
which tends to raise dust increases the number of bacteria in the
air greatly, and the dust and emanations from the clothes of
people crowded in a close room fill the air with bacteria in very
great numbers. They are found in excessive abundance in every bit
of decaying matter wherever it may be. Manure heaps, dead bodies
of animals, decaying trees, filth and slime and muck everywhere
are filled with them, for it is in such places that they find
their best nourishment. The bodies of animals contain them in the
mouth, stomach, and intestine in great numbers, and this is, of
course, equally true of man. On the surface of the body they cling
in great quantity; attached to the clothes, under the finger
nails, among the hairs, in every possible crevice or hiding place
in the skin, and in all secretions. They do not, however, occur in
the tissues of a healthy individual, either in the blood, muscle,
gland, or any other organ. Secretions, such as milk, urine, etc.,
always contain them, however, since the bacteria do exist in the
ducts of the glands which conduct the secretions to the exterior,
and thus, while the bacteria are never in the healthy gland
itself, they always succeed in contaminating the secretion as it
passes to the exterior. Not only higher animals, but the lower
animals also have their bodies more or less covered with bacteria.
Flies have them on their feet, bees among their hairs, etc.

In short, wherever on the face of Nature there is a lodging place
for dust there will be found bacteria. In most of these localities
they are dormant, or at least growing only a little. The bacteria
clinging to the dry hair can grow but little, if at all, and those
in pure water multiply very little. When dried as dust they are
entirely dormant. But each individual bacterium or spore has the
potential power of multiplication already noticed, and as soon as
it by accident falls upon a place where there is food and moisture
it will begin to multiply. Everywhere in Nature, then, exists this
group of organisms with its almost inconceivable power of
multiplication, but a power held in check by lack of food. Furnish
them with food and their potential powers become actual. Such food
is provided by the dead bodies of animals or plants, or by animal
secretions, or from various other sources. The bacteria which are
fortunate enough to get furnished with such food material continue
to feed upon it until the food supply is exhausted or their growth
is checked in some other way. They may be regarded, therefore, as
a constant and universal power usually held in check. With their
universal presence and their powers of producing chemical changes
in food material, they are ever ready to produce changes in the
face of Nature, and to these changes we will now turn.





CHAPTER II.

MISCELLANEOUS USE OF BACTERIA IN THE ARTS.


The foods upon which bacteria live are in endless variety, almost
every product of animal or vegetable life serving to supply their
needs. Some species appear to require somewhat definite kinds of
food, and have therefore rather narrow conditions of life, but the
majority may live upon a great variety of organic compounds. As
they consume the material which serves them as food they produce
chemical changes therein. These changes are largely of a nature
that the chemist knows as decomposition changes. By this is meant
that the bacteria, seizing hold of ingredients which constitute
their food, break them to pieces chemically. The molecule of the
original food matter is split into simpler molecules, and the food
is thus changed in its chemical nature. As a result, the compounds
which appear in the decomposing solution are commonly simpler than
the original food molecules. Such products are in general called
decomposition products, or sometimes cleavage products. Sometimes,
however, the bacteria have, in addition to their power of pulling
their food to pieces, a further power of building other compounds
out of the fragments, thus building up as well as pulling down.
But, however they do it, bacteria when growing in any food
material have the power of giving rise to numerous products which
did not exist in the food mass before. Because of their
extraordinary powers of reproduction they are capable of producing
these changes very rapidly and can give rise in a short time to
large amounts of the peculiar products of their growth.

It is to these powers of producing chemical changes in their food
that bacteria owe all their importance in the world. Their power
of chemically destroying the food products is in itself of no
little importance, but the products which arise as the result of
this series of chemical changes are of an importance in the world
which we are only just beginning to appreciate. In our attempt to
outline the agency which bacteria play in our industries and in
natural processes as well, we shall notice that they are sometimes
of value simply for their power of producing decomposition; but
their greatest value lies in the fact that they are important
agents because of the products of their life.

We may notice, in the first place, that in the arts there are
several industries which may properly be classed together as
maceration industries, all of which are based upon the
decomposition powers of bacteria. Hardly any animal or vegetable
substance is able to resist their softening influence, and the
artisan relies upon this power in several different directions.

BENEFITS DERIVED FROM POWERS OF DECOMPOSITION.

Linen.--Linen consists of certain woody fibres of the stem of the
flax. The flax stem is not made up entirely of the valuable
fibres, but largely of more brittle wood fibres, which are of no
use. The valuable fibres are, however, closely united with the
wood and with each other in such an intimate fashion that it is
impossible to separate them by any mechanical means. The whole
cellular substance of the stem is bound together by some cementing
materials which hold it in a compact mass, probably a salt of
calcium and pectinic acid. The art of preparing flax is a process
of getting rid of the worthless wood fibres and preserving the
valuable, longer, tougher, and more valuable fibres, which are
then made into linen. But to separate them it is necessary first
to soften the whole tissue. This is always done through the aid of
bacteria. The flax stems, after proper preparation, are exposed to
the action of moisture and heat, which soon develops a rapid
bacterial growth. Sometimes this is done by simply exposing the
flax to the dew and rain and allowing it to lie thus exposed for
some time. By another process the stems are completely immersed in
water and allowed to remain for ten to fourteen days. By a third
process the water in which the flax is immersed is heated from 75
degrees to 90 degrees F., with the addition of certain chemicals,
for some fifty to sixty hours. In all cases the effect is the
same. The moisture and the heat cause a growth of bacteria which
proceeds with more or less rapidity according to the temperature
and other conditions. A putrefactive fermentation is thus set up
which softens the gummy substance holding the fibres together. The
process is known as "retting," and after it is completed the
fibres are easily isolated from each other. A purely mechanical
process now easily separates the valuable fibres from the wood
fibres. The whole process is a typical fermentation. A
disagreeable odour arises from the fermenting flax, and the liquid
after the fermentation is filled with products which make valuable
manure. The process has not been scientifically studied until very
recently. The bacillus which produces the "retting" is known now,
however, and it has been shown that the "retting" is a process of
decomposition of the pectin cement. No method of separating the
linen fibres in the flax from the wood fibres has yet been devised
which dispenses with the aid of bacteria.

Jute and Hemp.--Almost exactly the same use is made of bacterial
action in the manufacture of jute und hemp. The commercial aspect
of the jute industry has grown to be a large one, involving many
millions of dollars. Like linen, jute is a fibre of the inner bark
of a plant, and is mixed in the bark with a mass of other useless
fibrous material. As in the case of linen, a fermentation by
bacteria is depended upon as a means of softening the material so
that the fibres can be disassociated. The process is called
"retting," as in the linen manufacture. The details of the process
are somewhat different. The jute is commonly fermented in tanks of
stagnant water, although sometimes it is allowed to soak in river
water for a sufficient length of time to produce the softening.
After the fermentation is thus started the jute fibre is separated
from the wood, and is of a sufficient flexibility and toughness to
be woven into sacking, carpets, curtains, table covers, and other
coarse cloth.

Practically the same method is used in separating the tough fibres
of the hemp. The hemp plant contains some long flexible fibres
with others of no value, and bacterial fermentation is relied upon
to soften the tissues so that they may be separated.

Cocoanut fibre, a somewhat similar material is obtained from the
husk of the cocoanut by the same means. The unripened husk is
allowed to steep and ferment in water for a long time, six months
or a year being required. By this time the husk has become so
softened that it can be beaten until the fibres separate and can
be removed. They are subsequently made into a number of coarse
articles, especially valuable for their toughness. Door mats,
brushes, ships' fenders, etc., are illustrations of the sort of
articles made from them.

In each of these processes the fermentation must have a tendency
to soften the desired fibres as well as the connecting substance.
Putrefaction attacks all kinds of vegetable tissue, and if this
"retting" continues too long the desired fibre is decidedly
injured by the softening effect of the fermentation. It is quite
probable that, even as commonly carried on, the fermentation has
some slight injurious effect upon the fibre, and that if some
purely mechanical means could be devised for separating the fibre
from the wood it would produce a better material. But such
mechanical means has not been devised, and at present a
putrefactive fermentation appears to be the only practical method
of separating the fibres.

Sponges.--A somewhat similar use is made of bacteria in the
commercial preparation of sponges. The sponge of commerce is
simply the fibrous skeleton of a marine animal. When it is alive
this skeleton is completely filled with the softer parts of the
animal, and to fit the sponge for use this softer organic material
must be got rid of. It is easily accomplished by rotting. The
fresh sponges are allowed to stand in the warm sun and very
rapidly decay. Bacteria make their way into the sponge and
thoroughly decompose the soft tissues. After a short putrefaction
of this sort the softened organic matter can be easily washed out
of the skeleton and leave the clean fibre ready for market.

Leather preparation.--The tanning of leather is a purely chemical
process, and in some processes the whole operation of preparing
the leather is a chemical one. In others, however, especially in
America, bacteria are brought into action at one stage. The dried
hide which comes to the tannery must first have the hair removed
together with the outer skin. The hide for this purpose must be
moistened and softened. In some tanneries this is done by steeping
it in chemicals. In others, however, it is put into water and
slightly heated until fermentation arises. The fermentation
softens it so that the outer skin can be easily removed with a
knife, and the removal of hair is accomplished at the same time.
Bacterial putrefaction in the tannery is thus an assistance in
preparing the skin for the tanning proper. Even in the subsequent
tanning a bacterial fermentation appears to play a part, but
little is yet known in regard to it.

Maceration of skeletons.--The making of skeletons for museums and
anatomical instruction in general is no very great industry, and
yet it is one of importance. In the making of skeletons the
process of maceration is commonly used as an aid. The maceration
consists simply in allowing the skeleton to soak in water for a
day or two after cleaning away the bulk of the muscles. The
putrefaction that arises softens the connective tissues so much
that the bones may be readily cleaned of flesh.

Citric acid.--Bacterial fermentation is employed also in the
ordinary preparation of citric acid. The acid is made chiefly from
the juice of the lemon. The juice is pressed from the fruit and
then allowed to ferment. The fermentation aids in separating a
mucilaginous mass and making it thus possible to obtain the citric
acid in a purer condition. The action is probably similar to the
maceration processes described above, although it has not as yet
been studied by bacteriologists.

BENEFITS DERIVED FROM THE PRODUCTS OF BACTERIAL LIFE.

While bacteria thus play a part in our industries simply from
their power of producing decomposition, it is primarily because of
the products of their action that they are of value. Wherever
bacteria seize hold of organic matter and feed upon it, there are
certain to be developed new chemical compounds, resulting largely
from decomposition, but partly also from constructive processes.
These new compounds are of great variety. Different species of
bacteria do not by any means produce the same compounds even when
growing in and decomposing the same food material. Moreover, the
same species of bacteria may give rise to different products when
growing in different food materials. Some of the compounds
produced by such processes are poisonous, others are harmless.
Some are gaseous, others are liquids. Some have peculiar odours,
as may be recognised from the smell arising from a bit of decaying
meat. Others have peculiar tastes, as may be realized in the gamy
taste of meat which is in the incipient stages of putrefaction. By
purely empirical means mankind has learned methods of encouraging
the development of some of these products, and is to-day making
practical use of this power, possessed by bacteria, of furnishing
desired chemical compounds. Industries involving the investment of
hundreds of millions of dollars are founded upon the products of
bacterial life, and they have a far more important relation to our
everyday life than is commonly imagined. In many cases the artisan
who is dependent upon this action of microscopic life is unaware
of the fact. His processes are those which experience has taught
produce desired results, but, nevertheless, his dependence upon
bacteria is none the less fundamental.

BACTERIA IN THE FERMENTATIVE INDUSTRIES.

We may notice, first, several miscellaneous instances of the
application of bacteria to various fermentative industries where
their aid is of more or less value to man. In some of the examples
to be mentioned the influence of bacteria is profound and
fundamental, while in others it is only incidental. The
fermentative industries of civilization are gigantic in extent,
and have come to be an important factor in modern civilized life.
The large part of the fermentation is based upon the growth of a
class of microscopic plants which we call yeasts. Bacteria and
yeasts are both microscopic plants, and perhaps somewhat closely
related to each other. The botanist finds a difference between
them, based upon their method of multiplication, and therefore
places them in different classes (Fig. 2, page 19). In their
general power of producing chemical changes in their food
products, yeasts agree closely with bacteria, though the kinds of
chemical changes are different. The whole of the great
fermentative industries, in which are invested hundreds of
millions of dollars, is based upon chemical decompositions
produced by microscopic plants. In the great part of commercial
fermentations alcohol is the product desired, and alcohol, though
it is sometimes produced by bacteria, is in commercial quantities
produced only by yeasts. Hence it is that, although the
fermentations produced by bacteria are more common in Nature than
those produced by yeasts and give rise to a much larger number of
decomposition products, still their commercial aspect is decidedly
less important than that of yeasts. Nevertheless, bacteria are not
without their importance in the ordinary fermentative processes.
Although they are of no importance as aids in the common
fermentative processes, they are not infrequently the cause of
much trouble. In the fermentation of malt to produce beer, or
grape juice to produce wine, it is the desire of the brewer and
vintner to have this fermentation produced by pure yeasts, unmixed
with bacteria. If the yeast is pure the fermentation is uniform
and successful. But the brewer and vintner have long known that
the fermentation is frequently interfered with by irregularities.
The troubles which arise have long been known, but the
bacteriologist has finally discovered their cause, and in general
their remedy. The cause of the chief troubles which arise in the
fermentation is the presence of contaminating bacteria among the
yeasts. These bacteria have been more or less carefully studied by
bacteriologists, and their effect upon the beer or wine
determined. Some of them produce acid and render the products
sour; others make them bitter; others, again, produce a slimy
material which makes the wine or beer "ropy." Something like a
score of bacteria species have been found liable to occur in the
fermenting material and destroy the value of the product of both
the wine maker and the beer brewer. The species of bacteria which
infect and injure wine are different from those which infect and
injure beer. They are ever present as possibilities in the great
alcoholic fermentations. They are dangers which must be guarded
against. In former years the troubles from these sources were much
greater than they are at present. Since it has been demonstrated
that the different imperfections in the fermentative process are
due to bacterial impurities, commonly in the yeasts which are used
to produce the fermentation, methods of avoiding them are readily
devised. To-day the vintner has ready command of processes for
avoiding the troubles which arise from bacteria, and the brewer is
always provided with a microscope to show him the presence or
absence of the contaminating bacteria. While, then, the alcoholic
fermentations are not dependent upon bacteria, the proper
management of these fermentations requires a knowledge of their
habits and characters.

There are certain other fermentative processes of more or less
importance in their commercial aspects, which are directly
dependent upon bacterial action, Some of them we should
unhesitatingly look upon as fermentations, while others would
hardly be thought of as belonging to the fermentation industries.

VINEGAR.

The commercial importance of the manufacture of vinegar, though
large, does not, of course, compare in extent with that of the
alcoholic fermentations. Vinegar is a weak solution of acetic
acid, together with various other ingredients which have come from
the materials furnishing the acid. In the manufacture of vinegar,
alcohol is always used as the source of the acetic acid. The
production of acetic acid from alcohol is a simple oxidation. The
equation C2H6O + O2 = C2H4O2 + H2O shows the chemical change that
occurs. This oxidation can be brought about by purely chemical
means. While alcohol will not readily unite with oxygen under
common conditions, if the alcohol is allowed to pass over a bit of
platinum sponge the union readily occurs and acetic acid results.
This method of acetic-acid production is possible experimentally,
but is impracticable on any large scale. In the ordinary
manufacture of vinegar the oxidation is a true fermentation, and
brought about by the growth of bacteria.

In the commercial manufacture of vinegar several different weak
alcoholic solutions are used. The most common of these are
fermented malt, weak wine, cider, and sometimes a weak solution of
spirit to which is added sugar and malt. If these solutions are
allowed to stand for a time in contact with air, they slowly turn
sour by the gradual conversion of the alcohol into acetic acid. At
the close of the process practically all of the alcohol has
disappeared. Ordinarily, however, not all of it has been converted
into acetic acid, for the oxidation does not all stop at this
step. As the oxidation goes on, some of the acid is oxidized into
carbonic dioxide, which is, of course, dissipated at once into the
air, and if the process is allowed to continue unchecked for a
long enough period much of the acetic acid will be lost in this
way.

The oxidation of the alcohol in all commercial production of
vinegar is brought about by the growth of bacteria in the liquid.
When the vinegar production is going on properly, there is formed
on the top of the liquid a dense felted mass known as the "mother
of vinegar." This mass proves to be made of bacteria which have
the power of absorbing oxygen from the air, or, at all events, of
causing the alcohol to unite with oxygen. It was at first thought
that a single species of bacterium was thus the cause of the
oxidation of alcohol, and this was named Mycoderma aceti. But
further study has shown that several have the power, and that even
in the commercial manufacture of vinegar several species play a
part (Fig. 18), although the different species are not yet very
thoroughly studied. Each appears to act best under different
conditions. Some of them act slowly, and others rapidly, the slow-
growing species appearing to produce the larger amount of acid in
the end. After the amount of acetic acid reaches a certain
percentage, the bacteria are unable to produce more, even though
there be alcohol still left unoxidized. A percentage as high as
fourteen per cent, commonly destroys all their power of growth.
The production of the acid is wholly dependent upon the growth of
the bacteria, and the secret of the successful vinegar manufacture
is the skilful manipulation of these bacteria so as to keep them
in the purest condition and to give them the best opportunity for
growth.

One method of vinegar manufacture which is quite rapid is carried
on in a slightly different manner. A tall cylindrical chamber is
filled with wood shavings, and a weak solution of alcohol is
allowed to trickle slowly through it. The liquid after passing
over the shavings comes out after a number of hours well charged
with acetic acid. This process at first sight appears to be a
purely chemical one, and reminds us of the oxidation which occurs
when alcohol is allowed to pass over a platinum sponge. It has
been claimed, indeed, that this is a chemical oxidation in which
bacteria play no part. But this appears to be an error. It is
always found necessary in this method to start the process by
pouring upon the shavings some warm vinegar. Unless in this way
the shavings become charged with the vinegar-holding bacteria the
alcohol will not undergo oxidation during its passage over them,
and after the bacteria thus introduced have grown enough to coat
the shavings thoroughly the acetic-acid production is much more
rapid than at first. If vinegar is allowed to trickle slowly down
a suspended string, so that its bacteria may distribute themselves
through the string, and then alcohol be allowed to trickle over it
in the same way, the oxidation takes place and acetic acid is
formed. From the accumulation of such facts it has come to be
recognised that all processes for the commercial manufacture of
vinegar depend upon the action of bacteria. While the oxidation of
alcohol into acetic acid may take place by purely chemical means,
these processes are not practical on a large scale, and vinegar
manufacturers everywhere depend upon bacteria as their agents in
producing the oxidation. These bacteria, several species in all,
feed upon the nitrogenous matter in the fermenting mass and
produce the desired change in the alcohol.

This vinegar fermentation is subject to certain irregularities,
and the vinegar manufacturers can not always depend upon its
occurring in a satisfactory manner. Just as in brewing, so here,
contaminating bacteria sometimes find their way into the
fermenting mass and interfere with its normal course. In
particular, the flavour of the vinegar is liable to suffer from
such causes. As yet our vinegar manufacturers have not applied to
acetic fermentation the same principle which has been so
successful in brewing--namely, the use, as a starter of the
fermentation, of a pure culture of the proper species of bacteria.
This has been done experimentally and proves to be feasible. In
practice, however, vinegar makers find that simpler methods of
obtaining a starter--by means of which they procure a culture
nearly though not absolutely pure--are perfectly satisfactory. It
is uncertain whether really pure cultures will ever be used in
this industry.

LACTIC ACID.

The manufacture of lactic acid is an industry of less extent than
that of acetic acid, and yet it is one which has some considerable
commercial importance. Lactic acid is used in no large quantity,
although it is of some value as a medicine and in the arts. For
its production we are wholly dependent upon bacteria. It is this
acid which, as we shall see, is produced in the ordinary souring
of milk, and a large number of species of bacteria are capable of
producing the acid from milk sugar. Any sample of sour milk may
therefore always be depended upon to contain plenty of lactic
organisms. In its manufacture for commercial purposes milk is
sometimes used as a source, but more commonly other substances.
Sometimes a mixture of cane sugar and tartaric acid is used. To
start the fermentation the mixture is inoculated with a mass of
sour milk or decaying cheese, or both, such a mixture always
containing lactic organisms. To be sure, it also contains many
other bacteria which have different effects, but the acid
producers are always so abundant and grow so vigorously that the
lactic fermentation occurs in spite of all other bacteria. Here
also there is a possibility of an improvement in the process by
the use of pure cultures of lactic organisms. Up to the present,
however, there has been no application of such methods. The
commercial aspects of the industry are not upon a sufficiently
large scale to call for much in this direction.

At the present time the only method we have for the manufacture of
lactic acid is dependent upon bacteria. Chemical processes for its
manufacture are known, but not employed commercially. There are
several different kinds of lactic acid. They differ from each
other in the relations of the atoms within their molecule, and in
their relation to polarized light, some forms rotating the plane
of polarized light to the right, others to the left, while others
are inactive in this respect. All the types are produced by
fermentation processes, different species of bacteria having
powers of producing the different types.

BUTYRIC ACID.

Butyric acid is another acid for which we are chiefly dependent
upon bacteria. This acid is of no very great importance, and its
manufacture can hardly be called an industry; still it is to a
certain extent made, and is an article of commerce. It is an acid
that can be manufactured by chemical means, but, as in the case of
the last two acids, its commercial manufacture is based upon
bacterial action. Quite a number of species of bacteria can
produce butyric acid, and they produce it from a variety of
different sources. Butyric acid is a common ingredient in old milk
and in butter, and its formation by bacteria was historically one
of the first bacterial fermentations to be clearly understood. It
can be produced also in various sugar and starchy solutions.
Glycerine may also undergo a butyric fermentation. The presence of
this acid is occasionally troublesome, since it is one of the
factors in the rancidity of butter and other similar materials.

INDIGO PREPARATION.

The preparation of indigo from the indigo plant is a fermentative
process brought about by a specific bacterium. The leaves of the
plant are immersed in water in a large vat, and a rapid
fermentation arises. As a result of the fermentation the part of
the plant which is the basis of the indigo is separated from the
leaves and dissolved in the water; and as a second feature of the
fermentation the soluble material is changed in its chemical
nature into indigo proper. As this change occurs the
characteristic blue colour is developed, and the material is
rendered insoluble in water. It therefore makes its appearance as
a blue mass separated from the water, and is then removed as
indigo.

Of the nature of the process we as yet know very little. That it
is a fermentation is certain, and it has been proved that it is
produced by a definite species of bacterium which occurs on the
indigo leaves. If the sterilized leaves are placed in sterile
water no fermentation occurs and no indigo is formed. If, however,
some of the specific bacteria are added to the mass the
fermentation soon begins and the blue colour of the indigo makes
its appearance. It is plain, therefore, that indigo is a product
of bacterial fermentation, and commonly due to a single definite
species of bacterium. Of the details of the formation, however, we
as yet know little, and no practical application of the facts have
yet been made.

BACTERIA IN TOBACCO CURING.

A fermentative process of quite a different nature, but of immense
commercial value, is found in the preparation of tobacco. The
process by which tobacco is prepared is a long and somewhat
complicated one, consisting of a number of different stages. The
tobacco, after being first dried in a careful manner, is
subsequently allowed to absorb moisture from the atmosphere, and
is then placed in large heaps to undergo a further change. This
process appears to be a fermentation, for the temperature of the
mass rises rapidly, and every indication of a fermentative action
is seen. The tobacco in these heaps is changed occasionally, the
heap being thrown down and built up again in such a way that the
portion which was first at the bottom comes to the top, and in
this way all parts of the heap may become equally affected by the
process. After this process the tobacco is sent to the different
manufacturers, who finish the process of curing. The further
treatment it receives varies widely according to the desired
product, whether for smoking or for snuff, etc. In all cases,
however, fermentations play a prominent part. Sometimes the leaves
are directly inoculated with fermenting material. In the
preparation of snuff the details of the process are more
complicated than in the preparation of smoking tobacco. The
tobacco, after being ground and mixed with certain ingredients, is
allowed to undergo a fermentation which lasts for weeks, and
indeed for months. In the different methods of preparing snuff the
fermentations take place in different ways, and sometimes the
tobacco is subjected to two or three different fermentative
actions. The result of the whole is the slow preparation of the
commercial product. It is during the final fermentative processes
that the peculiar colour and flavour of the snuff are developed,
and it is during the fermentation of the leaves of the smoking
tobacco--either the original fermentation or the subsequent ones--
that the special flavours and aromas of tobacco are produced.

It can not be claimed for a moment that these changes by which the
tobacco is cured and finally brought to a marketable condition are
due wholly to bacteria. There is no question that chemical and
physical phenomena play an important part in them. Nevertheless,
from the moment when the tobacco is cut in the fields until the
time it is ready for market the curing is very intimately
associated with bacteria and fermentative organisms in general.
Some of these processes are wholly brought about by bacterial
life; in others the micro-organisms aid the process, though they
perhaps can not be regarded as the sole agents.

At the outset the tobacco producer has to contend with a number of
micro-organisms which may produce diseases in his tobacco. During
the drying process, if the temperature or the amount of moisture
or the access of air is not kept in a proper condition, various
troubles arise and various diseases make their appearance, which
either injure or ruin the value of the product. These appear to be
produced by micro-organisms of different sorts. During the
fermentation which follows the drying the producer has to contend
with micro-organisms that are troublesome to him; for unless the
phenomena are properly regulated the fermentation that occurs
produces effects upon the tobacco which ruin its character. From
the time the tobacco is cut until the final stage in the curing
the persons engaged in preparing it for market must be on a
constant watch to prevent the growth within it of undesirable
organisms. The preparation of tobacco is for this reason a
delicate operation, and one that will be very likely to fail
unless the greatest care is taken. In the several fermentative
processes which occur in the preparation there is no question that
micro-organisms aid the tobacco producer and manufacturer.
Bacteria produce the first fermentation that follows the drying,
and it is these organisms too, in large measure, that give rise to
all the subsequent fermentations, although seemingly in some cases
purely chemical processes materially aid. Now the special quality
of the tobacco is in part dependent upon the peculiar type of
fermentation which occurs in one or another of these fermenting
actions. It is the fermentation that gives rise to the peculiar
flavour and to the aroma of the different grades of tobacco.
Inasmuch as the various flavours which characterize tobacco of
different grades are developed, at least to a large extent, during
the fermentation processes, it is a natural supposition that the
different qualities of the tobacco, so far as concerns flavour,
are due to the different types of fermentation. The number of
species of bacteria which are found upon the tobacco leaves in the
various stages of its preparation is quite large, and from what we
have already learned it is inevitable that the different kinds of
bacteria will produce different results in the fermenting process.
It would seem natural, therefore, to assume that the different
flavours of different grades may not unlikely be due to the fact
that the tobacco in the different cases has been fermented under
the influence of different kinds of bacteria.

Nor is this simply a matter of inference. To a certain extent
experimental evidence has borne out the conclusion, and has given
at least a slight indication of practical results in the future.
Acting upon the suggestion that the difference between the high
grades of tobacco and the poorer grades is due to the character of
the bacteria that produce the fermentation, certain
bacteriologists have attempted to obtain from a high quality of
tobacco the species of bacteria which are infesting it. These
bacteria have then been cultivated by bacteriological methods and
used in experiments for the fermentation of tobacco. If it is true
that the flavour of high grade tobacco is in large measure, or
even in part, due to the action of the peculiar microbes from the
soil where it grows, it ought to be possible to produce similar
flavours in the leaves of tobacco grown in other localities, if
the fermentation of the leaves is carried on by means of the pure
cultures of bacteria obtained from the high grade tobacco. Not
very much has been done or is known in this connection as yet. Two
bacteriologists have experimented independently in fermenting
tobacco leaves by the action of pure cultures of bacteria obtained
from such sources. Each of them reports successful experiments.
Each claims that they have been able to improve the quality of
tobacco by inoculating the leaves with a pure culture of bacteria
obtained from tobacco having high quality in flavour. In addition
to this, several other bacteriologists have carried on experiments
sufficient to indicate that the flavours of the tobacco and the
character of the ripening may be decidedly changed by the use of
different species of micro-organisms in the fermentations that go
on during the curing processes.

In regard to the whole matter, however, we must recognise that as
yet we have very little knowledge. The subject has been under
investigation for only a short time; and, while considerable
information has been derived, this information is not thoroughly
understood, and our knowledge in regard to the matter is as yet in
rather a chaotic condition. It seems certain, however, that the
quality of tobacco is in large measure dependent upon the
character of the fermentations that occur at different stages of
the curing. It seems certain also that these fermentations are
wholly or chiefly produced by microorganisms, and that the
character of the fermentation is in large measure dependent upon
the species of micro-organisms that produce it. If these are
facts, it would seem not improbable that a further study may
produce practical results for this great industry. The study of
yeasts and the methods of keeping yeast from contaminations has
revolutionised the brewing industry. Perhaps in this other
fermentative industry, which is of such great commercial extent,
the use of pure cultures of bacteria may in the future produce as
great revolutions in methods as it has in the industry of the
alcoholic fermentation.

It must not, however, be inferred that the differences in grades
of tobacco grown in different parts of the world are due solely to
variations in the curing processes and to the types of
fermentation. There are differences in the texture of the leaves,
differences in the chemical composition of the tobaccoes, which
are due undoubtedly to the soils and the climatic conditions in
which they grow, and these, of course, will never be affected by
changing the character of the ferment active processes. It is,
however, probable that in so far as the flavours that distinguish
the high and low grades of tobacco are due to the character of the
fermentative processes, they may be in the future, at least to a
large extent, controlled by the use of pure cultures in curing
processes. Seemingly, then, there is as great a future in the
development of this fermentative industry as there has been in the
past in the development of the fermentative industry associated
with brewing and vinting.

OPIUM.

Opium for smoking purposes is commonly allowed to undergo a curing
process which lasts several months. This appears to be somewhat
similar to the curing of tobacco. Apparently it is a fermentation
due to the growth of microorganisms. The organisms in question are
not, however, bacteria in this case, but a species of allied
fungus. The plant is a mould, and it is claimed that inoculation
of the opium with cultures of this mould hastens the curing.

TROUBLESOME FERMENTATIONS.

Before leaving this branch of the subject it is necessary to
notice some of the troublesome fermentations which are ever
interfering with our industries, requiring special methods, or,
indeed, sometimes developing special industries to meet them. As
agents of decomposition, bacteria will of course be a trouble
whenever they get into material which it is desired to preserve.
Since they are abundant everywhere, it is necessary to count upon
their attacking with certainty any fermentable substance which is
exposed to air and water. Hence they are frequently the cause of
much trouble. In the fermentative industries they occasionally
cause an improper sort of fermentation to occur unless care is
taken to prevent undesired species of bacteria from being present.
In vinegar making, improper species of bacteria obtaining access
to the solution give rise to undesirable flavours, greatly
injuring the product. In tobacco curing it is very common for the
wrong species of bacteria to gain access to the tobacco at some
stage of the curing and by their growth give rise to various
troubles. It is the ubiquitous presence of bacteria which makes it
impossible to preserve fruits, meats, or vegetables for any length
of time without special methods. This fact in itself has caused
the development of one of our most important industries. Canning
meats or fruits consists in nothing more than bringing them into a
condition in which they will be preserved from attack of these
micro-organisms. The method is extremely simple in theory. It is
nothing more than heating the material to be preserved to a high
temperature and then sealing it hermetically while it is still
hot. The heat kills all the bacteria which may chance to be lodged
in it, and the hermetical sealing prevents other bacteria from
obtaining access. Inasmuch as all organic decomposition is
produced by bacterial growth, such sterilized and sealed material
will be preserved indefinitely when the operation is performed
carefully enough. The methods of accomplishing this with
sufficient care are somewhat varied in different industries, but
they are all fundamentally the same. It is an interesting fact
that this method of preserving meats was devised in the last
century, before the relation of micro-organisms to fermentation
and putrefaction was really suspected. For a long time it had been
in practical use while scientists were still disputing whether
putrefaction could be avoided by preventing the access of
bacteria. The industry has, however, developed wonderfully within
the last few years, since the principles underlying it have been
understood. This understanding has led to better methods of
destroying bacterial life and to proper sealing, and these have of
course led to greater success in the preservation, until to-day
the canning industries are among those which involve capital
reckoned in the millions.

Occasionally bacteria are of some value in food products. The gamy
flavour of meats is nothing more than incipient decomposition.
Sauer Kraut is a food mass intentionally allowed to ferment and
sour. The value of bacteria in producing butter and cheese
flavours is noticed elsewhere. But commonly our aim must be to
prevent the growth of bacteria in foods. Foods must be dried or
cooked or kept on ice, or some other means adopted for preventing
bacterial growth in them. It is their presence that forces us to
keep our ice box, thus founding the ice business, as well as that
of the manufacture of refrigerators. It is their presence, again,
that forces us to smoke hams, to salt mackerel, to dry fish or
other meats, to keep pork in brine, and to introduce numerous
other details in the methods of food preparation and preservation.





CHAPTER III.

RELATION OF BACTERIA TO THE DAIRY INDUSTRY.


Dairying is one of the most primitive of our industries. From the
very earliest period, ever since man began to keep domestic
cattle, he has been familiar with dairying. During these many
centuries certain methods of procedure have been developed which
produce desired results. These methods, however, have been devised
simply from the accumulation of experience, with very little
knowledge as to the reasons underlying them. The methods of past
centuries are, however, ceasing to be satisfactory. The advance of
our civilization during the last half century has seen a marked
expansion in the extent of the dairy industry. With this expansion
has appeared the necessity for new methods, and dairymen have for
years been looking for them. The last few years have been teaching
us that the new methods are to be found along the line of the
application of the discoveries of modern bacteriology. We have
been learning that the dairyman is more closely related to
bacteria and their activities than almost any other class of
persons. Modern dairying, apart from the matter of keeping the
cow, consists largely in trying to prevent bacteria from growing
in milk or in stimulating their growth in cream, butter, and
cheese. These chief products of the dairy will be considered
separately.

SOURCES OF BACTERIA IN MILK.

The first fact that claims our attention is, that milk at the time
it is secreted from the udder of the healthy cow contains no
bacteria. Although bacteria are almost ubiquitous, they are not
found in the circulating fluids of healthy animals, and are not
secreted by their glands. Milk when first secreted by the milk
gland is therefore free from bacteria. It has taken a long time to
demonstrate this fact, but it has been finally satisfactorily
proved. Secondly, it has been demonstrated that practically all of
the normal changes which occur in milk after its secretion are
caused by the growth of bacteria. This, too, was long denied, and
for quite a number of years after putrefactions and fermentations
were generally acknowledged to be caused by the growth of micro-
organisms, the changes which occurred in milk were excepted from
the rule. The uniformity with which milk will sour, and the
difficulty, or seeming impossibility, of preventing this change,
led to the belief that the souring of milk was a normal change
characteristic of milk, just as clotting is characteristic of
blood. This was, however, eventually disproved, and it was finally
demonstrated that, beyond a few physical changes connected with
evaporation and a slight oxidation of the fat, milk, if kept free
from bacteria, will undergo no change. If bacteria are not
present, it will remain sweet indefinitely.

But it is impossible to draw milk from the cow in such a manner
that it will be free from bacteria except by the use of
precautions absolutely impracticable in ordinary dairying. As milk
is commonly drawn, it is sure to be contaminated by bacteria, and
by the time it has entered the milk pail it contains frequently as
many as half a million, or even a million, bacteria in every cubic
inch of the milk. This seems almost incredible, but it has been
demonstrated in many cases and is beyond question. Since these
bacteria are not in the secreted milk, they must come from some
external sources, and these sources are the following:

The first in importance is the cow herself; for while her milk
when secreted is sterile, and while there are no bacteria in her
blood, nevertheless the cow is the most prolific source of
bacterial contamination. In the first place, the milk ducts are
full of them. After each milking a little milk is always left in
the duct, and this furnishes an ideal place for bacteria to grow.
Some bacteria from the air or elsewhere are sure to get into these
ducts after the milking, and they begin at once to multiply
rapidly. By the next milking they become very abundant in the
ducts, and the first milk drawn washes most of them at once into
the milk pail, where they can continue their growth in the milk.
Again, the exterior of the cow's body contains them in abundance.
Every hair, every particle of dirt, every bit of dried manure, is
a lurking place for millions of bacteria. The hind quarters of a
cow are commonly in a condition of much filth, for the farmer
rarely grooms his cow, and during the milking, by her movements,
by the switching of her tail, and by the rubbing she gets from the
milker, no inconsiderable amount of this dirt and filth is brushed
off and falls into the milk pail The farmer understands this
source of dirt and usually feels it necessary to strain the milk
after the milking. But the straining it receives through a coarse
cloth, while it will remove the coarser particles of dirt, has no
effect upon the bacteria, for these pass through any strainer
unimpeded. Again, the milk vessels themselves contain bacteria, for
they are never washed absolutely clean. After the most thorough
washing which the milk pail receives from the kitchen, there will
always be left many bacteria clinging in the cracks of the tin or
in the wood, ready to begin to grow as soon as the milk once more
fills the pail The milker himself contributes to the supply, for
he goes to the milking with unclean hands, unclean clothes, and
not a few bacteria get from him to his milk pail. Lastly, we find
the air of the milking stall furnishing its quota of milk bacteria.
This source of bacteria is, how ever, not so great as was formerly
believed. That the air may contain many bacteria in its dust is
certain, and doubtless these fall in some quantity into the milk,
especially if the cattle are allowed to feed upon dusty hay before
and during the milking. But unless the air is thus full of dust
this source of bacteria is not very great, and compared with the
bacteria from the other sources the air bacteria are unimportant.

The milk thus gets filled with bacteria, and since it furnishes an
excellent food these bacteria begin at once to grow. The milk when
drawn is warm and at a temperature which especially stimulates
bacterial growth. They multiply with great rapidity, and in the
course of a few hours increase perhaps a thousandfold. The numbers
which may be found after twenty-four hours are sometimes
inconceivable; market milk may contain as many as five hundred
millions per cubic inch; and while this is a decidedly extreme
number, milk that is a day old will almost always contain many
millions in each cubic inch, the number depending upon the age of
the milk and its temperature. During this growth the bacteria
have, of course, not been without their effect. Recognising as we
do that bacteria are agents for chemical change, we are prepared
to see the milk undergoing some modifications during this rapid
multiplication of bacteria. The changes which these bacteria
produce in the milk and its products are numerous, and decidedly
affect its value. They are both advantageous and disadvantageous
to the dairyman. They are nuisances so far as concerns the milk
producer, but allies of the butter and cheese maker.

THE EFFECT OF BACTERIA ON MILK.

The first and most universal change effected in milk is its
SOURING. So universal is this phenomenon that it is generally
regarded as an inevitable change which can not be avoided, and, as
already pointed out, has in the past been regarded as a normal
property of milk. To-day, however, the phenomenon is well
understood. It is due to the action of certain of the milk
bacteria upon the milk sugar which converts it into lactic acid,
and this acid gives the sour taste and curdles the milk. After
this acid is produced in small quantity its presence proves
deleterious to the growth of the bacteria, and further bacterial
growth is checked. After souring, therefore, the milk for some
time does not ordinarily undergo any further changes.

Milk souring has been commonly regarded as a single phenomenon,
alike in all cases. When it was first studied by bacteriologists
it was thought to be due in all cases to a single species of
micro-organism which was discovered to be commonly present and
named Bacillus acidi lactici (Fig. 19). This bacterium has
certainly the power of souring milk rapidly, and is found to be
very common in dairies in Europe. As soon as bacteriologists
turned their attention more closely to the subject it was found
that the spontaneous souring of milk was not always caused by the
same species of bacterium. Instead of finding this Bacillus acidi
lactici always present, they found that quite a number of
different species of bacteria have the power of souring milk, and
are found in different specimens of soured milk. The number of
species of bacteria which have been found to sour milk has
increased until something over a hundred are known to have this
power. These different species do not affect the milk in the same
way. All produce some acid, but they differ in the kind and the
amount of acid, and especially in the other changes which are
effected at the same time that the milk is soured, so that the
resulting soured milk is quite variable. In spite of this variety,
however, the most recent work tends to show that the majority of
cases of spontaneous souring of milk are produced by bacteria
which, though somewhat variable, probably constitute a single
species, and are identical with the Bacillus acidi lactici (Fig.
19). This species, found common in the dairies of Europe,
according to recent investigations occurs in this country as well.
We may say, then, that while there are many species of bacteria
infesting the dairy which can sour the milk, there is one which is
more common and more universally found than others, and this is
the ordinary cause of milk souring.

When we study more carefully the effect upon the milk of the
different species of bacteria found in the dairy, we find that
there is a great variety of changes which they produce when they
are allowed to grow in milk. The dairyman experiences many
troubles with his milk. It sometimes curdles without becoming
acid. Sometimes it becomes bitter, or acquires an unpleasant
"tainted" taste, or, again, a "soapy" taste. Occasionally a
dairyman finds his milk becoming slimy, instead of souring and
curdling in the normal fashion. At such times, after a number of
hours, the milk becomes so slimy that it can be drawn into long
threads. Such an infection proves very troublesome, for many a
time it persists in spite of all attempts made to remedy it.
Again, in other cases the milk will turn blue, acquiring about the
time it becomes sour a beautiful sky-blue colour. Or it may become
red, or occasionally yellow. All of these troubles the dairyman
owes to the presence in his milk of unusual species of bacteria
which grow there abundantly.

Bacteriologists have been able to make out satisfactorily the
connection of all these infections with different species of the
bacteria. A large number of species have been found to curdle milk
without rendering it acid, several render it bitter, and a number
produce a "tainted" and one a "soapy" taste. A score or more have
been found which have the power of rendering the milk slimy. Two
different species at least have the power of turning the milk to
sky-blue colour; two or three produce red pigments (Fig. 20), and
one or two have been found which produce a yellow colour. In
short, it has been determined beyond question that all these
infections, which are more or less troublesome to dairymen, are
due to the growth of unusual bacteria in the milk.

These various infections are all troublesome, and indeed it may be
said that, so far as concerns the milk producer and the milk
consumer, bacteria are from beginning to end a source of trouble.
It is the desire of the milk producer to avoid them as far as
possible--a desire which is shared also by everyone who has
anything to do with milk as milk. Having recognised that the
various troubles, which occasionally occur even in the better
class of dairies, are due to bacteria, the dairyman is, at least
in a measure, prepared to avoid them. The avoiding of these
troubles is moderately easy as soon as dairymen recognise the
source from which the infectious organisms come, and also the fact
that low temperatures will in all cases remedy the evil to a large
extent. With this knowledge in hand the avoidance of all these
troubles is only a question of care in handling the dairy. It must
be recognised that most of these troublesome bacteria come from
some unusual sources of infection. By unusual sources are meant
those which the exercise of care will avoid. It is true that the
souring bacteria appear to be so universally distributed that they
can not be avoided by any ordinary means. But all other
troublesome bacteria appear to be within control. The milkman must
remember that the sources of the troubles which are liable to
arise in his milk are in some form of filth: either filth on the
cow, or dust in the hay which is scattered through the barn, or
dirt on cows' udders, or some other unusual and avoidable source.
These sources, from what we have already noticed, will always
furnish the milk with bacteria; but under common conditions, and
when the cow is kept in conditions of ordinary cleanliness, and
frequently even when not cleanly, will only furnish bacteria that
produce the universal souring. Recognising this, the dairyman at
once learns that his remedies for the troublesome infections are
cleanliness and low temperatures. If he is careful to keep his
milk vessels scrupulously clean; if he will keep his cow as
cleanly as he does his horse; and if he will use care in and
around the barn and dairy, and then apply low temperatures to the
milk, he need never be disturbed by slimy or tainted milk, or any
of these other troubles; or he can remove such infections speedily
should they once appear. Pure sweet milk is only a question of
sufficient care. But care means labour and expense. As long as we
demand cheap milk, so long will we be supplied with milk procured
under conditions of filth. But when we learn that cheap milk is
poor milk, and when we are willing to pay a little more for it,
then only may we expect the use of greater care in the handling of
the milk, resulting in a purer product.

Bacteriology has therefore taught us that the whole question of
the milk supply in our communities is one of avoiding the too
rapid growth of bacteria. These organisms are uniformly a nuisance
to the milkman. To avoid their evil influence have been designed
all the methods of caring for the dairy and the barn, all the
methods of distributing milk in ice cars. Moreover, all the
special devices connected with the great industry of milk supply
have for their foundation the attempt to avoid, in the first
place, the presence of too great a number of bacteria, and. in the
second place, the growth of these bacteria.

BACTERIA IN BUTTER MAKING.

CREAM RIPENING.--Passing from milk to butter, we find a somewhat
different story, inasmuch as here bacteria are direct allies to
the dairyman rather than his enemies. Without being aware of it,
butter makers have for years been making use of bacteria in their
butter making and have been profiting by the products which the
bacteria have furnished them. Cream, as it is obtained from milk,
will always contain bacteria in large quantity, and these bacteria
will grow as readily in the cream as they will in the milk. The
butter maker seldom churns his cream when it is freshly obtained
from the milk. There are, it is true, some places where sweet
cream butter is made and is in demand, but in the majority of
butter-consuming countries a different quality of butter is
desired, and the cream is subjected to a process known as
"ripening" or "souring" before it is churned. In ripening, the
cream is simply allowed to stand in a vat for a period varying
from twelve hours to two or three days, according to
circumstances. During this period certain changes take place
therein. The bacteria which were in the cream originally, get an
opportunity to grow, and by the time the ripening is complete they
become extremely numerous. As a result, the character of the cream
changes just as the milk is changed under similar circumstances.
It becomes somewhat soured; it becomes slightly curdled, and
acquires a peculiarly pleasant taste and an aroma which was not
present in the original fresh cream. After this ripening the cream
is churned. It is during the ripening that the bacteria produce
their effect, for after the churning they are of less importance.
Part of them collect in the butter, part of them are washed off
from the butter in the buttermilk and the subsequent processes.
Most of the bacteria that are left in the butter soon die, not
finding there a favourable condition for growth; some of them,
however, live and grow for some time and are prominent agents in
the changes by which butter becomes rancid. The butter maker is
concerned with the ripening rather than with later processes.

The object of the ripening of cream is to render it in a better
condition for butter making. The butter maker has learned by long-
experience that ripened cream churns more rapidly than sweet
cream, and that he obtains a larger yield of butter therefrom. The
great object of the ripening, however, is to develop in the butter
the peculiar flavour and aroma which is characteristic of the
highest product. Sweet cream butter lacks flavour and aroma,
having indeed a taste almost identically the same as cream.
Butter, however, that is made from ripened cream has a peculiar
delicate flavour and aroma which is well known to lovers of
butter, and which is developed during the ripening process.

Bacteriologists have been able to explain with a considerable
degree of accuracy the object of this ripening. The process is
really a fermentation comparable to the fermentation that takes
place in a brewer's malt. The growth of bacteria during the
ripening produces chemical changes of a somewhat complicated
character, and concerns each of the ingredients of the milk. The
lactic-acid organisms affect the milk sugar and produce lactic
acid; others act upon the fat, producing slight changes therein;
while others act upon the casein and the albumens of the milk. As
a result, various biproducts of decomposition arise, and it is
these biproducts of decomposition that make the difference between
the ripened and the unripened cream. They render it sour and
curdle it, and they also produce the flavours and aromas that
characterize it. Products of decomposition are generally looked
upon as undesirable for food, and this is equally true of these
products that arise in cream if the decomposition is allowed to
continue long enough. If the ripening, instead of being stopped at
the end of a day or two, is allowed to continue several days, the
cream becomes decayed and the butter made therefrom is decidedly
offensive. But under the conditions of ordinary ripening, when the
process is stopped at the right moment, the decomposition products
are pleasant rather than unpleasant, and the flavours and aromas
which they impart to the cream and to the subsequent butter are
those that are desired. It is these decomposition products that
give the peculiar character to a high quality of butter, and this
peculiar quality is a matter that determines the price which the
butter maker can obtain for his product.

But, unfortunately, the butter maker is not always able to depend
upon the ripening. While commonly it progresses in a satisfactory
manner, sometimes, for no reason that he can assign, the ripening
does not progress normally. Instead of developing the pleasant
aroma and flavour of the properly ripened cream, the cream develops
unpleasant tastes. It may be bitter or somewhat tainted, and just
as sure as these flavours develop in the cream, so sure does the
quality of the butter suffer. Moreover, it has been learned by
experience that some creameries are incapable of obtaining an
equally good ripening of their cream. While some of them will
obtain favourable results, others, with equal care, will obtain a
far less favourable flavour and aroma in their butter. The reason
for all this has been explained by modern bacteriology. In the
milk, and consequently in the cream, there are always found many
bacteria, but these are not always of the same kinds. There are
scores, and probably hundreds, of species of bacteria common in and
around our barns and dairies, and the bacteria that are abundant
and that grow in different lots of cream will not be always the
same. It makes a decided difference in the character of the
ripening, and in the consequent flavours and aromas, whether one or
another species of bacteria has been growing in the cream. Some
species are found to produce good results with desired flavours,
while others, under identical conditions, produce decidedly poor
results with undesired flavours. If the butter maker obtains cream
which is filled with a large number of bacteria capable of
producing good flavours, then the ripening of his cream will be
satisfactory and his butter will be of high quality. If, however,
it chances that his cream contains only the species which produce
unpleasant flavours, then the character of the ripening will be
decidedly inferior and the butter will be of a poorer grade.
Fortunately the majority of the kinds of bacteria liable to get
into the cream from ordinary sources are such as produce either
good effects upon the cream or do not materially influence the
flavour or aroma. Hence it is that the ripening of cream will
commonly produce good results. Bacteriologists have learned that
there are some species of bacteria more or less common around our
barns which produce undesirable effects upon flavour, and should
these become especially abundant in the cream, then the character
of the ripening and the quality of the subsequent butter will
suffer. These malign species of bacteria, however, are not very
common in properly kept barns and dairies. Hence the process that
is so widely used, of simply allowing cream to ripen under the
influence of any bacteria that happen to be in it, ordinarily
produces good results. But our butter makers sometimes find, at the
times when the cattle change from winter to summer or from summer
to winter feed, that the ripening is abnormal. The reason appears
to be that the cream has become infested with an abundance of
malign species. The ripening that they produce is therefore an
undesirable one, and the quality of the butter is sure to suffer.

So long as butter was made only in private dairies it was a matter
of comparatively little importance if there was an occasional
falling off in quality of this sort. When it was made a few pounds
at a time, and only once or twice a week, it was not a very
serious matter if a few churnings of butter did suffer in quality.
But to-day the butter-making industries are becoming more and more
concentrated into large creameries, and it is a matter of a good
deal more importance to discover some means by which a uniformly
high quality can be insured. If a creamery which makes five
hundred pounds of butter per day suffers from such an injurious
ripening, the quality of its butter will fall off to such an
extent as to command a lower price, and the creamery suffers
materially. Perhaps the continuation of such a trouble for two or
three weeks would make a difference between financial success and
failure in the creamery. With our concentration of the butter-
making industries it is becoming thus desirable to discover some
means of regulating this process more accurately.

The remedy of these occasional ill effects in cream ripening has
not been within the reach of the butter maker. The butter maker
must make butter with the cream that is furnished him, and if that
cream is already impregnated with malign species of bacteria he is
helpless. It is true that much can be done to remedy these
difficulties by the exercise of especial care in the barns of the
patrons of the creamery. If the barns, the cows, the dairies, the
milk vessels, etc., are all kept in condition of strict
cleanliness, if especial care is taken particularly at the seasons
of the year when trouble is likely to arise, and if some attention
is paid to the kind of food which the cattle eat, as a rule the
cream will not become infected with injurious bacteria. It may be
taken as a demonstrated fact that these malign bacteria come from
sources of filth, and the careful avoidance of all such sources of
filth will in a very large measure prevent their occurrence in the
cream. Such measures as these have been found to be practicable in
many creameries. Creameries which make the highest priced and the
most uniform quality of butter are those in which the greatest
care is taken in the barns and dairies to insure cleanliness and
in the handling of the milk and cream. With such attention a large
portion of the trouble which arises in the creameries from malign
bacteria may be avoided.

But these methods furnish no sure remedy against evils of improper
species of bacteria in cream ripening, and do not furnish any sure
means of obtaining uniform flavour in butter. Even under the very
best conditions the flavour of the butter will vary with the
season of the year. Butter made in the winter is inferior to that
made in the summer months; and while this is doubtless due in part
to the different food which the cattle have and to the character
of the cream resulting therefrom, these differences in the flavour
of the butter are also in part dependent upon the different
species of bacteria which are present in the ripening of cream at
different seasons. The species of bacteria in June cream are
different from those that are commonly present in January cream,
and this is certainly a factor in determining the difference
between winter and summer butter.

USE OF ARTIFICIAL BACTERIA CULTURES FOR CREAM RIPENING.

Bacteriologists have been for some time endeavouring to aid butter
makers in this direction by furnishing them with the bacteria
needful for the best results in cream ripening. The method of
doing this is extremely simple in principle, but proves to be
somewhat difficult in practice. It is only necessary to obtain the
species of bacteria that produce the highest results, and then to
furnish these in pure culture and in large quantity to the butter
makers, to enable them to inoculate their cream with the species
of bacteria which will produce the results that they desire. For
this purpose bacteriologists have been for several years searching
for the proper species of bacteria to produce the best results,
and there have been put upon the market for sale several distinct
"pure cultures" for this purpose. These have been obtained by
different bacteriologists and dairymen in the northern European
countries and also in the United States. These pure cultures are
furnished to the dairymen in various forms, but they always
consist of great quantities of certain kinds of bacteria which
experience has found to be advantageous for the purpose of cream
ripening.

There have hitherto appeared a number of difficulties in the way
of reaching complete success in these directions. The most
prominent arises in devising a method of using pure cultures in
the creamery. The cream which the butter makers desire to ripen
is, as we have seen, already impregnated with bacteria, and would
ripen in a fashion of its own even if no pure culture of bacteria
were added thereto. Pure cultures can not therefore be used as
simply as can yeast in bread dough. It is plain that the simple
addition of a pure culture to a mass of cream would not produce
the desired effects, because the cream would be ripened then, not
by the pure culture alone, but by the pure culture plus all of the
bacteria that were originally present. It would, of course, be
something of a question as to whether under these conditions the
results would be favourable, and it would seem that this method
would not furnish any means of getting rid of bad tastes and
flavours which have come from the presence of malign species of
bacteria. It is plainly desirable to get rid of the cream bacteria
before the pure culture is added. This can be readily done by
heating it to a temperature of 69 degrees C. (155 degrees F.) for
a short time, this temperature being sufficient to destroy most of
the bacteria. The subsequent addition of the pure culture of
cream-ripening bacteria will cause the cream to ripen under the
influence of the added culture alone. This method proves to be
successful, and in the butter making countries in Europe it is
becoming rapidly adopted.

In this country, however, this process has not as yet become very
popular, inasmuch as the heating of the cream is a matter of
considerable expense and trouble, and our butter makers have not
been very ready to adopt it. For this reason, and also for the
purpose of familiarizing butter makers with the use of pure
cultures, it has been attempted to produce somewhat similar though
less uniform results by the use of pure cultures in cream without
previous healing. In the use of pure cultures in this way, the
butter maker is directed to add to his cream a large amount of a
prepared culture of certain species of bacteria, upon the
principle that the addition of such a large number of bacteria to
the cream, even though the cream is already inoculated with
certain bacteria, will produce a ripening of the cream chiefly
influenced by the artificially added culture. The culture thus
added, being present in very much greater quantity than the other
"wild" species, will have a much greater effect than any of them.
This method, of course, cannot insure uniformity. While it may
work satisfactorily in many cases, it is very evident that in
others, when the cream is already filled with a large number of
malign species of bacteria, such an artificial culture would not
produce the desired results. This appears to be not only the
theoretical but the actual experience. The addition of such pure
cultures in many cases produces favourable results, but it does
not always do so, and the result is not uniform. While the use of
pure cultures in this way is an advantage over the method of
simply allowing the cream to ripen normally without such
additions, it is a method that is decidedly inferior to that which
first pasteurizes the cream and subsequently adds a starter.

There is still another method of adding bacteria to cream to
insure a more advantageous ripening, which is frequently used,
and, being simpler, is in many cases a decided advantage. This
method is by the use of what is called a natural starter. A
natural starter consists simply of a lot of cream which has been
taken from the most favourable source possible--that is, from the
cleanest and best dairy, or from the herd producing the best
quality of cream--and allowing this cream to stand in a warm place
for a couple of days until it becomes sour. The cream will by that
time be filled with large numbers of bacteria, and this is then
put as a starter into the vat of cream to be ripened. Of course,
in the use of this method the butter maker has no control over the
kinds of bacteria that will grow in the starter, but it is found,
practically, that if the cream is taken from a good source the
results are extremely favourable, and there is produced in this
way almost always an improvement in the butter.

The use of pure cultures is still quite new, particularly in this
country. In the European butter-making countries they have been
used for a longer period and have become very much better known.
What the future may develop along this line it is difficult to
say; but it seems at least probable that as the difficulties in
the details are mastered the time will come when starters will be
used by our butter makers for their cream ripening, just as yeast
is used by housewives for raising bread, or by brewers for
fermenting malt. These starters will probably in time be furnished
by bacteriologists. Bacteriology, in other words, is offering in
the near future to our butter makers a method of controlling the
ripening of the cream in such a way as to insure the obtaining of
a high and uniform quality of butter, so far, at least, as
concerns flavour and aroma.

BACTERIA IN CHEESE.

Cheese ripening.--The third great product of the dairy industry is
cheese, and in connection with this product the dairyman is even
more dependent upon bacteria than he is in the production of
butter. In the manufacture of cheese the casein of the milk is
separated from the other products by the use of rennet, and is
collected in large masses and pressed, forming the fresh cheese.
This cheese is then set aside for several weeks, and sometimes for
months, to undergo a process that is known as ripening. During the
ripening there are developed in the cheese the peculiar flavours
which are characteristic of the completed product. The taste of
freshly made cheese is extremely unlike that of the ripened
product. While butter made from unripened cream has a pleasant
flavour, and one which is in many places particularly enjoyed,
there is nowhere a demand for unripened cheese, for the freshly
made cheese has a taste that scarce any one regards as pleasant.
Indeed, the whole value of the cheese is dependent upon the
flavour of the product, and this flavour is developed during the
ripening.

The cheese maker finds in the ripening of his cheese the most
difficult part of his manufacture. It is indeed a process over
which he has very little control. Even when all conditions seem to
be correct, when cheese is made in the most careful manner, it not
infrequently occurs that the ripening takes place in a manner that
is entirely abnormal, and the resulting cheese becomes worthless.
The cheese maker has been at an entire loss to understand these
irregularities, nor has he possessed any means of removing them.
The abnormal ripening that occurs takes on various types.
Sometimes the cheese will become extraordinarily porous, filled
with large holes which cause the cheese to swell out of proper
shape and become worthless. At other times various spots of red or
blue appear in the manufactured cheese; while again unpleasant
tastes and flavours develop which render the product of no value.
Sometimes a considerable portion of the product of the cheese
factory undergoes such irregular ripening, and the product for a
long time will thus be worthless. If some means could be
discovered of removing these irregularities it would be a great
boon to the cheese manufacturer; and very many attempts have been
made in one way or another to furnish the cheese maker with some
details in the manufacture which will enable him in a measure to
control the ripening.

The ripening of the cheese has been subjected to a large amount of
study on the part of bacteriologists who have been interested in
dairy products. That the ripening of cheese is the result of
bacterial growth therein appears to be probable from a priori
grounds. Like the ripening of cream, it is a process that occurs
somewhat slowly. It is a chemical change which is accompanied by
the destruction of proteid matter; it takes place best at certain
temperatures, and temperatures which we know are favourable to the
growth of micro-organisms, all of which phenomena suggest to us
the action of bacteria. Moreover, the flavours and the tastes that
arise have a decided resemblance in many cases to the
decomposition products of bacteria, strikingly so in Limburger
cheese. When we come to study the matter of cheese ripening
carefully we learn beyond question that this a priori conclusion
is correct. The ripening of any cheese is dependent upon several
different factors. The method of preparation, the amount of water
left in the curd, the temperature of ripening, and other
miscellaneous factors connected with the mechanical process of
cheese manufacture, affect its character. But, in addition to all
these factors, there is undoubtedly another one, and that is the
number and the character of the bacteria that chance to be in the
curd when the cheese is made. While it is found that cheeses which
are treated by different processes will ripen in a different
manner, it is also found that two cheeses which have been made
under similar conditions and treated in identically the same way
may also ripen in a different manner, so that the resulting
flavour will vary. The variations between cheeses thus made may be
slight or they may be considerable, but variations certainly do
occur. Every one knows the great difference in flavours of
different cheeses, and these flavours are due in considerable
measure to factors other than the simple mechanical process of
making the cheese. The general similarity of the whole process to
a bacterial fermentation leads us to believe at the outset that
some of the differences in character are due to different kinds of
bacteria that multiply in the cheese and produce decomposition
therein.

When the matter comes to be studied by bacteriology, the
demonstration of this position becomes easy. That the ripening of
cheese is due to growth of bacteria is very easily proved by
manufacturing cheeses from milk which is deprived of bacteria. For
instance, cheeses have been made from milk that has been either
sterilized or pasteurized--which processes destroy most of the
bacteria therein--and, treated otherwise in a normal manner, are
set aside to ripen. These cheeses do NOT ripen, but remain for
months with practically the same taste that they had originally.
In other experiments the cheese has been treated with a small
amount of disinfective, which is sufficient to prevent bacteria
from growing, and again ripening is found to be absolutely
prevented. Furthermore, if the cheese under ordinary conditions is
studied during the ripening process, it is found that bacteria are
growing during the whole time. These facts all taken together
plainly prove that the ripening of cheese is a fermentation due to
bacteria. It will be noticed, however, that the conditions in the
cheese are not favourable for very rapid bacterial growth. It is
true that there is plenty of food in the cheese for bacterial
life, but the cheese is not very moist; it is extremely dense,
being subjected in all cases to more or less pressure. The
penetration of oxygen into the centre of the mass must be
extremely slight. The density, the lack of a great amount of
moisture, and the lack of oxygen furnish conditions in which
bacteria will not grow very rapidly. The conditions are far less
favourable than those of ripening cream, and the bacteria do not
grow with anything like the rapidity that they grow in cream.
Indeed, the growth of these organisms during the ripening is
extremely slow compared to the possibilities of bacterial growth
that we have already noticed. Nevertheless, the bacteria do
multiply in the cheese, and as the ripening goes on they become
more and more abundant, although the number fluctuates, rising and
falling under different conditions.

When the attempt is made to determine the relation of the
different kinds of ripening to different kinds of bacteria, it has
thus far met with extremely little success. That different
flavours are due to the ripening produced by different kinds of
bacteria would appear to be almost certain when we remember, as we
have already noticed, the different kinds of decomposition
produced by different species of bacteria. It would seem,
moreover, that it ought not to be very difficult to separate from
the ripened cheese the bacteria which are present, and thus obtain
the kind of bacteria necessary to produce the desired ripening.
But for some reason this does not prove to be so easy in practice
as it seems to be in theory. Many different species of bacteria
have been separated from cheeses. One bacteriologist, studying
several cheeses, separated about eighty different species
therefrom, and others have found perhaps as many more from
different sources. Moreover, experiments have been made with a
considerable number of these different kinds of bacteria to
determine whether they are capable of producing normal ripening.
These experiments consist of making cheese out of milk that has
been deprived of its bacteria, and which has been inoculated with
large quantities of the species in question. Hitherto these
experiments have not been very satisfactory. In some cases the
cheese appears to ripen scarcely at all; in other cases the
ripening occurs, but the resulting cheese is of a peculiar
character, entirely unlike the cheese that it is desired to
imitate. There have been one or two experiments in recent times
that give a little more promise of success than the earlier ones,
for a few species of bacteria have been used in ripening with what
the authors have thought to be promising success. The cheese made
from the milk artificially inoculated with these species ripens in
a satisfactory manner and gives some of the character desired,
though up to the present time in no case has the typical normal
ripening been produced in any of these experiments.

But these experiments have demonstrated beyond question that the
abnormal ripening which is common in cheese factories is due to
the presence of undesirable species of bacteria in the milk. Many
of the experiments in making cheeses by means of artificial
cultures of bacteria have resulted in decidedly abnormal cheeses.
Many of the cheeses thus manufactured have shown imperfections in
ripening which are identical with those actually occurring in the
cheese factory. Several different species of bacteria have been
found which, when artificially used thus for ripening cheese, will
give rise to the porosity and the abnormal swelling of the cheese
already referred to (Fig. 24). Others produced bad tastes and
flavours, and enough has been done in this line to demonstrate
beyond peradventure that the abnormal ripening of cheese is due
primarily to the growth of improper species therein. Quite a long
list of species of bacteria which produce abnormal ripening have
been isolated from cheeses, and have been studied and experimented
with by bacteriologists. As a result of this study of abnormal
ripening, there has been suggested a method of partially
controlling these--remedying them. The method consists simply in
testing the fermenting qualities of the milk used. A small sample
of milk from different dairies is allowed to stand in the cheese
factory by itself until it undergoes its normal souring. If the
fermentation or souring that thus occurs is of a normal character,
the milk is regarded as proper for cheese making. But if the
fermentation that occurs in any particular sample of milk is
unusual; if an extraordinary amount of gas bubbles are produced,
or if unpleasant smells and tastes arise, the sample is regarded
as unfavourable for cheese making, and as likely to produce
abnormal ripening in the cheeses. Milk from this source would
therefore be excluded from the milk that is to be used in cheese
making. This, of course, is a tentative and an unsatisfactory
method of controlling the ripening, and yet it is one of some
practical value to cheese makers. It is the only method that has
yet been suggested of controlling the ripening.

Our bacteriologists, of course, are quite confident that in the
future more practical results will be obtained along this line
than in the past. If it is true that cheeses are ripened by
bacteria; if it is true that different qualities in the cheese are
due to the growth of different species of bacteria during the
ripening, it would seem to be possible to obtain the proper kind
of bacteria and to furnish them to the cheese maker for
artificially inoculating his cheese, just as it has been possible
to furnish artificially cultivated yeasts to the brewer, and as it
has become possible to furnish artificially cultivated bacteria to
the butter maker. We must, however, recognise this to be a matter
for the future. Up to the present time no practical results along
the lines of bacteria have been obtained which our cheese
manufacturers can make use of in the way of controlling with any
accuracy this process of cheese ripening.

Thus it will be seen that in this last dairy product bacteria play
even a more important part than in any of the others. The food
value of cheese is dependent upon the casein which is present. The
market price, however, is controlled entirely by the flavour, and
this flavour is a product of bacterial growth. Upon the action of
bacteria, then, the cheese maker is absolutely dependent; and when
our bacteriologists are able in the future to investigate this
matter further, it seems to be at least possible that they may
obtain some means of enabling the cheese maker to control the
ripening accurately. Not only so, but recognising the great
variety in the flavours of cheese, and recognising that different
kinds of bacteria undoubtedly produce different kinds of
decomposition products, it seems to be at least possible that a
time will come when the cheese maker will be able to produce at--
will any particularly desired flavour in his cheese by the
addition to it of particular species of bacteria, or particular
mixtures of species of bacteria which have been discovered to
produce the desired effects.





CHAPTER IV.

BACTERIA IN NATURAL PROCESSES.--AGRICULTURE.


Thus far, in considering the relations of bacteria to mankind, we
have taken into account only the arts and manufactures, and have
found bacteria playing no unimportant part in many of the
industries of our modern civilized life. So important are they
that there is no one who is not directly affected by them. There
is hardly a moment in our life when we are not using some of the
direct or indirect products of bacterial action. We turn now,
however, to the consideration of a matter of even more fundamental
importance; for when we come to study bacteria in Nature, we find
that there are certain natural processes connected with the life
of animals and plants that are fundamentally based upon their
powers. Living Nature appears limitless, for life processes have
been going on in the world through countless centuries with
seemingly unimpaired vigour. At the very bottom we find this
never-ending exhibition of vital power dependent upon certain
activities of micro-organisms. So thoroughly is this true that, as
we shall find after a short consideration, the continuance of life
upon the surface of the world would be impossible if bacterial
action were checked for any considerable length of time. The life
of the globe is, in short, dependent upon these micro-organisms.

BACTERIA AS SCAVENGERS.

In the first place, we may notice the value of these organisms
simply as scavengers, keeping the surface of the earth in the
proper condition for the growth of animals and plants. A large
tree in the forest dies and falls to the ground. For a while the
tree trunk lies there a massive structure, but in the course of
months a slow change takes place in it. The bark becomes softened
and falls from the wood. The wood also becomes more or less
softened; it is preyed upon then by insect life; its density
decreases more and more, until finally it crumbles into a soft,
brownish, powdery mass, and eventually the whole sinks into the
soil, is overgrown by mosses and other vegetation, and the tree
trunk has disappeared from view. In the same way the body of the
dead animal undergoes the process of the softening of its tissues
by decay. The softer parts of the body rapidly dissipate, and even
the bones themselves eventually are covered with the soil and
disintegrated, until in time they, too, disappear from any visible
existence. This whole process is one of decay, and the result is
that the solid mass of the body of the tree or of the animal has
been decomposed. What has become of it? The answer holds the
secret of Nature's eternal freshness. Part of it has dissipated
into the air in the form of gases and water vapour; part of it has
changed its composition and has become incorporated into the soil,
the final result being that the body of the plant or animal
disappears as such, and its substance is converted into gaseous
form, which is dissipated in the air or into simple compounds
which sink into the earth.

This whole process of decay of organic life is one in which
bacteria play the most important part. In the case of the
decomposition of the woody matter of the tree trunk, the process
is begun by the agency of moulds, for this group of organisms
alone appears to be capable of attacking such hard woody
structure. The later part of the decay, however, is largely carried
on by bacterial life. In the decomposition of the animal tissues,
bacteria alone are the agents. Thus the process by which organic
matter is dissipated into the air or incorporated into the soil is
one which is primarily presided over by bacterial life.

Viewing this matter in a purely mechanical light, the importance
of bacteria in thus acting as scavengers can hardly be
overestimated. If we think for a moment of the condition of the
world were there no such decomposing agents to rid the earth's
surface of the dead bodies of animals and plants, we shall see
that long since the earth would have been uninhabitable. If the
dead bodies of plants and animals of past ages simply accumulated
on the surface of the ground without any forces to reduce them
into simple compounds for dissipation, by their very bulk they
would have long since completely covered the surface of the earth
so as to afford no possible room for further growth of plants and
animals. In a purely mechanical way, then, bacteria as
decomposition agents are necessary to keep the surface of the
earth fresh and unencumbered so that life can continue.

BACTERIA AS AGENTS IN NATURE'S FOOD CYCLE.

But the matter by no means ends here. When we come to think of it,
it is a matter of considerable surprise that the surface of the
earth has been able to continue producing animals and plants for
the many millions of years during which life has been in
existence. Plants and animals both require food, animals depending
wholly upon plants therefor. Plants, however, equally with
animals, require food, and although they obtain a considerable
portion of their food from the air, yet no inconsiderable part of
it is obtained from the soil. The question is forced upon us,
therefore, as to why the soil has not long since become exhausted
of food. How could the soil continue to support plants year after
year for millions of years, and yet remain as fertile as ever?

The explanation of this phenomenon is in the simple fact that the
processes of Nature are such that the same food is used over and
over again, first by the plant, then by the animal, and then again
by the plant, and there is no necessity for any end of the process
so long as the sun furnishes energy to keep the circulation
continuous. One phase of this transference of food from animal to
plant and from plant to animal is familiar to nearly every one. It
is a well-known fact that animals in their respiration consume
oxygen, but exhale it again in combination with carbon as carbonic
dioxide. On the other hand, plants in their life consume the
carbonic dioxide and exhale the oxygen again as free oxygen. Thus
each of these kingdoms makes use of the excreted product of the
other, and this process can go on indefinitely, the animals
furnishing our atmosphere with plenty of carbonic acid for plant
life, and the plants excreting into the atmosphere at the same
time an abundant sufficiency of oxygen for animal life. The oxygen
thus passes in an endless round from animal to plant and from
plant to animal.

A similar cycle is true of all the other foods of animal and plant
life, though in regard to the others the operation is more complex
and more members are required to complete the chain. The
transference of matter through a series of changes by which it is
brought from a condition in which it is proper food for plants
back again into a condition when it is once more a proper food for
plants, is one of the interesting discoveries of modern science,
and one in which, as we shall see, bacteria play a most important
part. This food cycle is illustrated roughly by the accompanying
diagram; but in order to understand it, an explanation of the
various steps in this cycle is necessary.

It will be noticed that at the bottom of the circle represented in
Fig. 25, at A, are given various ingredients which are found in
the soil and which form plant foods. Plant foods, as may be seen
there, are obtained partly from the air as carbonic dioxide and
water; but another portion comes from the soil. Among the soil
ingredients the most prominent are nitrates, which are the forms
of nitrogen compounds most easily made use of by plants as a
source of this important element. It should be stated also that
there are other compounds in the soil which furnish plants with
part of their food--compounds containing potassium, phosphorus,
and some other elements. For simplicity's sake, however, these
will be left out of consideration. Beginning at the bottom of the
cycle (Fig. 25 A), plant life seizes the gases from the air and
these foods from the soil, and by means of the energy furnished it
by the sun's rays builds these simple chemical compounds into more
complex ones. This gives us the second step, as shown in Fig. 25
B, the products of plant life. These products of plant life
consist of such materials as sugar, starches, fats, and proteids,
all of which have been manufactured by the plant from the
ingredients furnished it from the soil and air, and through the
agency of the sun's rays. These products of plant life now form
foods for the animal kingdom. Starches, fats, and proteids are
animal foods, and upon such complex bodies alone can the animal
kingdom be fed. Animal life, standing high up in the circle, is
not capable of extracting its nutriment from the soil, but must
take the more complex foods which have been manufactured by plant
life. These complex foods enter now into the animal and take their
place in the animal body. By the animal activities, some of the
foods are at once decomposed into carbonic acid and water, which,
being dissipated into the air, are brought back at once into the
condition in which they can serve again as plant food. This part
of the food is thus brought back again to the bottom of the circle
(Fig. 25, dotted lines). But while it is true that animals do thus
reduce some of their foods to the simple condition of carbonic
acid and water, this is not true of most of the foods which
contain nitrogen. The nitrogenous foods are as necessary for the
life as the carbon foods, and animals do not reduce their
nitrogenous foods to the condition in which plants can prey upon
them. While plants furnish them with nitrogenous food, they can
not give it back to the plants. Part of the nitrogenous foods
animals build into new albumins (Fig. 25 C); but a part of them
they reduce at once into a somewhat simpler condition known as
urea. Urea is the form in which the nitrogen is commonly excreted
from the animal body. But urea is not a plant food; for ordinary
plants are entirely unable to make use of it. Part of the nitrogen
eaten by the animal is stored up in its body, and thus the body of
the animal, after it has died, contains these nitrogen compounds
of high complexity. But plants are not able to use these
compounds. A plant can not be fed upon muscle tissue, nor upon
fats, nor bones, for these are compounds so complex that the
simple plant is unable to use them at all. So far, then, in the
food cycle the compounds taken from the soil have been built up
into compounds of greater and greater complexity; they have
reached the top of this circle, and no part of them, except part
of the carbon and oxygen, has become reduced again to plant food.
In order that this material should again become capable of
entering into the life of plants so as to go over the circle
again, it is necessary for it to be once more reduced from its
highly complex condition into a simpler one.

Now come into play these decomposition agencies which we have been
studying under the head of scavengers. It will be noticed that the
next step in the food cycle is taken by the decomposition
bacteria. These organisms, existing, as we have already seen, in
the air, in the soil, in the water, and always ready to seize hold
of any organic substance that may furnish them with food, feed
upon the products of animal life, whether they are such products
as muscle tissue, or fat, or sugar, or whether they are the
excreted products of animal life, such as urea, and produce
therein the chemical decomposition changes already noticed. As a
result of this chemical decomposition, the complex bodies are
broken into simpler and simpler compounds, and the final result is
a very thorough destruction of the animal body or the material
excreted by animal life, and its reduction into forms simple
enough for plants to use again as foods. Thus the bacteria come in
as a necessary link to connect the animal body, or the excretion
from the animal body, with the soil again, and therefore with that
part of the circle in which the material can once more serve as
plant food.

But in the decomposition that thus occurs through the agency of
the putrefactive bacteria it very commonly happens that some of
the food material is broken down into compounds too simple for use
as plant food. As will be seen by a glance at the diagram (Fig. 25
D), a portion of the cleavage products resulting from the
destruction of these animal foods takes the form of carbonic-acid
gas and water. These ingredients are at once in condition for
plant life, as shown by the dotted lines. They pass off into the
air, and the green leaves of vegetation everywhere again seize
them, assimilate them, and use them as food. Thus it is that the
carbon and the oxygen have completed the cycle, and have come back
again to the position in the circle where they started. In regard
to the nitrogen portion of the food, however, it very commonly
happens that the products which arise as the result of the
decomposition processes are not yet in proper condition for plant
food. They are reduced into a condition actually too simple for
the use of plants. As a result of these putrefactive changes, the
nitrogen products of animal life are broken frequently into
compounds as simple as ammonia (NH3), or into compounds which the
chemists speak of as nitrites (Fig. 25 at D). Now these compounds
are not ordinarily within the reach of plant life. The luxuriant
vegetation of the globe extracts its nitrogen from the soil in a
form more complex than either of the compounds here mentioned;
for, as we have seen, it is nitrates chiefly that furnish plants
with their nitrogen food factor. But nitrates contain considerable
oxygen. Ammonia, which is one of the products of putrefactive de-
composition, contains no oxygen, and nitrites, another factor,
contains less oxygen than nitrates. These bodies are thus too
simple for plants to make use of as a source of nitrogen. The
chemical destruction of the food material which results from the
action of the putrefactive bacteria is too thorough, and the
nitrogen foods are not yet in condition to be used by plants.

Now comes in the agency of still another class of micro-organisms,
the existence of which has been demonstrated to us during the last
few years. In the soil everywhere, especially in fertile soil, is
a class of bacteria which has received the name of nitrifying
bacteria (Fig. 26). These organisms grow in the soil and feed upon
the soil ingredients. In the course of their life they have
somewhat the same action upon the simple nitrogen cleavage
products just mentioned as we have already noticed the vinegar-
producing species have upon alcohol, viz., the bringing about a
union with oxygen. There are apparently several different kinds of
nitrifying bacteria with different powers. Some of them cause an
oxidation of the nitrogen products by means of which the ammonia
is united with oxygen and built up into a series of products
finally resulting in nitrates (Fig. 26). By the action of other
species still higher nitrogen compounds, including the nitrites,
are further oxidized and built up into the form of nitrates. Thus
these nitrifying organisms form the last link in the chain that
binds the animal kingdom to the vegetable kingdom (Fig. 25 at 4).
For after the nitrifying organisms have oxidized nitrogen cleavage
products, the results of the oxidation in the form of nitrates or
nitric acid are left in the soil, and may now be seized upon by
the roots of plants, and begin once more their journey around the
food cycle. In this way it will be seen that while plants, by
building up compounds, form the connecting link between the soil
and animal life, bacteria in the other half of the cycle, by
reducing them again, give us the connecting link between animal
life and the soil. The food cycle would be as incomplete without
the agency of bacterial life as it would be without the agency of
plant life.

But even yet the food cycle is not complete. Some of the processes
of decomposition appear to cause a portion of the nitrogen to fly
out of the circle at a tangent. In the process of decomposition
which is going on through the agency of micro-organisms, a
considerable part of the nitrogen is dissipated into the air in
the form of free nitrogen. When a bit of meat decays, part of the
meat is, indeed, converted into ammonia or other nitrogen
compounds, but if the putrefaction is allowed to go on, in the end
a considerable portion of it will be broken into still simpler
forms, and the nitrogen will finally be dissipated into the air in
the form of free nitrogen. This dissipation of free nitrogen into
the air is going on in the world wherever putrefaction takes
place. Wherever decomposition of nitrogen products occurs some
free nitrogen is eliminated. Now, this part of the nitrogen has
passed beyond the reach of plants, for plants can not extract free
nitrogen from the air. In the diagram this is represented as a
portion of the material which, through the agency of the
decomposition bacteria, has been thrown out of the cycle at a
tangent (Fig. 25 E). It will, of course, be plain from this that
the store of nitrogen food must be constantly diminishing. The
soil may have been originally supplied with a given quantity of
nitrogen compound, but if the decomposition products are causing
considerable quantities of this nitrogen to be dissipated in the
air, it plainly follows that the total amount of nitrogen food
upon which the animal and vegetable kingdoms can depend is
becoming constantly reduced by such dissipation.

There are still other methods by which nitrogen is being lost from
the food cycle. First, we may notice that the ordinary processes
of vegetation result in a gradual draining of the soil and a
throwing of its nitrogen into the ocean. The body of any animal or
any plant that chances to fall into a brook or river is eventually
carried to the sea, and the products of its decomposition pass
into the ocean and are, of course, lost to the soil. Now, while
this gradual extraction of nitrogen from the soil by drainage is a
slow one, it is nevertheless a sure one. It is far more rapid in
these years of civilized life than in former times, since the
products of the soil are given to the city, and then are thrown
into its sewage Our cities, then, with our present system of
disposing of sewage, are draining from the soil the nitrogen
compounds and throwing them away.

In yet another direction must it be noticed that our nitrogen
compounds are being lost to plant life--viz., by the use of various
nitrogen compounds to form explosives. Gunpowder, nitro-glycerine,
dynamite, in fact, nearly all the explosives that are used the
world over for all sorts of purposes, are nitrogen compounds. When
they are exploded the nitrogen of the compound is dissipated into
the air in the form of gas, much of it in the form of free
nitrogen. The basis from which explosive compounds are made
contains nitrogen in the form in which it can be used by plants.
Saltpetre, for example, is equally good as a fertilizer and as a
basis for gunpowder. The products of the explosion are gases no
longer capable of use by plants, and thus every explosion of
nitrogen compounds aids in this gradual dissipation of nitrogen
products, taking them from the store of plant foods and throwing
them away.

All of these agencies contribute to reduce the amount of material
circulating in the food cycle of Nature, and thus seem to tend
inevitably in the end toward a termination of the processes of
life; for as soon as the soil becomes exhausted of its nitrogen
compounds, so soon will plant life cease from lack of nutrition,
and the disappearance of animal life will follow rapidly. It is
this loss of nitrogen in large measure that is forcing our
agriculturists to purchase fertilizers. The last fifteen years
have shown us, however, that here again we may look upon our
friends, the bacteria, as agents for counteracting this
dissipating tendency in the general processes of Nature. Bacterial
life in at least two different ways appears to have the function
of reclaiming from the atmosphere more or less of this dissipated
free nitrogen.

In the first place, it has been found in the last few years that
soil entirely free from all common plants, but containing certain
kinds of bacteria, if allowed to stand in contact with the air,
will slowly but surely gain in the amount of nitrogen compounds
that it contains. These nitrogen compounds are plainly
manufactured by the bacteria in the soil; for unless the bacteria
are present they do not accumulate, and they do accumulate
inevitably if the bacteria are present in the proper quantity and
the proper species. It appears that, as a rule, this fixation of
nitrogen is not performed by any one species of microorganisms,
but by two or three of them acting together. Certain combinations
of bacteria have been found which, when inoculated in the soil,
will bring about this fixation of nitrogen, but no one of the
species is capable of producing this result alone. We do not know
to what extent these organisms are distributed in the soil, nor
how widely this nitrogen fixation through bacterial life is going
on. It is only within a short time that it has been demonstrated
to exist, but we must look upon bacteria in the soil as one of the
factors in reclaiming from the atmosphere the dissipated free
nitrogen.

The second method by which bacteria aid in the reclaiming of this
lost nitrogen is by a combined action of certain species of
bacteria and some of the higher plants. Ordinary green plants, as
already noted, are unable to make use of the free nitrogen of the
atmosphere It was found, however, some fifteen years ago that some
species of plants, chiefly the great family of legumes, which
contains the pea plant, the bean, the clover, etc, are able, when
growing in soil that is poor in nitrogen, to obtain nitrogen from
some source other than the soil in which they grow. A pea plant in
soil that contains no nitrogen products and watered with water
that contains no nitrogen, will, after sprouting and growing for a
length of time, be found to have accumulated a considerable
quantity of fixed nitrogen in its tissues The only source of this
nitrogen has been evidently from the air which bathes the leaves
of the plant or permeates the soil and bathes its roots This fact
was at first disputed, but subsequently demonstrated to be true,
and was found later to be associated with the combined action of
these legumes and certain soil bacteria. When a legume thus gains
nitrogen from the air, it develops upon its roots little bunches
known as root nodules or root tubercles. The nodules are sometimes
the size of the head of a pm, and sometimes much larger than this,
occasionally reaching the size of a large pea, or even larger.
Upon microscopic examination they are found to be little nests of
bacteria In some way the soil organisms (Fig 27) make their way
into the roots of the sprouting plant, and finding there congenial
environment, develop in considerable quantities and produce root
tubercles in the root. Now, by some entirely unknown process, the
legume and the bacteria growing together succeed in extracting
the nitrogen from the atmosphere which permeates the soil, and
fixing this nitrogen in the tubercles and the roots in the form of
nitrogen compounds. The result is that, after a proper period of
growth, the amount of fixed nitrogen in the plant is found to have
very decidedly increased (Fig 25 E).

This, of course, furnishes a starting point for the reclaiming of
the lost atmospheric nitrogen. The legume continues to live its
usual life, perhaps increasing the store of nitrogen in its roots
and stems and leaves during the whole of its normal growth.
Subsequently, after having finished its ordinary life, the plant
will die, and then the roots and stems and leaves, falling upon
the ground and becoming buried, will be seized upon by the
decomposition bacteria already mentioned. The nitrogen which has
thus become fixed in their tissues will undergo the destructive
changes already described. This will result eventually in the
production of nitrates. Thus some of the lost nitrogen is restored
again to the soil in the form of nitrates, and may now start on
its route once more around the cycle of food.

It will be seen, then, that the food cycle is a complete one.
Beginning with the mineral ingredients in the soil, the food
matter may start on its circulation from the soil to the plant,
from the plant to the animal, from the animal to the bacterium and
from the bacterium through a series of other bacteria back again
to the soil in the condition in which it started. If, perchance,
in this progress around the circle some of the nitrogen is thrown
off at a tangent, this, too, is brought back again to the circle
through the agency of bacterial life. And so the food material of
animals and plants continues in this never-ceasing circulation. It
is the sunlight that furnishes the energy for the motion. It is
the sunlight that forces the food around the circle and keeps up
the endless change; and so long as, the sun continues to shine
upon the earth there seems to be no reason why the process should
ever cease. It is this repeated circulation that has made the
continuation of life possible for the millions and millions of
years of the earth's history. It is this continued circulation
that makes life possible still, and it is only this fact that the
food is thus capable of ever circulating from animal to plant and
from plant to animal that makes it possible for the living world
to continue its existence. But, ah we have seen, one half of this
great circle of food change is dependent upon bacterial life.
Without the bacterial life the animal body and the animal
excretion could never be brought back again within the reach of
the plant; and thus, were it not for the action of these micro-
organisms the food cycle would be incomplete and life could not
continue indefinitely upon the surface of the earth. At the very
foundation, the continuation of the present condition of Nature
and the existence of life during the past history of the world has
been fundamentally based upon the ubiquitous presence of bacteria
and upon their continual action in connection with both
destructive and constructive processes.

RELATION OF BACTERIA TO AGRICULTURE.

We have already noticed that bacteria play an important part in
some of the agricultural industries, particularly in the dairy.
From the consideration of the matters just discussed, it is
manifest that these organisms must have an even more intimate
relation to the farmer's occupation. At the foundation, farming
consists in the cultivation of plants and animals, and we have
already seen how essential are the bacteria in the continuance of
animal and plant life. But aside from these theoretical
considerations, a little study shows that in a very practical
manner the farmer is ever making use of bacteria, as a rule, quite
unconsciously, but none the less positively.

SPROUTING OF SEEDS.

Even in the sprouting of seeds after they are sown in the soil
bacterial life has its influence. When seeds are placed m moist
soil they germinate under the influence of heat. The rich
albuminous material in the seeds furnishes excellent food, and
inasmuch as bacteria abound in the soil, it is inevitable that
they should grow in and feed upon the seed. If the moisture is
excessive and the heat considerable, they very frequently grow so
rapidly in the seed as to destroy its life as a seedling. The seed
rots in the ground as a result. This does not commonly occur,
however, in ordinary soil. But even here bacteria do grow in the
seed, though not so abundantly as to produce any injury. Indeed,
it has been claimed that their presence in the seed in small
quantities is a necessity for the proper sprouting of the seed. It
has been claimed that their growth tends to soften the food
material in the seed, so that the young seedling can more readily
absorb it for its own food, and that without such a softening the
seed remains too hard for the plant to use. This may well be
doubted, however, for seeds can apparently sprout well enough
without the aid of bacteria. But, nevertheless, bacteria do grow
in the seed during its germination, and thus do aid the plant in
the softening of the food material. We can not regard them as
essential to seed germination. It may well be claimed that they
ordinarily play at least an incidental part in this fundamental
life process, although it is uncertain whether the growth of
seedlings is to any considerable extent aided thereby.

THE SILO.

In the management of a silo the farmer has undoubtedly another
great bacteriological problem. In the attempt to preserve his
summer-grown food for the winter use of his animals, he is
hindered by the activity of common bacteria. If the food is kept
moist, it is sure to undergo decomposition and be ruined in a
short time as animal food. The farmer finds it necessary,
therefore, to dry some kinds of foods, like hay. While he can thus
preserve some foods, others can not be so treated. Much of the
rank growth of the farm, like cornstalks, is good food while it is
fresh, but is of little value when dried. The farmer has from
experience and observation discovered a method of managing
bacterial growth which enables him to avoid their ordinary evil
effects. This is by the use of the silo. The silo is a large,
heavily built box, which is open only at the top. In the silo the
green food is packed tightly, and when full all access of air is
excluded, except at its surface. Under these conditions the food
remains moist, but nevertheless does not undergo its ordinary
fermentations and putrefactions, and may be preserved for months
without being ruined. The food in such a silo may be taken out
months after it is packed, and will still be found to be in good
condition for food. It is true that it has changed its character
somewhat, but it is not decayed, and is eagerly eaten by cattle.

We are yet very ignorant of the nature of the changes which occur
m the food while in the silo. The food is not preserved from
fermentation. When the siloxis packed slowly, a very decided
fermentation occurs by which the mass is raised to a high
temperature (140 degrees F. to 160 degrees F.). This heating is
produced by certain species of bacteria which grow readily even at
this high temperature. The fermentation uses up the air in the
silo to a certain extent and produces a settling of the material
which still further excludes air. The first fermentation soon
ceases, and afterward only slow changes occur. Certain acid-
producing bacteria after a little begin to grow slowly, and in
time the silage is rendered somewhat sour by the production of
acetic acid. But the exclusion of air, the close packing, and the
small amount of moisture appear to prevent the growth of the
common putrefactive bacteria, and the silage remains good for a
long time. In other methods of filling the silo, the food is very
quickly packed and densely crowded together so as to exclude as
much air as possible from the beginning. Under these conditions
the lack of moisture and air prevents fermentative action very
largely. Only certain acid-producing organisms grow, and these
very slowly. The essential result in either case is that the
common putrefactive bacteria are prevented from growing, probably
by lack of sufficient oxygen and moisture, and thus the decay is
prevented. The closely packed food offers just the same
unfavourable condition for the growth of common putrefactive
bacteria that we have already seen offered by the hard-pressed
cheese, and the bacteria growth is in the same way held in check.
Our knowledge of the matter is as yet very slight, but we do know
enough to understand that the successful management of a silo is
dependent upon the manipulation of bacteria.

THE FERTILITY OF THE SOIL.

The farmer's sole duty is to extract food from the soil. This he
does either directly by raising crops, or indirectly by raising
animals which feed upon the products of the soil. In either case
the fertility of the soil is the fundamental factor in his
success. This fertility is a gift to him from the bacteria.

Even in the first formation of soil he is in a measure dependent
upon bacteria. Soil, as is well known, is produced in large part
by the crumbling of the rocks into powder. This crumbling we
generally call weathering, and regard it as due to the effect of
moisture and cold upon the rocks, together with the oxidizing
action of the air. Doubtless this is true, and the weathering
action is largely a physical and chemical one. Nevertheless, in
this fundamental process of rock disintegration bacterial action
plays a part, though perhaps a small one. Some species of
bacteria, as we have seen, can live upon very simple foods,
finding in free nitrogen and carbonates sufficiently highly
complex material for their life. These organisms appear to grow on
the bare surface of rocks, assimilating nitrogen from the air, and
carbon from some widely diffused carbonates or from the CO2 in the
air. Their secreted products of an acid nature help to soften the
rocks, and thus aid in performing the first step in weathering.

The soil is not, however, all made up of disintegrated rocks. It
contains, besides, various ingredients which combine to make it
fertile. Among these are various sulphates which form important
parts of plant foods. These sulphates appear to be formed, in
part, at least, by bacterial agency. The decomposition of proteids
gives rise, among other things, to hydrogen sulphide (H2S). This
gas, which is of common occurrence in the atmosphere, is oxidized
by bacterial growth into sulphuric acid, and this is the basis of
part of the soil sulphates. The deposition of iron phosphates and
iron silicates is probably also in a measure aided by bacterial
action. All of these processes are factors in the formation of
soil. Beyond much question the rock disintegration which occurs
everywhere in Nature is chiefly the result of physical and
chemical changes, but there is reason for believing that the
physical and chemical processes are, to a slight extent at least,
assisted by bacterial life.

A more important factor of soil fertility is its nitrogen content,
without which it is completely barren. The origin of these
nitrogen ingredients has been more or less of a puzzle. Fertile
soil everywhere contains nitrates and other nitrogen compounds,
and in certain parts of the world there are large accumulations of
these compounds, like the nitrate beds of Chili. That they have
come ultimately from the free atmospheric nitrogen seems certain,
and various attempts have been made to explain a method of this
nitrogen fixation. It has been suggested that electrical
discharges in the air may form nitric acid, which would readily
then unite with soil ingredients to form nitrates. There is little
reason, however, for believing this to be a very important factor
But in the soil bacteria we find undoubtedly an efficient agency m
this nitrogen fixation. As already seen, the bacteria are able to
seize the free atmospheric nitrogen, converting it into nitrite
and nitrates. We have also learned that they can act in connection
with legumes and some other plants, enabling them to fix
atmospheric nitrogen and store it m their roots. By these two
means the nitrogen ingredient in the soil is prevented from
becoming exhausted by the processes of dissipation constantly
going on. Further, by some such agency must we imagine the
original nitrogen soil ingredient to have been derived. Such an
organic agency is the only one yet discerned which appears to have
been efficient in furnishing virgin soil with its nitrates, and we
must therefore look upon bacteria as essential to the original
fertility of the soil. But in another direction still does the
farmer depend directly upon bacteria The most important factor in
the fertility of the soil is the part of it called humus. This
humus is very complex, and never alike in different soils It
contains nitrogen compounds in abundance, together with sulphates,
phosphates, sugar, and many other substances. It is this which
makes the garden soil different from sand, or the rich soil
different from the sterile soil. If the soil is cultivated year
after year, its food ingredients are slowly but surely exhausted.
Something is taken from the humus each year, and unless this be
replaced the soil ceases to be able to support life. To keep up a
constant yield from the soil the farmer understands that he must
apply fertilizers more or less constantly.

This application of fertilizers is simply feeding the crops. Some
of these fertilizers the farmer purchases, and knows little or
nothing as to their origin. The most common method of feeding the
crops is, however, by the use of ordinary barnyard manure. The
reason why this material contains plant food we can understand,
since it is made of the undigested part of food, together with all
the urea and other excretions of animals, and contains, therefore,
besides various minerals, all of the nitrogenous waste of animal
life. These secretions are not at first fit for plant food. The
farmer has learned by experience that such excretions, before they
are of any use on his fields, must undergo a process of slow
change, which is sometimes called ripening. Fresh manure is
sometimes used on the fields, but it is only made use of by the
plants after the ripening process has occurred. Fresh animal
excretions are of little or no value as a fertilizer. The farmer,
therefore, commonly allows it to remain in heaps for some time,
and it undergoes a slow change, which gradually converts it into a
condition in which it can be used by plants. This ripening is
readily explained by the facts already considered The fresh animal
secretions consist of various highly complex compounds of
nitrogen, and the ripening is a process of their decomposition.
The proteids are broken to pieces, and their nitrogen elements
reduced to the form of nitrates, leucin, etc, or even to ammonia
or free nitrogen. Further, a second process occurs, the process of
oxidation of these nitrogen compounds already noticed, and the
ammonia and nitrites resulting from the decomposition are built
into nitrates. In short, in this ripening manure the processes
noticed in the first part of this chapter are taking place, by
which the complex nitrogenous bodies are first reduced and then
oxidized to form plant food. The ripening of manure is both an
analytical and a synthetical process. By the analysis, proteids
and other bodies are broken into very simple compounds, some of
them, indeed, being dissipated into the air, but other portions
are retained and then oxidized, and these latter become the real
fertilizing materials. Through the agency of bacteria the compost
heap thus becomes the great source of plant food to the farmer.
Into this compost heap he throws garbage, straw, vegetable and
animal substances in general, or any organic refuse which may be
at hand. The various bacteria seize it all, and cause the
decomposition which converts it into plant food again. The rotting
of the compost heap is thus a gigantic cultivation of bacteria.

This knowledge of the ripening process is further teaching the
farmer how to prevent waste. In the ordinary decomposition of the
compost heap not an inconsiderable portion of the nitrogen is lost
in the air by dissipation as ammonia or free nitrogen. Even his
nitrates may be thus lost by bacterial action. This portion is
lost to the farmer completely, and he can only hope to replace it
either by purchasing nitrates in the form of commercial
fertilizers, or by reclaiming it from the air by the use of the
bacterial agencies already noticed. With the knowledge now at his
command he is learning to prevent this waste. In the decomposition
one large factor of loss is the ammonia, which, being a gas, is
readily dissipated into the air. Knowing this common result of
bacterial action, the scientist has told the farmer that, by
adding certain common chemicals to his decomposing manure heap,
chemicals which will readily unite with ammonia, he may retain
most of the nitrogen in this heap in the form of ammonia salts,
which, once formed, no longer show a tendency to dissipate into
the air. Ordinary gypsum, or superphosphates, or plaster will
readily unite with ammonia, and these added to the manure heap
largely counteract the tendency of the nitrogen to waste, thus
enabling the farmer to put back into his soil most of the nitrogen
which was extracted from it by his crops and then used by his
stock. His vegetable crops raise the nitrates into proteids. His
animals feed upon the proteids, and perform his work or furnish
him with milk. Then his bacteria stock take the excreted or refuse
nitrogen, and in his manure heap turn it back again into nitrates
ready to begin the circle once more. This might go on almost
indefinitely were it not for two facts, the farmer sends
nitrogenous material off his farm in the milk or grains or other
nitrogenous products, which he sells, and the decomposition
processes, as we have seen, dissipate some of the nitrogen into
the air as free nitrogen.

To meet this emergency and loss the farmer has another method of
enriching the soil, again depending upon bacteria. This is the so-
called green manuring. Here certain plants which seize nitrogen
from the air are cultivated upon the field to be fertilized, and,
instead of harvesting a crop, it is ploughed into the soil. Or
perhaps the tops may be harvested, the rest being ploughed into
the soil. The vegetable material thus ploughed in lies over a
season and enriches the soil. Here the bacteria of the soil come
into play in several directions. First, if the crop sowed be a
legume, the soil bacteria assist it to seize the nitrogen from the
air. The only plants which are of use in this green manuring are
those which can, through the agency of bacteria, obtain nitrogen
from the air and store it in their roots. Second, after the crop
is ploughed into the soil various decomposing bacteria seize upon
it, pulling the compounds to pieces. The carbon is largely
dissipated into the air as carbonic dioxide, where the next
generation of plants can get hold of it. The minerals and the
nitrogen remain in the soil. The nitrogenous portions go through
the same series of decomposition and synthetical changes already
described, and thus eventually the nitrogen seized from the air by
the combined action of the legumes and the bacteria is converted
into nitrates, and will serve for food for the next set of plants
grown on the same soil. Here is thus a practical method of using
the nitrogen assimilation powers of bacteria, and reclaiming
nitrogen from the air to replace that which has been lost. Thus it
is that the farmer's nitrogen problem of the fertile soil appears
to resolve itself into a proper handling of bacteria. These
organisms have stocked his soil in the first place. They convert
all of his compost heap wastes into simple bodies, some of which
are changed into plant foods, while others are at the same time
lost. Lastly, they may be made to reclaim this lost nitrogen, and
the fanner, so soon as he has requisite knowledge of these facts,
will be able to keep within his control the supply of this
important element. The continued fertility of the soil is thus a
gift from the bacteria.

BACTERIA AS SOURCES OF TROUBLE TO THE FARMER.

While the topics already considered comprise the most important
factors in agricultural bacteriology, the farmer's relations to
bacteria do not end here. These organisms come incidentally into
his life in many ways. They are not always his aids as they are in
most of the instances thus far cited. They produce disease in his
cattle, as will be noticed in the next chapter. Bacteria are
agents of decomposition, and they are just as likely to decompose
material which the farmer wishes to preserve as they are to
decompose material which the farmer desires to undergo the process
of decay. They are as ready to attack his fruits and vegetables as
to ripen his cream. The skin of fruits and vegetables is a
moderately good protection of the interior from the attack of
bacteria; but if the skin be broken in any place, bacteria get in
and cause decay, and to prevent it the farmer uses a cold cellar.
The bacteria prevent the farmer from preserving meats for any
length of time unless he checks their growth in some way. They get
into the eggs of his fowls and ruin them. Their troublesome nature
in the dairy in preventing the keeping of milk has already been
noticed. If he plants his seeds in very moist, damp weather, the
soil bacteria cause too rapid a decomposition of the seeds and
they rot in the ground instead of sprouting. They produce
disagreeable odours, and are the cause of most of the peculiar
smells, good and bad, around the barn. They attack the organic
matter which gets into his well or brook or pond, decomposing it,
filling the water with disagreeable and perhaps poisonous products
which render it unfit to drink. They not only aid in the decay of
the fallen tree in his forests; but in the same way attack the
timber which he wishes to preserve, especially if it is kept in a
moist condition. Thus they contribute largely to the gradual
destruction of wooden structures. It is therefore the presence of
these organisms which forces him to dry his hay, to smoke his
hams, to corn his beef, to keep his fruits and vegetables cool and
prevent skin bruises, to ice his dairy, to protect his timber from
rain, to use stone instead of wooden foundations for buildings,
etc. In general, when the farmer desires to get rid of any organic
refuse, he depends upon bacteria, for they are his sole agents
(aside from fire) for the final destruction of organic matter.
When he wishes to convert waste organic refuse into fertilizing
material, he uses the bacteria of his compost heap. On the other
hand, whenever he desires to preserve organic material, the
bacteria are the enemies against which he must carefully guard.

Thus the farmer's life from year's end to year's end is in most
intimate association with bacteria. Upon them he depends to insure
the continued fertility of his soil and the constant continued
production of good crops. Upon them he depends to turn into plant
food all the organic refuse from his house or from his barn. Upon
them he depends to replenish his stock of nitrogen. It is these
organisms which furnish his dairy with its butter flavours and
with the taste of its cheese. But, on the other hand, against them
he must be constantly alert. All his food products must be
protected from their ravages. A successful farmer's life, then,
largely resolves itself into a skilful management of bacterial
activity. To aid them in destroying or decomposing everything
which he does not desire to preserve, and to prevent their
destroying the organic material which he wishes to keep for future
use, is the object of a considerable portion of farm labour; and
the most successful farmer to-day, and we believe the most
successful farmer of the future, is the one who most intelligently
and skilfully manipulates these gigantic forces furnished him by
the growth of his microscopical allies.

RELATION OF BACTERIA TO COAL. Another one of Nature's processes in
which bacteria have played an important part is in the formation
of coal. It is unnecessary to emphasize the importance of coal in
modern civilization. Aside from its use as fuel, upon which
civilization is dependent, coal is a source of an endless variety
of valuable products. It is the source of our illuminating gas,
and ammonia is one of the products of the gas manufacture. From
the coal also comes coal tar, the material from which such a long
series of valuable materials, as aniline colours, carbolic acid,
etc, is derived. The list of products which we owe to coal is very
long, and the value of this material is hardly to be overrated. In
the preparation of these ingredients from coal bacteria do not
play any part. Most of them are derived by means of distillation.
But when asked for the agents which have given us the coal of the
coal beds, we shall find that here, too, we owe a great debt to
bacteria.

Coal, as is well known, has come from the accumulation of the
luxuriant vegetable growth of the past geological ages. It has
therefore been directly furnished us by the vegetation of the
green plants of the past, and, in general, it represents so much
carbonic dioxide which these plants have extracted from the
atmosphere. But while the green plants have been the active agents
in producing this assimilation, bacteria have played an important
part in coal manufacture in two different directions. The first
appears to be in furnishing these plants with nitrogen. Without a
store of fixed nitrogen in the soil these carboniferous plants
could not have grown. This matter has already been considered. We
have no very absolute knowledge as to the agency of bacteria in
furnishing nitrogen for this vegetation in past ages, but there is
every reason to believe that in the past, as in the present, the
chief source of organic nitrogen has been from the atmosphere and
derived from the atmosphere through the agency of bacteria. In the
absence of any other known factor we may be pretty safe in the
assumption that bacteria played an important part in this nitrogen
fixation, and that bacteria must therefore be regarded as the
agents which have furnished us the nitrogen stored in the coal.

But in a later stage of coal formation bacteria have contributed
more directly to the formation of coal. Coal is not simply
accumulated vegetation. The coal of our coal beds is very
different in its chemical composition from the wood of the trees.
It contains a much higher percentage of carbon and a lower
percentage of hydrogen and oxygen than ordinary vegetable
substances. The conversion of the vegetation of the carboniferous
ages into coal was accompanied by a gradual loss of hydrogen and a
consequent increase in the percentage of carbon. It is this change
that has added to the density of the substance and makes the
greater value of coal as fuel. There is little doubt now as to the
method by which this woody material of the past has been converted
into coal. The same process appears to be going on in a similar
manner to-day in the peat beds of various northern countries. The
fallen vegetation, trees, trunks, branches, and leaves, accumulate
in masses, and, when the conditions of moisture and temperature
are right, begin to undergo a fermentation. Ordinarily this action
of bacteria, as already noticed, produces an almost complete
though slow oxidation of the carbon, and results in the total
decay of the vegetable matter. But if the vegetable mass be
covered by water and mud under proper conditions of moisture and
temperature, a different kind of fermentation arises which does
not produce such complete decay. The covering of water prevents
the access of oxygen to the fermenting mass, an oxidation of the
carbon is largely prevented, and the vegetable matter slowly
changes its character. Under the influence of this slow
fermentation, aided, probably by pressure, the mass becomes more
and more solid and condensed, its woody character becomes less and
less distinct, and there is a gradual loss of the hydrogen and the
oxygen. Doubtless there is a loss of carbon also, for there is an
evolution of marsh gas which contains carbon. But, in this slow
fermentation taking place under the water in peat bogs and marshes
the carbon loss is relatively small; the woody material does not
become completely oxidized, as it does in free operations of
decay. The loss of hydrogen and oxygen from the mass is greater
than that of carbon, and the percentage of carbon therefore
increases. This is not the ordinary kind of fermentation that goes
on in vegetable accumulations. It requires special conditions and
possibly special kinds of fermenting organisms. Peat is not formed
in all climates. In warm regions, or where the woody matter is
freely exposed to the air, the fermentation of vegetable matter is
more complete, and it is entirely destroyed by oxidation. It is
only in colder regions and when covered with water that the
destruction of the organic matter stops short of decay. But such
incomplete fermentation is still going on in many parts of the
world, and by its means vegetable accumulations are being
converted into peat.

This formation of peat appears to be a first step in the formation
of denser coal. By a continuation of the same processes the mass
becomes still more dense and solid. As we pass from the top to the
bottom of such an accumulation of peat, we find it becoming denser
and denser, and at the bottom it is commonly of a hard consistence,
brownish in colour, and with only slight traces of the
original woody structure. Such material is called lignite. It
contains a higher percentage of carbon than peat, but a lower
percentage than coal, and is plainly a step in coal formation. But
the process goes on, the hydrogen and oxygen loss continuing until
there is finally produced true coal.

If this is the correct understanding of the formation of coal, we
see that we have plainly a process in which bacterial life has had
a large and important share. We are, of course, densely ignorant
of the exact processes going on. We know nothing positively as to
the kind of microorganisms which produce this slow, peculiar
fermentation. As yet, the fermentation going on in the formation
of the peat has not been studied by the bacteriologists, and we do
not know from direct experiment that it is a matter of bacterial
action. It has been commonly regarded as simply a slow chemical
change, but its general similarity to other fermentative processes
is so great that we can have little hesitation in attributing it
to micro-organisms, and doubtless to some forms of plants allied
to bacteria. There is no reason for doubting that bacteria existed
in the geological ages with essentially the same powers as they
now possess, and to some forms of bacteria which grow in the
absence of oxygen can we probably attribute the slow change which
has produced coal. Here, then, is another great source of wealth
in Nature for which we are dependent upon bacteria. While, of
course, water and pressure were very essential factors in the
deposition of coal, it was a peculiar kind of fermentation
occurring in the vegetation that brought about the chemical
changes in it which resulted in its transformation into coal. The
vegetation of the carboniferous age was dependent upon the
nitrogen fixed by the bacteria, and to these organisms also do we
owe the fact that this vegetation was stored for us in the rocks.





CHAPTER V.

PARASITIC BACTERIA AND THEIR RELATION TO DISEASE.


Perhaps the most universally known fact in regard to bacteria is
that they are the cause of disease. It is this fact that has made
them objects of such wide interest. This is the side of the
subject that first attracted attention, has been most studied, and
in regard to which there has been the greatest accumulation of
evidence. So persistently has the relation of bacteria to disease
been discussed and emphasized that the majority of readers are
hardly able to disassociate the two. To most people the very word
bacteria is almost equivalent to disease, and the thought of
swallowing microbes in drinking water or milk is decidedly
repugnant and alarming. In the public mind it is only necessary to
demonstrate that an article holds bacteria to throw it under
condemnation.

We have already seen that bacteria are to be regarded as agents
for good, and that from their fundamental relation to plant life
they must be looked upon as our friends rather than as our
enemies. It is true that there is another side to the story which
relates to the parasitic species. These parasitic forms may do us
direct or indirect injury. But the species of bacteria which are
capable of doing us any injury, the pathogenic bacteria, are
really very few compared to the great host of species which are
harmless. A small number of species, perhaps a score or two, are
pathogenic, while a much larger number, amounting to hundreds and
perhaps thousands of species, are perfectly harmless. This latter
class do no injury even though swallowed by man in thousands. They
are not parasitic, and are unable to grow in the body of man.
Their presence is entirely consistent with the most perfect
health, and, indeed, there are some reasons for believing that
they are sometimes directly beneficial to health. It is entirely
unjust to condemn all bacteria because a few chance to produce
mischief. Bacteria in general are agents for good rather than ill.

There are, however, some species which cause mankind much trouble
by interfering in one way or another with the normal processes of
life. These pathogenic bacteria, or disease germs, do not all act
alike, but bring about injury to man in a number of different
ways. We may recognise two different classes among them, which,
however, we shall see are connected by intermediate types. These
two classes are, first, the pathogenic bacteria, which are not
strictly parasitic but live free in Nature; and, second, those
which live as true parasites in the bodies of man or other
animals. To understand the real relation of these two classes, we
must first notice the method by which bacteria in general produce
disease.

METHOD BY WHICH BACTERIA PRODUCE DISEASE.

Since it was first clearly recognised that certain species of
bacteria have the power of producing disease, the question as to
how they do so has ever been a prominent one Even if they do grow
in the body, why should their presence give rise to the symptoms
characterizing disease? Various answers to this question have been
given in the past It has been suggested that in their growth they
consume the food of the body and thus exhaust it, that they
produce an oxidation of the body tissues, or that they produce a
reduction of these tissues, or that they mechanically interfere
with the circulation None of these suggestions have proved of much
value Another view was early advanced, and has stood the test of
time. This claim is that the bacteria while growing in the body
produce poisons, and these poisons then have a direct action on
the body We have already noticed that bacteria during their growth
in any medium produce a large number of biproducts of
decomposition. We noticed also that among these biproducts there
are some which have a poisonous nature; so poisonous are they that
when inoculated into the body of an animal they may produce
poisoning and death. We have only to suppose that the pathogenic
bacteria, when growing as parasites in man, produce such poisons,
and we have at once an explanation of the method by which they
give rise to disease.

This explanation of germ disease is more than simple theory. It
has been in many cases clearly demonstrated. It has been found
that the bacteria which cause diphtheria, tetanus, typhoid,
tuberculosis, and many other diseases, produce, even when growing
in common culture media, poisons which are of a very violent
nature. These poisons when inoculated into the bodies of animals
give rise to much the same symptoms as the bacteria do themselves
when growing as parasites in the animals. The chief difference in
the results from inoculating an animal with the poison and with
the living bacteria is in the rapidity of the action. When the
poison is injected the poisoning symptoms are almost immediately
seen, but when the living bacteria are inoculated the effect is
only seen after several days or longer, not, in short, until the
inoculated bacteria have had time enough to grow in the body and
produce the poison in quantity. It has not by any means been shown
that all pathogenic germs produce their effect in this way, but it
has been proved to be the real method in quite a number of cases,
and is extremely probable in others. While some bacteria perhaps
produce results by a different method, we must recognise the
production of poisons as at all events the common direct cause of
the symptoms of disease. This explanation will enable us more
clearly to understand the relation of different bacteria to
disease.

PATHOGENIC GERMS WHICH ARE NOT STRICTLY PARASITIC

Recognising that bacteria may produce poisons, we readily see that
it is not always necessary that they should be parasites in order
to produce trouble. In their ordinary growth in Nature such
bacteria will produce no trouble The poisons will be produced in
decaying material but will seldom be taken into the human body.
These poisons, produced in the first stages of putrefaction, are
oxidized by further stages of decomposition into harmless
products. But should it happen that some of these bacteria
obtained a chance to grow vigorously for a while in organic
products that are subsequently swallowed as man's food, it is
plain that evil results might follow. If such food is swallowed by
man after the bacteria have produced their poisonous bodies, it
will tend to produce an immediate poisoning of his system. The
effect may be sudden and severe if considerable quantity of the
poisonous material is swallowed, or slight but protracted if small
quantities are repeatedly consumed in food. Such instances are not
uncommon. Well-known examples are cases of ice-cream poisoning,
poisoning from eating cheese or from drinking milk, or in not a
few instances from eating fish or meats within which bacteria have
had opportunity for growth. In all these cases the poison is
swallowed in quantity sufficient to give rise quickly to severe
symptoms, sometimes resulting fatally, and at other times passing
off as soon as the body succeeds in throwing off the poisons. In
other cases still, however, the amount of poison swallowed may be
very slight, too slight to produce much effect unless the same be
consumed repeatedly. All such trouble may be attributed to
fermented or partly decayed food. It is difficult to distinguish
such instances from others produced in a slightly different way,
as follows:

It may happen that the bacteria which grow in food products
continue to grow in the food even after it is swallowed and has
passed into the stomach or intestines. This appears particularly
true of milk bacteria. Under these conditions the bacteria are not
in any proper sense parasitic, since they are simply living in and
feeding upon the same food which they consume outside the body,
and are not feeding upon the tissues of man. The poisons which
they produce will continue to be developed as long as the bacteria
continue to grow, whether in a milk pail or a human stomach. If
now the poisons are absorbed by the body, they may produce a mild
or severe disease which will be more or less lasting, continuing
perhaps as long as the same food and the same bacteria are
supplied to the individual. The most important disease of this
class appears to be the dreaded cholera infantum, so common among
infants who feed upon cow's milk in warm weather. It is easy to
understand the nature of this disease when we remember the great
number of bacteria in milk, especially in hot weather, and when we
remember that the delicate organism of the infant will be thrown
at once into disorder by slight amounts of poison which would have
no appreciable effect upon the stronger adult. We can easily
understand, further, how the disease readily yields to treatment
if care is taken to sterilize the milk given to the patient.

We do not know to-day the extent of the troubles which are
produced by bacteria of this sort. They will, of course, be
chiefly connected with our food products, and commonly, though not
always, will affect the digestive functions. It is probable that
many of the cases of summer diarrhoea are produced by some such
cause, and if they could be traced to their source would be found
to be produced by bacterial poisons swallowed with food or drink,
or by similar poisons produced by bacteria growing in such food
after it is swallowed by the individual. In hot weather, when
bacteria are so abundant everywhere and growing so rapidly, it is
impossible to avoid such dangers completely without exercising
over all food a guard which would be decidedly oppressive. It is
well to bear in mind, however, that the most common and most
dangerous source of such poisons is milk or its products, and for
this reason one should hesitate to drink milk in hot weather
unless it is either quite fresh or has been boiled to destroy its
bacteria.

PATHOGENIC BACTERIA WHICH ARE TRUE PARASITES.

This class of pathogenic bacteria includes those which actually
invade the body and feed upon its tissues instead of living simply
upon swallowed food. It is difficult, however, to draw any sharp
line separating the two classes. The bacteria which cause
diphtheria (Fig. 28), for instance, do not really invade the body.
They grow in the throat, attached to its walls, and are confined
to this external location or to the superficial tissues. This
bacillus is, in short, only found in the mouth and throat, and is
practically confined to the so-called false membranes. It never
enters any of the tissues of the body, although attached to the
mucous membrane. It grows vigorously in this membrane, and there
secretes or in some way produces extremely violent poisons. These
poisons are then absorbed by the body and give rise to the general
symptoms of the disease. Much the same is true of the bacillus
which causes tetanus or lockjaw (Fig. 29). This bacillus is
commonly inoculated into the flesh of the victim by a wound made
with some object which has been lying upon the earth where the
bacillus lives. The bacillus grows readily after being inoculated,
but it is localized at the point of the wound, without invading
the tissue to any extent. It produces, however, during its growth
several poisons which have been separated and studied. Among them
are some of the most violent poisons of which we have any
knowledge. While the bacillus grows in the tissues around the
wound it secretes these poisons, which are then absorbed by the
body generally. Their poisoning effects produce the violent
symptoms of the disease. Of much the same nature is Asiatic
cholera. This is caused by a bacillus which is able to grow
rapidly in the intestines, feeding perhaps in part on the food in
the intestines and perhaps in part upon the body secretions. To a
slight extent also it appears to be able to invade the tissues of
the body, for the bacilli are found in the walls of the
intestines. But it is not a proper parasite, and the fatal disease
it produces is the result of the absorption of the poisons
secreted in the intestines.

It is but a step from this to the true parasites. Typhoid fever,
for example, is a disease produced by bacteria which grow in the
intestines, but which also invade the tissues more extensively
than the cholera germs (Fig. 30). They do not invade the body
generally, however, but become somewhat localized in special
glands like the liver, the spleen, etc. Even here they do not
appear to find a very favourable condition, for they do not grow
extensively in these places. They are likely to be found in the
spleen in small groups or centres, but not generally distributed
through it. Wherever they grow they produce poison, which has been
called typhotoxine, and it is this poison chiefly which gives rise
to the fever.

Quite a considerable number of the pathogenic germs are, like the
typhoid bacillus, more or less confined to special places. Instead
of distributing themselves through the body after they find
entrance, they are restricted to special organs. The most common
example of a parasite of this sort is the tuberculosis bacillus,
the cause of consumption, scrofula, white swelling, lupus, etc.
(Fig. 31). Although this bacillus is very common and is able to
attack almost any organ in the body, it is usually very restricted
in growth. It may become localized in a small gland, a single
joint, a small spot in the lungs, or in the glands of the
mesentery, the other parts of the body remaining free from
infection. Not infrequently the whole trouble is thus confined to
such a small locality that nothing serious results. But in other
instances the bacilli may after a time slowly or rapidly
distribute themselves from these centres, attacking more and more
of the body until perhaps fatal results follow in the end. This
disease is therefore commonly of very slow progress.

Again, we have still other parasites which are not thus confined,
but which, as soon as they enter the body, produce a general
infection, attacking the blood and perhaps nearly all tissues
simultaneously. The most typical example of this sort is anthrax
or malignant pustule, a disease fortunately rare in man (Fig. 32).
Here the bacilli multiply in the blood, and very soon a general
and fatal infection of the whole body arises, resulting from the
abundance of the bacilli everywhere. Some of the obscure diseases
known as blood poisoning appear to be of the same general nature,
these diseases resulting from a very general invasion of the whole
body by certain pathogenic bacteria.

In general, then, we see that the so-called germ diseases result
from the action upon the body of poisons produced by bacterial
growth. Differences in the nature of these poisons produce
differences in the character of the disease, and differences in
the parasitic powers of the different species of bacteria produce
wide differences in the course of the diseases and their relation
to external phenomena.

WHAT DISEASES ARE DUE TO BACTERIA?

It is, of course, an extremely important matter to determine to
what extent human diseases are caused by bacteria. It is not easy,
nor indeed possible, to do this to-day with accuracy. It is no
easy matter to prove that any particular disease is caused by
bacteria. To do this it is necessary to find some particular
bacterium present in all cases of the disease; to find some method
of getting it to grow outside the body in culture media; to
demonstrate its absence in healthy animals, or healthy human
individuals if it be a human disease; and, finally, to reproduce
the disease in healthy animals by inoculating them with the
bacterium. All of these steps of proof present difficulties, but
especially the last one. In the study of animals it is
comparatively easy to reproduce a disease by inoculation. But
experiments upon man are commonly impossible, and in the case of
human diseases it is frequently very difficult or impossible to
obtain the final test of the matter. After finding a specific
bacterium associated with a disease, it is usually possible to
experiment with it further upon animals only. But some human
diseases do not attack animals, and in the case of diseases that
may be given to animals it is frequently uncertain whether the
disease produced in the animal by such inoculation is identical
with the human disease in question, owing to the difference of
symptoms in the different animals. As a consequence, the proof of
the germ nature of different diseases varies all the way from
absolute demonstration to mere suspicion. To give a complete and
correct list of the diseases caused by bacteria, or to give a list
of the bacteria species pathogenic to man, is therefore at present
impossible.

The difficulty of giving such a list is rendered greater from the
fact that we have in recent years learned that the same species of
pathogenic bacterium may produce different results under different
conditions. When the subject of germ disease was first studied and
the connection between bacteria and disease was first
demonstrated, it was thought that each particular species of
pathogenic bacteria produced a single definite disease; and
conversely, each germ disease was supposed to have its own
definite species of bacterium as its cause. Recent study has
shown, however, that this is not wholly true. It is true that some
diseases do have such a definite relation to definite bacteria.
The anthrax germ, for example, will always produce anthrax, no
matter where or how it is inoculated into the body. So, also, in
quite a number of other cases distinct specific bacteria are
associated with distinct diseases. But, on the other hand, there
are some pathogenic bacteria which are not so definite in their
action, and produce different results in accordance with
circumstances, the effect varying both with the organ attacked and
with the condition of the individual. For instance, a considerable
number of different types of blood poisoning, septicaemia,
pyaemia, gangrene, inflammation of wounds, or formation of pus
from slight skin wounds--indeed, a host of miscellaneous troubles,
ranging all the way from a slight pus formation to a violent and
severe blood poisoning--all appear to be caused by bacteria, and
it is impossible to make out any definite species associated with
the different types of these troubles. There are three common
forms of so-called pus cocci, and these are found almost
indiscriminately with various types of inflammatory troubles.
Moreover, these species of bacteria are found with almost absolute
constancy in and around the body, even in health. They are on the
clothing, on the skin, in the mouth and alimentary canal. Here
they exist, commonly doing no harm. They have, however, the power
of doing injury if by chance they get into wounds. But their power
of doing injury varies both with the condition of the individual
and with variations in the bacteria themselves. If the individual
is in a good condition of health these bacteria have little power
of injuring him even when they do get into such wounds, while at
times of feeble vitality they may do much more injury, and take
the occasion of any little cut or bruise to enter under the skin
and give rise to inflammation and pus. Some people will develop
slight abscesses or slight inflammations whenever the skin is
bruised, while with others such bruises or cuts heal at once
without trouble. Both are doubtless subject to the same chance of
infection, but the one resists, while the other does not. In
common parlance, we say that such a tendency to abscesses
indicates a bad condition of the blood--a phrase which means
nothing. Further, we find that the same species of bacterium may
have varying powers of producing disease at different times. Some
species are universal inhabitants of the alimentary canal and are
ordinarily harmless, while under other conditions of unknown
character they invade the tissues and give rise to a serious and
perhaps fatal disease. We may thus recognise some bacteria which
may be compared to foreign invaders, while others are domestic
enemies. The former, like the typhoid bacillus, always produce
trouble when they succeed in entering the body and finding a
foothold. The latter, like the normal intestinal bacilli, are
always present but commonly harmless, only under special
conditions becoming troublesome. All this shows that there are
other factors in determining the course of a disease, or even the
existence of a disease, than the simple presence of a peculiar
species of pathogenic bacterium.

From the facts just stated it will be evident that any list of
germ diseases will be rather uncertain. Still, the studies of the
last twenty years or more have disclosed some definite relations
of bacteria and disease, and a list of the diseases more or less
definitely associated with distinct species of bacteria is of
interest. Such a list, including only well-known diseases, is as
follows:

 Name of disease.              Name of bacterium producing the disease.
 Anthrax (Malignant pustule).     Bacillus anthracis.
 Cholera.                         Spirillum cholera: asiaticae
 Croupous pneumonia.              Micrococcus pneumonia crouposa.
 Diphtheria.                      Bacillus diphtheria.
 Glanders.                        Bacillus mallei.
 Gonorrhoea.                      Micrococcus gonorrhaeae
 Influenza.                       Bacillus of influenza.
 Leprosy.                         Bacillus leprae.
 Relapsing fever.                 Spirillum Obermeieri.
 Tetanus (lockjaw).               Bacillus tetani.
 Tuberculosis (including
 consumption, scrofula, etc.)     Bacillus tuberculosis.
 Typhoid fever.                   Bacillus typhi abdominalis.

Various wound infections, including septicaemia, pyaemia, acute
abscesses, ulcers, erysipelas, etc., are produced by a few forms
of micrococci, resembling each other in many points but differing
slightly. They are found almost indiscriminately in any of these
wound infections, and none of them appears to have any definite
relation to any special form of disease unless it be the
micrococcus of erysipelas. The common pus micrococci are grouped
under three species, Staphylococcus pyogenes aureus,
Staphylococcus pyogenes, and Streptococcus pyogenes. These three
are the most common, but others are occasionally found.

In addition to these, which may be regarded as demonstrated, the
following diseases are with more or less certainty regarded as
caused by distinct specific bacteria: Bronchitis, endocarditis,
measles, whooping-cough, peritonitis, pneumonia, syphilis.

Still another list might be given of diseases whose general nature
indicates that they are caused by bacteria, but in connection with
which no distinct bacterium has yet been found. As might be
expected also, a larger list of animal diseases has been
demonstrated to be caused by these organisms. In addition, quite a
number of species of bacteria have been found in such material as
faeces, putrefying blood, etc., which have been shown by
experiment to be capable of producing diseases in animals, but in
regard to which we have no evidence that they ever do produce
actual disease under any normal conditions. These may contribute,
perhaps, to the troubles arising from poisonous foods, but can not
be regarded as disease germs proper.

VARIABILITY OF PATHOGENIC POWERS.

As has already been stated, our ideas of the relation of bacteria
to disease have undergone quite a change since they were first
formulated, and we recognise other factors influencing disease
besides the actual presence of the bacterium. These we may briefly
consider under two heads, viz., variation in the bacterium, and
variation in the susceptibility of the individual. The first will
require only a brief consideration.

That the same species of pathogenic bacteria at different times
varies in its powers to produce disease has long been known.
Various conditions are known to affect thus the virulence of
bacteria. The bacillus which is supposed to give rise to pneumonia
loses its power to produce the disease after having been
cultivated for a short time in ordinary culture media in the
laboratory. This is easily understood upon the suggestion that it
is a parasitic bacillus and does not thrive except under parasitic
conditions. Its pathogenic powers can sometimes be restored by
passing it again through some susceptible animal. One of the most
violent pathogenic bacteria is that which produces anthrax, but
this loses its pathogenic powers if it is cultivated for a
considerable period at a high temperature. The micrococcus which
causes fowl cholera loses its power if it be cultivated in common
culture media, care being taken to allow several days to elapse
between the successive inoculations into new culture flasks. Most
pathogenic bacteria can in some way be so treated as to suffer a
diminution or complete loss of their powers of producing a fatal
disease. On the other hand, other conditions will cause an
increase in the virulence of a pathogenic germ. The virus which
produces hydrophobia is increased in violence if it is inoculated
into a rabbit and subsequently taken from the rabbit for further
inoculation. The fowl cholera micrococcus, which has been weakened
as just mentioned, may be restored to its original violence by
inoculating it into a small bird, like a sparrow, and inoculating
a second bird from this. A few such inoculations will make it as
active as ever. These variations doubtless exist among the species
in Nature as well as in artificial cultures. The bacteria which
produce the various wound infections and abscesses, etc., appear
to vary under normal conditions from a type capable of producing
violent and fatal blood poisoning to a type producing only a
simple abscess, or even to a type that is entirely innocuous. It
is this factor, doubtless, which in a large measure determines the
severity of any epidemic of a bacterial contagious disease.

SUSCEPTIBILITY OF THE INDIVIDUAL.

The very great modification of our early views has affected our
ideas as to the power which individuals have of resisting the
invasion of pathogenic bacteria. It has from the first been
understood that some individuals are more susceptible to disease
than others, and in attempting to determine the significance of
this fact many valuable and interesting discoveries have been
made. After the exposure to the disease there follows a period of
some length in which there are no discernible effects. This is
followed by the onset of the disease and its development to a
crisis, and, if this be passed, by a recovery. The general course
of a germ disease is divided into three stages: the stage of
incubation, the development of the disease, and the recovery. The
susceptibility of the body to a disease may be best considered
under the three heads of Invasion, Resistance, Recovery.

Means of Invasion.--In order that a germ disease should arise in
an individual, it is first necessary that the special bacterium
which causes the disease should get into the body. There are
several channels through which bacteria can thus find entrance;
these are through the mouth, through the nose, through the skin,
and occasionally through excretory ducts. Those which come through
the mouth come with the food or drink which we swallow; those
which enter through the nose must be traced to the air; and those
which enter through the skin come in most cases through contact
with some infected object, such as direct contact with the body of
an infected person or his clothing or some objects he has handled,
etc. Occasionally, perhaps, the bacteria may get into the skin
from the air, but this is certainly uncommon and confined to a few
diseases. There are here two facts of the utmost importance for
every one to understand: first, that the chance of disease
bacteria being carried to us through the air is very slight and
confined to a few diseases, such as smallpox, tuberculosis,
scarlet fever; etc., and, secondly, that the uninjured skin and
the uninjured mucous membrane also is almost a sure protection
against the invasion of the bacteria. If the skin is whole,
without bruises or cuts, bacteria can seldom, if ever, find
passage through it. These two facts are of the utmost importance,
since of all sources of infection we have the least power to guard
against infection through the air, and since of all means 'of
entrance we can guard the skin with the greatest difficulty. We
can easily render food free from pathogenic bacteria by heating
it. The material we drink can similarly be rendered harmless, but
we can not by any known means avoid breathing air, nor is there
any known method of disinfecting the air, and it is impossible for
those who have anything to do with sick persons to avoid entirely
having contact either with the patient or with infected clothing
or utensils.

From the facts here given it will be seen that the individual's
susceptibility to disease produced by parasitic bacteria will
depend upon his habits of cleanliness, his care in handling
infectious material, or care in cleansing the hands after such
handling, upon his habit of eating food cooked or raw, and upon
the condition of his skin and mucous membranes, since any kind of
bruises will increase susceptibility. Slight ailments, such as
colds, which inflame the mucous membrane, will decrease its
resisting power and render the individual more susceptible to the
entrance of any pathogenic germs should they happen to be present.
Sores in the mouth or decayed teeth may in the same way be
prominent factors in the individual's susceptibility. Thus quite a
number of purely physical factors may contribute to an
individual's susceptibility.

Resisting Power of the Body.--Even after the bacteria get into the
body it is by no means certain that they will give rise to
disease, for they have now a battle to fight before they can be
sure of holding their own. It is now, indeed, that the actual
conflict between the powers of the body and these microscopic
invaders begins. After they have found entrance into the body the
bacteria have arrayed against them strong resisting forces of the
human organism, endeavouring to destroy and expel them. Many of
them are rapidly killed, and sometimes they are all destroyed
without being able to gain a foothold. In such cases, of course,
no trouble results. In other cases the body fails to overcome the
powers of the invaders and they eventually multiply rapidly. In
this struggle the success of the invaders is not necessarily a
matter of numbers. They are simply struggling to gain a position
in the body, where they can feed and grow. A few individuals may
be entirely sufficient to seize such a foothold, and then these by
multiplying may soon become indefinitely numerous. To protect
itself, therefore, the human body must destroy every individual
bacterium, or at least render them all incapable of growth. Their
marvellous reproductive powers give the bacteria an advantage in
the battle. On the other hand, it takes time even for these
rapidly multiplying beings to become sufficiently numerous to do
injury. There is thus an interval after their penetration into the
body when these invaders are weak in numbers. During this
interval--the period of incubation--the body may organize a
resistance sufficient to expel them.

We do not as yet thoroughly understand the forces which the human
organism is able to array against these invading foes. Some of its
methods of defence are, however, already intelligible to us, and
we know enough, at all events, to give us an idea of the intensity
of the conflict that is going on, and of the vigorous and powerful
forces which the human organism is able to bring against its
invading enemies.

In the first place, we notice that a majority of bacteria are
utterly unable to grow in the human body even if they do find
entrance. There are known to bacteriologists to-day many hundreds,
even thousands of species, but the vast majority of these find in
the human tissues conditions so hostile to their life that they
are utterly unable to grow therein. Human flesh or human blood
will furnish excellent food for them if the individual be dead,
but living human flesh and blood in some way exerts a repressing
influence upon them which is fatal to the growth of a vast
majority of species. Some few species, however, are not thus
destroyed by the hostile agencies of the tissues of the animal,
but are capable of growing and multiplying in the living body.
These alone are what constitute the pathogenic bacteria, since, of
course, these are the only bacteria which can produce disease by
growing in the tissues of an animal. The fact that the vast
majority of bacteria can not grow in the living organism shows
clearly enough that there are some conditions existing in the
living tissue hostile to bacterial life. There can be little
doubt, moreover, that it is these same hostile conditions, which
enable the body to resist the attack of the pathogenic species in
cases where resistance is successfully made.

What are the forces arrayed against these invaders? The essential
nature of the battle appears to be a production of poisons and
counter poisons. It appears to be an undoubted fact that the first
step in repelling these bacteria is to flood them with certain
poisons which check their growth. In the blood and lymph of man
and other animals there are present certain products which have a
direct deleterious influence upon the growth of micro-organisms.
The existence of these poisons is undoubted, many an experiment
having directly attested to their presence in the blood of
animals. Of their nature we know very little, but of their
repressing influence upon bacterial growth we are sure. They have
been named alexines, and they are produced in the living tissue,
although as to the method of their production we are in ignorance.
By the aid of these poisons the body is able to prevent the growth
of the vast majority of bacteria which get into its tissues.
Ordinary micro-organisms are killed at once, for these alexines
act as antiseptics, and common bacteria can no more grow in the
living body than they could in a solution containing other poisons
Thus the body has a perfect protection against the majority of
bacteria. The great host of species which are found in water,
milk, air, in our mouths or clinging to our skin, and which are
almost omnipresent in Nature, are capable of growing well enough
in ordinary lifeless organic foods, but just as soon as they
succeed in finding entrance into living human tissue their growth
is checked at once by these antiseptic agents which are poured
upon them. Such bacteria are therefore not pathogenic germs, and
not sources of trouble to human health.

There are, on the other hand, a few species of bacteria which may
be able to retain their lodgment in the body m spite of this
attempt of the individual to get rid of them. These, of course,
constitute the pathogenic species, or so called "disease germs".
Only such species as can overcome this first resistance can be
disease germs, for they alone can retain their foothold in the
body.

But how do these species overcome the poisons, which kill the
other harmless bacteria? They, as well as the harmless forms, find
these alexines injurious to their growth, but in some way they are
able to counteract the poisons. In this general discussion of
poisons we are dealing with a subject which is somewhat obscure,
but apparently the pathogenic bacteria are able to overcome the
alexines of the body by producing in their turn certain other
products which neutralize the alexines, thus annulling their
action. These pathogenic bacteria, when they get into the body,
give rise at once to a group of bodies which have been named
lysines. These lysines are as mysterious to us as the alexines,
but they neutralize the effect of the alexines and thus overcome
the resistance the body offers to bacterial growth The invaders
can now multiply rapidly enough to get a lasting foothold in the
body and then soon produce the abnormal symptoms which we call
disease Pathogenic bacteria thus differ from the non-pathogenic
bacteria primarily in this power of secreting products which can
neutralize the ordinary effects of the alexines, and so overcome
the body's normal resistance to their parasitic life.

Even if the bacteria do thus overcome the alexines the battle is
not yet over, for the individual has another method of defence
which is now brought into activity to check the growth of the
invading organisms. This second method of resistance is by means
of a series of active cells found in the blood, known as white
blood-corpuscles (Fig. 33 a, b). They are minute bits of
protoplasm present in the blood and lymph in large quantities.
They are active cells, capable of locomotion and able to crawl out
of the blood-vessels Not infrequently they are found to take into
their bodies small objects with which they come in contact. One of
their duties is thus to engulf minute irritating bodies which may
be in the tissues, and to carry them away for excretion. They thus
act as scavengers These corpuscles certainly have some agency in
warding off the attacks of pathogenic bacteria Very commonly they
collect in great numbers in the region of the body where invading
bacteria are found. Such invading bacteria exist upon them a
strong attraction, and the corpuscles leave the blood-vessels and
sometimes form a solid phalanx completely surrounding the invading
germs. Their collection at these points may make itself seen
externally by the phenomenon we call inflammation.

There is no question that the corpuscles engage in conflict with
the bacteria when they thus surround them. There has been not a
little dispute, however, as to the method by which they carry on
the conflict. It has been held by some that the corpuscles
actually take the bacteria into their bodies, swallow them, as it
were, and subsequently digest them (Fig. 33 c, d, e). This idea
gave rise to the theory of phagocytosis, and the corpuscles were
consequently named phagocytes. The study of several years has,
however, made it probable that this is not the ordinary method by
which the corpuscles destroy the bacteria. According to our
present knowledge the method is a chemical one. These cells, when
they thus collect in quantities around the invaders, appear to
secrete from their own bodies certain injurious products which act
upon the bacteria much as do the alexines already mentioned. These
new bodies have a decidedly injurious effect upon the multiplying
bacteria; they rapidly check their growth, and, acting in union
with the alexines, may perhaps entirely destroy them.

After the bacteria are thus killed, the white blood-corpuscles may
load themselves with their dead bodies and carry them away (Fig.
33 d, e). Sometimes they pass back into the blood stream and carry
the bacteria to various parts of the body for elimination. Not
infrequently the white corpuscles die in the contest, and then may
accumulate in the form of pus and make their way through the skin
to be discharged directly. The battle between these phagocytes and
the bacteria goes on vigorously. If in the end the phagocytes
prove too strong for the invaders, the bacteria are gradually all
destroyed, and the attack is repelled. Under these circumstances
the individual commonly knows nothing--of the matter. This
conflict has taken place entirely without any consciousness on his
part, and he may not even know that he has been exposed to the
attack of the bacteria. In other cases the bacteria prove too
strong for the phagocytes. They multiply too rapidly, and
sometimes they produce secretions which actually drive the
phagocytes away. Commonly, as already noticed, the corpuscles are
attracted to the point of invasion, but in some cases, when a
particularly deadly and vigorous species of bacteria invades the
body, the secretions produced by them are so powerful as actually
to drive the corpuscles away. Under these circumstances the
invading hosts have a chance to multiply unimpeded, to distribute
themselves over the body, and the disease rapidly follows as the
result of their poisoning action on the body tissues.

It is plain, then, that the human body is not helpless in the
presence of the bacteria of disease, but that it is supplied with
powerful resistant forces. It must not be supposed, however, that
the outline of the action of these forces just given is anything
like a complete account of the matter; nor must it be inferred
that the resistance is in all respects exactly as outlined. The
subject has only recently been an object of investigation, and we
are as yet in the dark in regard to many of the facts. The future
may require us to modify to some extent even the brief outline
which has been given. But while we recognise this uncertainty in
the details, we may be assured of the general facts. The living
body has some very efficacious resistant forces which prevent most
bacteria from growing within its tissues, and which in large
measure may be relied upon to drive out the true pathogenic
bacteria. These resistant forces are in part associated with the
productions of body poisons, and are in part associated with the
active powers of special cells which have been called phagocytes.
The origin of the poisons and the exact method of action of the
phagocytes we may well leave to the future to explain.

These resisting powers of the body will vary with conditions. It
is evident that they are natural powers, and they will doubtless
vary with the general condition of vigour of the individual.
Robust health, a body whose powers are strong, well nourished, and
vigorous, will plainly furnish the conditions for the greatest
resistance to bacterial diseases. One whose bodily activities are
weakened by poor nutrition can offer less resistance. The question
whether one shall suffer from a germ disease is not simply the
question whether he shall be exposed, or even the question whether
the bacteria shall find entrance into his body. It is equally
dependent upon whether he has the bodily vigour to produce
alexines in proper quantity, or to summon the phagocytes in
sufficient abundance and vigour to ward off the attack. We may do
much to prevent disease by sanitation, which aids in protecting
the individual from attack; but we must not forget that the other
half of the battle is of equal importance, and hence we must do
all we can to strengthen the resisting forces of the organism.

RECOVERY FROM GERM DISEASES.

These resisting forces are not always sufficient to drive off the
invaders. The organisms may retain their hold in the body for a
time and eventually break down the resistance. After this they may
multiply unimpeded and take entire possession of the body. As they
become more numerous their poisonous products increase and begin
to produce direct poisoning effects on the body. The incubation
period is over and the disease comes on. The disease now runs its
course. It becomes commonly more and more severe until a crisis is
reached. Then, unless the poisoning is so severe that death
occurs, the effects pass away and recovery takes place.

But why should not a germ disease be always fatal? If the bacteria
thus take possession of the body and can grow there, why do they
not always continue to multiply until they produce sufficient
poison to destroy the life of the individual? Such fatal results
do, of course, occur, but in by far the larger proportion of cases
recovery finally takes place.

Plainly, the body must have another set of resisting forces which
is concerned in the final recovery. Although weakened by the
poisoning and suffering from the disease, it does not yield the
battle, but somewhat slowly organizes a new attack upon the
invaders. For a time the multiplying bacteria have an unimpeded
course and grow rapidly; but finally their further increase is
checked, their vigour impaired, and after this they diminish in
numbers and are finally expelled from the body entirely. Of the
nature of this new resistance but little is yet known. We notice,
in the first place, that commonly after such a recovery the
individual has decidedly increased resistance to the disease. This
increased resistance may be very lasting, and may be so
considerable as to give almost complete immunity from the disease
for many years, or for life. One attack of scarlet fever gives the
individual great immunity for the future. On the other hand, the
resistance thus derived may be very temporary, as in the case of
diphtheria. But a certain amount of resistance appears to be
always acquired. This power of resisting the activities of the
parasites seems to be increased during the progress of the
disease, and, if it becomes sufficient, it finally drives off the
bacteria before they have produced death. After this, recovery
takes place. To what this newly acquired resisting power is due is
by no means clear to bacteriologists, although certain factors
are already known. It appears beyond question that in the case of
certain diseases the cells of the body after a time produce
substances which serve as antidotes to the poisons produced by the
bacteria during their growth in the body-antitoxines. In the case
of diphtheria, for instance, the germs growing in the throat
produce poisons which are absorbed by the body and give rise to
the symptoms of the disease; but after a time the body cells
react, and themselves produce a counter toxic body which
neutralizes the poisonous effect of the diphtheria poison. This
substance has been isolated from the blood of animals that have
recovered from an attack of diphtheria, and has been called
diphtheria antitoxine. But even with this knowledge the recovery
is not fully explained. This antitoxine neutralizes the effects of
the diphtheria toxine, and then the body develops strength to
drive off the bacteria which have obtained lodgment in the throat.
How they accomplish this latter achievement we do not know as yet.
The antitoxme developed simply neutralizes the effects of the
toxine. Some other force must be at work to get rid of the
bacteria, a force which can only exert itself after the poisoning
effect of the poison is neutralized. In these cases, then, the
recovery is due, first, to the development in the body of the
natural antidotes to the toxic poisons, and, second, to some other
unknown force which drives off the parasites.

These facts are certainly surprising. If one had been asked to
suggest the least likely theory to explain recovery from disease,
he could hardly have found one more unlikely than that the body
cells developed during the disease an antidote to the poison which
the disease bacteria were producing. Nevertheless, it is beyond
question that such antidotes are formed during the course of the
germ diseases. It has not yet been shown in all diseases, and it
would be entirely too much to claim that this is the method of
recovery in all cases. We may say, however, in regard to bacterial
diseases in general, that after the bacteria enter the body at
some weak point they have first a battle to fight with the
resisting powers of the body, which appear to be partly biological
and partly chemical. These resisting powers are in many cases
entirely sufficient to prevent the bacteria from obtaining a
foothold. If the invading host overcome the resisting powers, then
they begin to multiply rapidly, and take possession of the body or
some part of it. They continue to grow until either the individual
dies or something occurs to check their growth. After the
individual develops the renewed powers of checking their growth,
recovery takes place, and the individual is then, because of these
renewed powers of resistance, immune from a second attack of the
disease for a variable length of time.

This, in the merest outline, represents the relation of bacterial
parasites to the human body But while this is a fair general
expression of the matter, it must be recognised that different
diseases differ much in their relations, and no general outline
will apply to all They differ in their method of attack and in the
point of attack. Not only do they produce different kinds of
poisons giving rise to different symptoms of poisoning; not only
do they produce different results in different animals; not only
do the different pathogenic species differ much in their power
to develop serious disease, but the different species are very
particular as to what species of animal they attack. Some of them
can live as parasites in man alone; some can live as parasites
upon man and the mouse and a few other animals; some can live in
various animals but not in man; some appear to be able to live in
the field mouse, but not in the common mouse; some live in the
horse; some in birds, but not in warm-blooded mammals; while
others, again, can live almost equally well in the tissues of a
long list of animals. Those which can live as parasites upon man
are, of course, especially related to human disease, and are of
particular interest to the physician, while those which live in
animals are in a similar way of interest to veterinarians.

Thus we see that parasitic bacteria show the widest variations.
They differ in point of attack, in method of attack, and in the
part of the body which they seize upon as a nucleus for growth.
They differ in violence and in the character of the poisons they
produce, as well as in their power of overcoming the resisting
powers of the body. They differ at different times in their powers
of producing disease. In short, they show such a large number of
different methods of action that no general statements can be made
which will apply universally, and no one method of guarding
against them or in driving them off can be hoped to apply to any
extended list of diseases.

DISEASES CAUSED BY OTHER ORGANISMS THAN BACTERIA.

Although the purpose of this work is to deal primarily with the
bacterial world, it would hardly be fitting to leave the subject
without some reference to diseases caused by organisms which do
not belong to the group of bacteria. While most of the so-called
germ diseases are caused by the bacteria which we have been
studying in the previous chapters, there are some whose inciting
cause is to be found among organisms belonging to other groups.
Some of these are plants of a higher organization than bacteria,
but others are undoubtedly microscopic animals. Their life habits
are somewhat different from those of bacteria, and hence the
course of the diseases is commonly different. Of the diseases thus
produced by microscopic animals or by higher plants, one or two
are of importance enough to deserve special mention here.

Malaria.--The most important of these diseases is malaria in its
various forms, and known under various names--chills and fever,
autumnal fever, etc. This disease, so common almost everywhere,
has been studied by physicians and scientists for a long time, and
many have been the causes assigned to it. At one time it was
thought to be the result of the growth of a bacterium, and a
distinct bacillus was described as producing it. It has finally
been shown, however, to be caused by a microscopic organism
belonging to the group of unicellular animals, and somewhat
closely related to the well-known amoeba. This organism is shown
in Fig. 34. The whole history of the malarial organism is not yet
known. The following statements comprise the most important facts
known in regard to it, and its relation to the disease in man.

Undoubtedly the malarial germ has some home outside the human
body, but it is not yet very definitely known what this external
home is; nor do we know from what source the human parasite is
derived. It appears probable that water serves in some cases as
its means of transference to man, and air in other cases. From
some external source it gains access to man and finds its way into
the blood. Here it attacks the red blood-corpuscles, each malarial
organism making its way into a single one (Fig. 340). Here it now
grows, increasing in size at the expense of the substance of the
corpuscle. As it becomes larger it becomes granular, and soon
shows a tendency to separate into a number of irregular masses.
Finally it breaks up into many minute bodies called spores. These
bodies break out of the corpuscle and for a time live a free life
in the blood. After a time they make their way into other red
blood-corpuscles, develop into new malarial amoeboid parasites,
and repeat the growth and sporulation. This process can apparently
be repeated many times without check.

These organisms are thus to be regarded as parasites of the red
corpuscles. It is, of course, easy to believe that an extensive
parasitism and destruction of the corpuscles would be disastrous
to the health of the individual, and the severity of the disease
will depend upon the extent of the parasitism. Corresponding to
this life history of the organism, the disease malaria is commonly
characterized by a decided intermittency, periods of chill and
fever alternating with periods of intermission in which these
symptoms are abated. The paroxysms of the disease, characterized
by the chill, occur at the time that the spores are escaping from
the blood-corpuscles and floating in the blood. After they have
again found their way into a blood-corpuscle the fever diminishes,
and during their growth in the corpuscle until the next
sporulation the individual has a rest from the more severe
symptoms.

There appears to be more than one variety of the malarial
organism, the different types differing in the length of time it
takes for their growth and sporulation. There is one variety, the
most common one, which requires two days for its growth, thus
giving rise to the paroxysm of the disease about once in forty-
eight hours; another variety appears to require three days for its
growth; while still another variety appears to be decidedly
irregular in its period of growth and sporulation. These facts
readily explain some of the variations in the disease. Certain
other irregularities appear to be due to a different cause. More
than one brood of parasites may be in the blood of the individual
at the same time, one producing sporulation at one time and
another at a different time. Such a simultaneous growth of two
independent broods may plainly produce almost any kind of
modification in the regularity of the disease.

The malarial organism appears to be very sensitive to quinine, a
very small quantity being sufficient to kill it. Upon this point
depends the value of quinine as a medicine. If the drug be present
in the blood at the time when the spores are set free from the
blood-corpuscle, they are rapidly killed by it before they have a
chance to enter another corpuscle. During their growth in the
corpuscle they are far less sensitive to quinine than when they
exist in the free condition as spores, and at this time the drug
has little effect.

The malarial organism is an animal, and can not be cultivated in
the laboratory by any artificial method yet devised. Its whole
history is therefore not known. It doubtless has some home outside
the blood of animals, and very likely it may pass through other
stages of a metamorphosis in the bodies of other animals. Most
parasitic animals have two or more hosts upon which they live,
alternating from one to the other, and that such is the case with
the malarial parasite is at least probable. But as yet
bacteriologists have been unable to discover anything very
definite in regard to the matter. Until we can learn something in
regard to its life outside the blood of man we can do little in
the way of devising methods to avoid it.

Malaria differs from most germ diseases in the fact that the
organisms which produce it are not eliminated from the body in any
way. In most germ diseases the germs are discharged from the
patient by secretions or excretions of some kind, and from these
excretions may readily find their way into other individuals. The
malarial organism is not discharged from the body in any way, and
hence is not contagious. If the parasite does pass part of its
history in some other animal than man, there must be some means by
which it passes from man to its other host. It has been suggested
that some of the insects which feed upon human blood may serve as
the second host and become inoculated when feeding upon such
blood. This has been demonstrated with startling success in regard
to the mosquito (Anopheles), some investigators going so far as to
say that this is the only way in which the disease can be
communicated.

Several other microscopic animals occur as parasites upon man, and
some of them are so definitely associated with certain diseases as
to lead to the belief that they are the cause of these diseases.
The only one of very common occurrence is a species known as
Amaeba coli, which is found in cases of dysentery. In a certain
type of dysentery this organism is so universally found that there
is little doubt that it is in some very intimate way associated
with the cause of the disease. Definite proof of the matter is,
however, as yet wanting.

On the side of plants, we find that several plants of a higher
organization than bacteria may become parasitic upon the body of
man and produce various types of disease. These plants belong
mostly to the same group as the moulds, and they are especially
apt to attack the skin. They grow in the skin, particularly under
the hair, and may send their threadlike branches into some of the
subdermal tissues. This produces irritation and inflammation of
the skin, resulting in trouble, and making sores difficult to
heal. So long as the plant continues to grow, the sores, of
course, can not be healed, and when the organisms get into the
skin under the hair it is frequently difficult to destroy them.
Among the diseases thus caused are ringworm, thrush, alopecia,
etc.





CHAPTER VI.

METHODS OF COMBATING PARASITIC BACTERIA.


The chief advantage of knowing the cause of disease is that it
gives us a vantage ground from which we may hope to find means of
avoiding its evils. The study of medicine in the past history of
the world has been almost purely empirical, with a very little of
scientific basis. Great hopes are now entertained that these new
facts will place this matter upon a more strictly scientific
foundation. Certainly in the past twenty-five years, since
bacteriology has been studied, more has been done to solve
problems connected with disease than ever before. This new
knowledge has been particularly directed toward means of avoiding
disease. Bacteriology has thus far borne fruit largely in the line
of preventive medicine, although to a certain extent also along
the line of curative medicine. This chapter will be devoted to
considering how the study of bacteriology has contributed directly
and indirectly to our power of combating disease.

PREVENTIVE MEDICINE.

In the study of medicine in the past centuries the only aim has
been to discover methods of curing disease; at the present time a
large and increasing amount of study is devoted to the methods of
preventing disease. Preventive medicine is a development of the
last few years, and is based almost wholly upon our knowledge of
bacteria. This subject is yearly becoming of more importance.
Forewarned is forearmed, and it has been found that to know the
cause of a disease is a long step toward avoiding it. As some of
our contagious and epidemic diseases have been studied in the
light of bacteriological knowledge, it has been found possible to
determine not only their cause, but also how infection is brought
about, and consequently how contagion may be avoided. Some of the
results which have grown up so slowly as to be hardly appreciated
are really great triumphs. For instance, the study of bacteriology
first led us to suspect, and then demonstrated, that tuberculosis
is a contagious disease, and from the time that this was thus
proved there has been a slow, but, it is hoped, a sure decline in
this disease. Bacteriological study has shown that the source of
cholera infection in cases of raging epidemics is, in large part
at least, our drinking water; and since this has been known,
although cholera has twice invaded Europe, and has been widely
distributed, it has not obtained any strong foothold or given rise
to any serious epidemic except in a few cases where its ravages
can be traced to recognised carelessness. It is very significant
to compare the history of the cholera epidemics of the past few
years with those of earlier dates. In the epidemics of earlier
years the cholera swept ruthlessly through communities without
check. In the last few years, although it has repeatedly knocked
at the doors of many European cities, it has been commonly
confined to isolated cases, except in a few instances where these
facts concerning the relation to drinking water were ignored.

The study of preventive medicine is yet in its infancy, but it has
already accomplished much. It has developed modern systems of
sanitation, has guided us in the building of hospitals, given
rules for the management of the sick-room which largely prevent
contagion from patient to nurse; it has told us what diseases are
contagious, and in what way; it has told us what sources of
contagion should be suspected and guarded against, and has thus
done very much to prevent the spread of disease. Its value is seen
in the fact that there has been a constant decrease in the death
rate since modern ideas of sanitation began to have any influence,
and in the fact that our general epidemics are less severe than in
former years, as well as in the fact that more people escape the
diseases which were in former times almost universal.

The study of preventive medicine takes into view several factors,
all connected with the method and means of contagion. They are the
following:

The Source of Infectious Material.--t has been learned that for
most diseases the infectious material comes from individuals
suffering with the disease, and that except in a few cases, like
malaria, we must always look to individuals suffering from disease
for all sources of contagion. It is found that pathogenic bacteria
are in all these cases eliminated from the patient in some way,
either from the alimentary canal or from skin secretions or
otherwise, and that any nurse with common sense can have no
difficulty in determining in what way the infectious material is
eliminated from her patients. When this fact is known and taken
into consideration it is a comparatively easy matter to devise
valuable precautions against distribution of such material. It is
thus of no small importance to remember that the simple presence
of bacteria in food or drink is of no significance unless these
bacteria have come from some source of disease infection.

The Method of Distribution.--The bacteria must next get from the
original source of the disease to the new susceptible individual.
Bacteria have no independent powers of distribution unless they be
immersed in liquids, and therefore their passage from individual
to individual must be a passive one. They are readily
transferred, however, by a number of different means, and the
study of these means is aiding much in checking contagion Study
along this line has shown that the means by which bacteria are
carried are several. First we may notice food as a distributor.
Food may become contaminated by infectious material in many ways;
for example, by contact with sewage, or with polluted water, or
even with eating utensils which have been used by patients. Water
is also likely to be contaminated with infectious material, and is
a fertile source for distributing typhoid and cholera. Milk may
become contaminated in a variety of ways, and be a source of
distributing the bacteria which produce typhoid fever,
tuberculosis, diphtheria, scarlet fever, and a few other less
common diseases. Again, infected clothing, bedding, or eating
utensils may be taken from a patient and be used by another
individual without proper cleansing. Direct contact, or contact
with infected animals, furnishes another method. Insects sometimes
carry the bacteria from person to person, and in some diseases
(tuberculosis, and perhaps scarlet fever and smallpox) we must
look to the air as a distributor of the infectious material.
Knowledge of these facts is helping to account for multitudes of
mysterious cases of infection, especially when we combine them
with the known sources of contagious matter.

Means of Invasion.--Bacteriology has shown us that different
species of parasitic bacteria have different means of entering the
body, and that each must enter the proper place in order to get a
foothold. After we learn that typhoid infectious material must
enter the mouth in order to produce the disease; that tuberculosis
may find entrance through the nose in breathing, while types of
blood poisoning enter only through wounds or broken skin, we learn
at once fundamental facts as to the proper methods of meeting
these dangers. We learn that with some diseases care exercised to
prevent the swallowing of infectious material is sufficient to
prevent contagion, while with others this is entirely
insufficient. When all these facts are understood it is almost
always perfectly possible to avoid contagion; and as these facts
become more and more widely known direct contagion is sure to
become less frequent.

Above all, it is telling us what becomes of the pathogenic
bacteria after being eliminated from the body of the patient; how
they may exist for a long time still active; how they may lurk in
filth or water dormant but alive, or how they may even multiply
there. Preventive medicine is telling us how to destroy those thus
lying in wait for a chance of infection, by discovering
disinfectants and telling us especially where and when to use
them. It has already taught us how to crush out certain forms of
epidemics by the proper means of destroying bacteria, and is
lessening the dangers from contagious diseases. In short, the
study of bacteriology has brought us into a condition where we are
no longer helpless in the presence of a raging epidemic. We no
longer sit in helpless dismay, as did our ancestors, when an
epidemic enters a community, but, knowing their causes and
sources, set about at once to remove them. As a result, severe
epidemics are becoming comparatively short-lived.

BACTERIA IN SURGERY.

In no line of preventive medicine has bacteriology been of so much
value and so striking in its results as in surgery. Ever since
surgery has been practised surgeons have had two difficulties to
contend with. The first has been the shock resulting from the
operation. This is dependent upon the extent of the operation, and
must always be a part of a surgical operation. The second has been
secondary effects following the operation. After the operation,
even though it was successful, there were almost sure to arise
secondary complications known as surgical fever, inflammation,
blood poisoning, gangrene, etc., which frequently resulted
fatally. These secondary complications were commonly much more
serious than the shock of the operation, and it used to be the
common occurrence for the patient to recover entirely from the
shock, but yield to the fevers which followed. They appeared to be
entirely unavoidable, and were indeed regarded as necessary parts
of the healing of the wound. Too frequently it appeared that the
greater the care taken with the patient the more likely he was to
suffer from some of these troubles. The soldier who was treated on
the battlefield and nursed in an improvised field hospital would
frequently recover, while the soldier who had the fortune to be
taken into the regular hospital, where greater care was possible,
succumbed to hospital gangrene. All these facts were clearly
recognised, but the surgeon, through ignorance of their cause, was
helpless in the presence of these inflammatory troubles, and felt
it always necessary to take them into consideration.

The demonstration that putrefaction and decay were caused by
bacteria, and the early proof that the silkworm disease was
produced by a micro-organism, led to the suggestion that the
inflammatory diseases accompanying wounds were similarly caused.
There are many striking similarities between these troubles and
putrefaction, and the suggestion was an obvious one. At first,
however, and for quite a number of years, it was impossible to
demonstrate the theory by finding the distinct species of micro-
organisms which produced the troubles. We have already seen that
there are several different species of bacteria which are
associated with this general class of diseases, but that no
specific one has any particular relation to a definite type of
inflammation. This fact made discoveries in this connection a slow
matter from the microscopical standpoint. But long before this
demonstration was finally reached the theory had received
practical application in the form of what has developed into
antiseptic or aseptic surgery.

Antiseptic surgery is based simply upon the attempt to prevent the
entrance of bacteria into the surgical wound. It is assumed that
if these organisms are kept from the wound the healing will take
place without the secondary fevers and inflammations which occur
if they do get a chance to grow in the wound. The theory met with
decided opposition at first, but accumulating facts demonstrated
its value, and to-day its methods have been adopted everywhere in
the civilized world. As the evidence has been accumulating,
surgeons have learned many important facts, foremost among which
is a knowledge of the common sources from which the infection of
wounds occurs. At first it was thought that the air was the great
source of infection, but the air bacteria have been found to be
usually harmless. It has appeared that the more common sources are
the surgeon's instruments, or his hands, or the clothing or
sponges which are allowed to come in contact with the wounds. It
has also appeared that the bacteria which produce this class of
troubles are common species, existing everywhere and universally
present around the body, clinging to the clothing or skin, and
always on hand to enter the wound if occasion offers. They are
always present, but commonly harmless. They are not foreign
invaders like the more violent pathogenic species, such as those
of Asiatic cholera, but may be compared to domestic enemies at
hand. It is these ever-present bacteria which the surgeon must
guard against. The methods by which he does this need not detain
us here. They consist essentially in bacteriological cleanliness.
The operation is performed with sterilized instruments under most
exacting conditions of cleanliness.

The result has been a complete revolution in surgery. As the
methods have become better understood and more thoroughly adopted,
the instances of secondary troubles following surgical wounds have
become less and less frequent until they have practically
disappeared in all simple cases. To-day the surgeon recognises
that when inflammatory troubles of this sort follow simple
surgical wounds it is a testimony to his carelessness. The skilful
surgeon has learned that with the precautions which he is able to
take to-day he has to fear only the direct effect of the shock of
the wound and its subsequent direct influence; but secondary
surgical fevers, blood poisoning, and surgical gangrene need not
be taken into consideration at all. Indeed, the modern surgeon
hardly knows what surgical gangrene is, and bacteriologists have
had practically no chance to study it. Secondary infections have
largely disappeared, and the surgeon is concerned simply with the
effect of the wound itself, and the power of the body to withstand
the shock and subsequently heal the wound.

With these secondary troubles no longer to disturb him, the
surgeon has become more and more bold. Operations formerly not
dreamed of are now performed without hesitation. In former years
an operation which opened the abdominal cavity was not thought
possible, or at least it was so nearly certain to result fatally
that it was resorted to only on the last extremity; while to-day
such operations are hardly regarded as serious. Even brain surgery
is becoming more and more common. Possibly our surgeons are
passing too far to the other extreme, and, feeling their power of
performing so many operations without inconvenience or danger,
they are using the knife in cases where it would be better to
leave Nature to herself for her own healing. But, be this as it
may, it is impossible to estimate the amount of suffering
prevented and the number of lives saved by the mastery of the
secondary inflammatory troubles which used to follow surgical
wounds.

Preventive medicine, then, has for its object the prevention
rather than the cure of disease. By showing the causes of disease
and telling us where and how they are contracted, it is telling us
how they may to a large extent be avoided. Unlike practical
medicine, this subject is one which has a direct relation to the
general public. While it may be best that the knowledge of
curative methods be confined largely to the medical profession, it
is eminently desirable that a knowledge of all the facts bearing
upon preventive medicine should be distributed as widely as
possible. One person can not satisfactorily apply his knowledge of
preventive medicine, if his neighbour is ignorant of or careless
of the facts. We can not hope to achieve the possibilities lying
along this line until there is a very wide distribution of
knowledge. Every epidemic that sweeps through our communities is a
testimony to the crying need of education in regard to such simple
facts as the source of infectious material, the methods of its
distribution, and the means of rendering it harmless.

PREVENTION IN INOCULATION.

It has long been recognised that in most cases recovery from one
attack of a contagious disease renders an individual more or less
immune against a second attack. It is unusual for an individual to
have the same contagious disease twice. This belief is certainly
based upon fact, although the immunity thus acquired is subject to
wide variations. There are some diseases in which there is little
reason for thinking that any immunity is acquired, as in the case
of tuberculosis, while there are others in which the immunity is
very great and very lasting, as in the case of scarlet fever.
Moreover, the immunity differs with individuals. While some
persons appear to acquire a lasting immunity by recovery from a
single attack, others will yield to a second attack very readily.
But in spite of this the fact of such acquired immunity is beyond
question. Apparently all infectious diseases from which a real
recovery takes place are followed by a certain amount of
protection from a second attack; but with some diseases the
immunity is very fleeting, while with others it is more lasting.
Diseases which produce a general infection of the whole system
are, as a rule, more likely to give rise to a lasting immunity
than those which affect only small parts. Tuberculosis, which, as
already noticed, is commonly quite localized in the body, has
little power of conveying immunity, while a disease like scarlet
fever, which affects the whole system, conveys a more lasting
protection.

Such immunity has long been known, and in the earlier years was
sometimes voluntarily acquired; even to-day we find some
individuals making use of the principle. It appears that a mild
attack of such diseases produces immunity equally well with a
severe attack, and acting upon this fact mothers have not
infrequently intentionally exposed their children to certain
diseases at seasons when they are mild, in order to have the
disease "over with" and their children protected in the future.
Even the more severe diseases have at times been thus voluntarily
acquired. In China it has sometimes been the custom thus to
acquire smallpox. Such methods are decidedly heroic, and of course
to be heartily condemned. But the principle that a mild type of
the disease conveys protection has been made use of in a more
logical and defensible way.

The first instance of this principle was in vaccination against
smallpox, now practised for more than a century. Cowpox is
doubtless closely related to smallpox, and an attack of the former
conveys a certain amount of protection against the latter. It was
easy, therefore, to inoculate man with some of the infectious
material from cowpox, and thus give him some protection against
the more serious smallpox. This was a purely empirical discovery,
and vaccination was practised long before the principle underlying
it was understood, and long before the germ nature of disease was
recognised. The principle was revived again, however, by Pasteur,
and this time with a logical thought as to its value. While
working upon anthrax among animals, he learned that here, as in
other diseases, recovery, when it occurred, conveyed immunity.
This led him to ask if it were not possible to devise a method of
giving to animals a mild form of the disease and thus protect them
from the more severe type. The problem of giving a mild type of
this extraordinarily severe disease was not an easy one. It could
not be done, of course, by inoculating the animals with a small
number of the bacteria, for their power of multiplication would
soon make them indefinitely numerous. It was necessary in some way
to diminish their violence. Pasteur succeeded in doing this by
causing them to grow in culture fluids for a time at a high
temperature. This treatment diminished their violence so much that
they could be inoculated into cattle, where they produced only the
mildest type of indisposition, from which the animals speedily
recovered. But even this mild type of the disease was triumphantly
demonstrated to protect the animals from the most severe form of
anthrax. The discovery was naturally hailed as a most remarkable
one, and one which promised great things in the future. If it was
thus possible, by direct laboratory methods, to find a means of
inoculating against a serious disease like anthrax, why could not
the same principle be applied to human diseases? The enthusiasts
began at once to look forward to a time when all diseases should
be thus conquered.

But the principle has not borne the fruit at first expected. There
is little doubt that it might be applied to quite a number of
human diseases if a serious attempt should be made. But several
objections arise against its wide application. In the first place,
the inoculation thus necessary is really a serious matter. Even
vaccination, as is well known, sometimes, through faulty methods,
results fatally, and it is a very serious thing to experiment upon
human beings with anything so powerful for ill as pathogenic
bacteria. The seriousness of the disease smallpox, its
extraordinary contagiousness, and the comparatively mild results
of vaccination, have made us willing to undergo vaccination at
times of epidemics to avoid the somewhat great probability of
taking the disease. But mankind is unwilling to undergo such an
operation, even though mild, for the purpose of avoiding other
less severe diseases, or diseases which are less likely to be
taken. We are unwilling to be inoculated against mild diseases, or
against the more severe ones which are uncommon. For instance, a
method has been devised for rendering animals immune against
lockjaw, which would probably apply equally well to man. But
mankind in general will never adopt it, since the danger from
lockjaw is so small. Inoculation must then be reserved for
diseases which are so severe and so common, or which occur in
periodical epidemics of so great severity, as to make people in
general willing to submit to inoculation as a protection. A
further objection arises from the fact that the immunity acquired
is not necessarily lasting. The cattle inoculated against anthrax
retain their protective powers for only a few months. How long
similar immunity might be retained in other cases we can not say,
but plainly this fact would effectually prevent this method of
protecting mankind from being used except in special cases. It is
out of the question to think of constant and repeated inoculations
against various diseases.

As a result, the principle of inoculation as an aid in preventive
medicine has not proved of very much value. The only other human
disease in which it has been attempted seriously is Asiatic
cholera. This disease in times of epidemics is so severe and the
chance of infection is so great as to justify such inoculation.
Several bacteriologists have in the last few years been trying to
discover a harmless method of inoculating against this disease.
Apparently they have succeeded, for experiments in India, the home
of the cholera, have been as successful as could be anticipated.
Bacteriological science has now in its possession a means of
inoculation against cholera which is perhaps as efficacious as
vaccination is against smallpox. Whether it will ever be used to
any extent is doubtful, since, as already pointed out, we are in a
position to avoid cholera epidemics by other means. If we can
protect our communities by guarding the water supply, it is not
likely that the method of inoculation will ever be widely used.

Another instance of the application of preventive inoculation has
been made, but one based upon a different principle. Hydrophobia
is certainly one of the most horrible of diseases, although
comparatively rare. Its rarity would effectually prevent mankind
from submitting to a general inoculation against it, but its
severity would make one who had been exposed to it by the bite of
a rabid animal ready to submit to almost any treatment that
promised to ward off the disease. In the attempt to discover a
means of inoculating against this disease it was necessary,
therefore, to find a method that could be applied after the time
of exposure--i.e., after the individual had been bitten by the
rabid animal. Fortunately, the disease has a long period of
incubation, and one that has proved long enough for the purpose. A
method of inoculation against this disease has been devised by
Pasteur, which can be applied after the individual has been bitten
by the rabid animal. Apparently, however, this preventive
inoculation is dependent upon a different principle from
vaccination or inoculation against anthrax. It does not appear to
give rise to a mild form of the disease, thus protecting the
individual, but rather to an acquired tolerance of the chemical
poisons produced by the disease. It is a well-known physiological
fact that the body can become accustomed to tolerate poisons if
inured to them by successively larger and larger doses. It is by
this power, apparently, that the inoculation against hydrophobia
produces its effect. Material containing the hydrophobia poison
(taken from the spinal cord of a rabbit dead with the disease) is
injected into the individual after he has been bitten by a rabid
animal. The poisonous material in the first injection is very
weak, but is followed later by a more powerful inoculation. The
result is that after a short time the individual has acquired the
power of resisting the hydrophobia poisons. Before the incubation
period of the original infectious matter from the bite of the
rabid animal has passed, the inoculated individual has so
thoroughly acquired a tolerance of the poison that he successfully
resists the attack of the infection. This method of inoculation
thus neutralizes the effects of the disease by anticipating them.

The method of treatment of hydrophobia met with extraordinarily
violent opposition. For several years it was regarded as a
mistake. But the constantly accumulating statistics from the
Pasteur Institute have been so overwhelmingly on one side as to
quiet opposition and bring about a general conviction that the
method is a success.

The method of preventive inoculation has not been extensively
applied to human diseases in addition to those mentioned. In a few
cases a similar method has been used to guard against diphtheria.
Among animals, experiment has shown that such methods can quite
easily be obtained, and doubtless the same would be true of
mankind if it was thought practical or feasible to apply them.
But, for reasons mentioned, this feature of preventive medicine
will always remain rather unimportant, and will be confined to a
few of the more violent diseases.

It may be well to raise the question as to why a single attack
with recovery conveys immunity. This question is really a part of
the one already discussed as to the method by which the body cures
disease. We have seen that this is in part due to the development
of chemical substances which either neutralize the poisons or act
as germicide upon the bacteria, or both, and perhaps due in part
to an active destruction of bacteria by cellular activity
(phagocytosis). There is little reason to doubt that it is the
same set of activities which renders the animal immune. The forces
which drive off the invading bacteria in one case are still
present to prevent a second attack of the same species of
bacterium. The length of time during which these forces are active
and sufficient to cope with any new invaders determines the length
of time during which the immunity lasts. Until, therefore, we can
answer with more exactness just how cure is brought about in case
of disease, we shall be unable to explain the method of immunity.

LIMITS OF PREVENTIVE MEDICINE.

With all the advance in preventive medicine we can not hope to
avoid disease entirely. We are discovering that the sources of
disease are on all sides of us, and so omnipresent that to avoid
them completely is impossible. If we were to apply to our lives
all the safeguards which bacteriology has taught us should be
applied in order to avoid the different diseases, we would
surround ourselves with conditions which would make life
intolerable. It would be oppressive enough for us to eat no food
except when it is hot, to drink no water except when boiled, and
to drink no milk except after sterilization; but these would not
satisfy the necessary conditions for avoiding disease. To meet all
dangers, we should handle nothing which has not been sterilized,
or should follow the handling by immediately sterilizing the
hands; we should wear only disinfected clothes, we should never
put our fingers in our mouths or touch our food with them; we
should cease to ride in public conveyances, and, indeed, should
cease to breathe common air. Absolute prevention of the chance of
infection is impossible. The most that preventive medicine can
hope for is to point out the most common and prolific sources of
infection, and thus enable civilized man to avoid some of his most
common troubles. It becomes a question, therefore, where we will
best draw the line in the employment of safeguards. Shall we drink
none except sterilized milk, and no water unless boiled? or shall
we put these occasional sources of danger in the same category
with bicycle and railroad accidents, dangers which can be avoided
by not using the bicycle or riding on the rail, but in regard to
which the remedy is too oppressive for application?

Indeed, when viewed in a broad philosophical light it may not be
the best course for mankind to shun all dangers. Strength in the
organism comes from the use rather than the disuse of our powers.
It is certain that the general health and vigour of mankind is to
be developed by meeting rather than by shunning dangers.
Resistance to disease means bodily vigour, and this is to be
developed in mankind by the application of the principle of
natural selection. In accordance with this principle, disease will
gradually remove the individuals of weak resisting powers, leaving
those of greater vigour. Parasitic bacteria are thus a means of
preventing the continued life of the weaker members of the
community, and so tend to strengthen mankind. By preventive
medicine many a weak individual who would otherwise succumb
earlier in the struggle is enabled to live a few years longer.
Whatever be our humanitarian feeling for the individual, we can
not fail to admit that this survival of the weak is of no benefit
to the race so far as the development of physical nature is
concerned. Indeed, if we were to take into consideration simply
the physical nature of man we should be obliged to recommend a
system such as the ancient Spartans developed, of exposing to
death all weakly individuals, that only the strong might live to
become the fathers of future generations. In this light, of
course, parasitic diseases would be an assistance rather than a
detriment to the human race. Of course such principles will never
again be dominant among men, and our conscience tells us to do all
we can to help the weak. We shall doubtless do all possible to
develop preventive medicine in order to guard the weak against
parasitic organisms. But it is at all events well for us to
remember that we can never hope to develop the strength of the
human race by shunning evil, but rather by combating it, and the
power of the human race to resist the invasions of these organisms
will never be developed by the line of action which guards us from
attack. Here, as in other directions, the principles of modern
humanity have, together with their undoubted favourable influence
upon mankind, certain tendencies toward weakness. While we shall
still do our utmost to develop preventive medicine in a proper
way, it may be well for us to remember these facts when we come to
the practical question of determining where to draw the limits of
the application of methods for preventing infectious diseases.

CURATIVE MEDICINE.

Bacteriology has hitherto contributed less to curative than to
preventive medicine. Nevertheless, its contributions to curative
medicine have not been unimportant, and there is promise of much
more in the future. It is, of course, unsafe to make predictions
for the future, but the accomplishments of the last few years give
much hope as to further results.

DRUGS.

It was at first thought that a knowledge of the specific bacteria
which cause a disease would give a ready means of finding specific
drugs for the cure of such disease. If a definite species of
bacterium causes a disease and we can cultivate the organism in
the laboratory, it is easy to find some drugs which will be fatal
to its growth, and these same drugs, it would seem, should be
valuable as medicines in these diseases. This hope has, however,
proved largely illusive. It is very easy to find some drug which
proves fatal to the specific germs while growing in the culture
media of the laboratory, but commonly these are of little or no
use when applied as medicines. In the first place, such substances
are usually very deadly poisons. Corrosive sublimate is a
substance which destroys all pathogenic germs with great rapidity,
but it is a deadly poison, and can not be used as a drug in
sufficient quantity to destroy the parasitic bacteria in the body
without at the same time producing poisonous effects on the body
itself. It is evident that for any drug to be of value in thus
destroying bacteria it must have some specially strong action upon
the bacteria. Its germicide action on the bacteria should be so
strong that a dose which would be fatal or very injurious to them
would be too small to have a deleterious influence on the body of
the individual. It has not proved an easy task to discover drugs
which will have any value as germicides when used in quantities so
small as to produce no injurious effect on the body.

A second difficulty is in getting the drug to produce its effect
at the right point. A few diseases, as we have noticed, are
produced by bacteria which distribute themselves almost
indiscriminately over the body; but the majority are somewhat
definitely localized in special points. Tuberculosis may attack a
single gland or a single lobe of the lung. Typhoid germ is
localized in the intestines, liver, spleen, etc. Even if it were
possible to find some drug which would have a very specific effect
upon the tuberculosis bacillus, it is plain that it would be a
very questionable method of procedure to introduce this into the
whole system simply that it might have an effect upon a very small
isolated gland. Sometimes such a bacterial affection may be
localized in places where it can be specially treated, as in the
case of an attack on a dermal gland, and in these cases some of
the germicides have proved to be of much value. Indeed, the use of
various disinfectants connected with abscesses and superficial
infections has proved of much value. To this extent, in
disinfecting wounds and as a local application, the development of
our knowledge of disinfectants has given no little aid to curative
medicine.

Very little success, however, has resulted in the attempt to find
specific drugs for specific diseases, and it is at least doubtful
whether many such will ever be found. The nearest approach to it
is quinine as a specific poison for malarial troubles. Malarious
diseases are not, however, produced by bacteria but by a
microscopic organism of a very different nature, thought to be an
animal rather than a plant. Besides this there has been little or
no success in discovering specifics in the form of drugs which can
be given as medicines or inoculated with the hope of destroying
special kinds of pathogenic bacteria without injury to the body.
While it is unwise to make predictions as to future discoveries,
there seems at present little hope for a development of curative
medicine along these lines.

VIS MEDICATRIX NATURAE.

The study of bacterial diseases as they progress in the body has
emphasized above all things the fact that diseases are eventually
cured by a natural rather than by an artificial process. If a
pathogenic bacterium succeeds in passing the outer safeguards and
entering the body, and if it then succeeds in overcoming the
forces of resistance which we have already noticed, it will begin
to multiply and produce mischief. This multiplication now goes on
for a time unchecked, and there is little reason to expect that we
can ever do much toward checking it by means of drugs. But after a
little, conditions arise which are hostile to the further growth
of the parasite. These hostile conditions are produced perhaps in
part by the secretions from the bacteria, for bacteria are unable
to flourish in a medium containing much of their own secretions.
The secretions which they produce are poisons to them as well as
to the individual in which they grow, and after these have become
quite abundant the further growth of the bacterium is checked and
finally stopped. Partly, also, must we conclude that these hostile
conditions are produced by active vital powers in the body of the
individual attacked. The individual, as we have seen, in some
cases develops a quantity of some substance which neutralizes the
bacterial poisons and thus prevents their having their maximum
effect. Thus relieved from the direct effects of the poisons, the
resisting powers are recuperated and once more begin to produce a
direct destruction of the bacteria. Possibly the bacteria, being
now weakened by the presence of their own products of growth, more
readily yield to the resisting forces of the cell life of the
body. Possibly the resisting forces are decidedly increased by the
reactive effect of the bacteria and their poisons. But, at all
events, in cases where recovery from parasitic diseases occurs,
the revived powers of resistance finally overcome the bacteria,
destroy them or drive them off, and the body recovers.

All this is, of course, a natural process. The recovery from a
disease produced by the invasion of parasitic bacteria depends
upon whether the body can resist the bacterial poisons long enough
for the recuperation of its resisting powers. If these poisons are
very violent and produced rapidly, death will probably occur
before the resisting powers are strong enough to drive off the
bacteria. In the case of some diseases the poisons are so violent
that this practically always occurs, recovery being very
exceptional. The poison produced by the tetanus bacillus is of
this nature, and recovery from lockjaw is of the rarest
occurrence. But in many other diseases the body is able to
withstand the poison, and later to recover its resisting powers
sufficiently to drive off the invaders. In all cases, however, the
process is a natural one and dependent upon the vital activity of
the body. It is based at the foundation, doubtless, upon the
powers of the body cells, either the phagocytes or other active
cells. The body has, in short, its own forces for repelling
invasions, and upon these forces must we depend for the power to
produce recovery.

It is evident that all these facts give us very little
encouragement that we shall ever be able to cure diseases directly
by means of drugs to destroy bacteria, but, on the contrary, that
we must ever depend upon the resisting powers of the body. They
teach us, moreover, along what line we must look for the future
development of curative medicine. It is evident that scientific
medicine must turn its attention toward the strengthening and
stimulating of the resisting and curative forces of the body. It
must be the physician's aim to enable the body to resist the
poisons as well as possible and to stimulate it to re-enforce its
resistant forces. Drugs have a place in medicine, of course, but
this place is chiefly to stimulate the body to react against its
invading hosts. They are, as a rule, not specific against definite
diseases. We can not hope for much in the way of discovering
special medicines adapted to special diseases. We must simply look
upon them as means which the physician has in hand for stimulating
the natural forces of the body, and these may doubtless vary with
different individual natures. Recognising this, we can see also
the logic of the small dose as compared to the large dose. A small
dose of a drug may serve as a stimulant for the lagging forces,
while a larger dose would directly repress them or produce
injurious secondary effects. As soon as we recognise that the aim
of medicine is not to destroy the disease but rather to stimulate
the resisting forces of the body, the whole logic of therapeutics
assumes a new aspect.

Physicians have understood this, and, especially in recent years,
have guided their practice by it. If a moderate dose of quinine
will check malaria in a few days, it does not follow that twice
the dose will do it in half the time or with twice the certainty.
The larger doses of the past, intended to drive out the disease,
have been everywhere replaced by smaller doses designed to
stimulate the lagging body powers. The modern physician makes no
attempt to cure typhoid fever, having long since learned his
inability to do this, at least if the fever once gets a foothold;
but he turns his attention to every conceivable means of
increasing the body's strength to resist the typhoid poison,
confident that if he can thus enable the patient to resist the
poisoning effects of the typhotoxine his patient will in the end
react against the disease and drive off the invading bacteria. The
physician's duty is to watch and guard, but he must depend upon
the vital powers of his patient to carry on alone the actual
battle with the bacterial invaders.

ANTITOXINES.

In very recent times, however, our bacteriologists have been
pointing out to the world certain entirely new means of assisting
the body to fight its battles with bacterial diseases. As already
noticed, one of the primal forces in the recovery, from some
diseases, at least, is the development in the body of a substance
which acts as an antidote to the bacterial poison. So long as this
antitoxine is not present the poisons produced by the disease will
have their full effect to weaken the body and prevent the revival
of its resisting powers to drive off the bacteria. Plainly, if it
is possible to obtain this antitoxine in quantity and then
inoculate it into the body when the toxic poisons are present, we
have a means for decidedly assisting the body in its efforts to
drive off the parasites. Such an antidote to the bacterial poison
would not, indeed, produce a cure, but it would perhaps have the
effect of annulling the action of the poisons, and would thus give
the body a much greater chance to master the bacteria. It is upon
this principle that is based the use of antitoxines in diphtheria
and tetanus

It will be clear that to obtain the antitoxine we must depend upon
some natural method for its production. We do not know enough of
the chemical nature of the antitoxines to manufacture them
artificially. Of course we can not deny the possibility of their
artificial production, and certain very recent experiments
indicate that perhaps they may be made by the agency of
electricity. At present, however, we must use natural methods, and
the one commonly adopted is simple. Some animal is selected whose
blood is harmless to man and that is subject to the disease to be
treated. For diphtheria a horse is chosen. This animal is
inoculated with small quantities of the diphtheria poison without
the diphtheria bacillus. This poison is easily obtained by causing
the diphtheria bacillus to grow in common media in the laboratory
for a while, and the toxines develop in quantity; then, by proper
filtration, the bacteria themselves can be removed, leaving a pure
solution of the toxic poison. Small quantities of this poison are
inoculated into the horse at successive intervals. The effect on
the horse is the same as if the animal had the disease. Its cells
react and produce a considerable quantity of the antitoxine which
remains in solution in the blood of the animal. This is not
theory, but demonstrated fact. The blood of a horse so treated is
found to have the effect of neutralizing the diphtheria poison,
although the blood of the horse before such treatment has no such
effect. Thus there is developed in the horse's blood a quantity of
the antitoxine, and now it may be used by physicians where needed.
If some of this horse's blood, properly treated, be inoculated
into the body of a person who is suffering from diphtheria, its
effect, provided the theory of antitoxines is true, will be to
counteract in part, at least, the poisons which are being produced
in the patient by the diphtheria bacillus. This does not cure the
disease nor in itself drive off the bacilli, but it does protect
the body from the poisons to such an extent as to enable it more
readily to assert its own resisting powers.

This method of using antitoxines as a help in curing disease is
very recent, and we can not even guess what may come of it. It has
apparently been successfully applied in diphtheria. It has also
been used in tetanus with slight success. The same principle has
been used in obtaining an antidote for the poison of snake bites,
since it has appeared that in this kind of poisoning the body will
develop an antidote to the poison if it gets a chance. Horses have
been treated in the same way as with the diphtheria poison, and in
the same way they develop a substance which neutralizes the snake
poison. Other diseases are being studied to-day with the hope of
similar results. How much further the principle will go we can not
say, nor can we be very confident that the same principle will
apply very widely. The parasitic diseases are so different in
nature that we can hardly expect that a method which is
satisfactory in meeting one of the diseases will be very likely to
be adapted to another. Vaccination has proved of value in
smallpox, but is not of use in other human diseases. Inoculation
with weakened germs has proved of value in anthrax and fowl
cholera, but will not apply to all diseases. Each of these
parasites must be fought by special methods, and we must not
expect that a method that is of value in one case must necessarily
be of use elsewhere. Above all, we must remember that the
antitoxines do not cure in themselves; they only guard the body
from the weakening effects of the poisons until it can cure
itself, and, unless the body has resisting powers, the antitoxine
will fail to produce the desired results.

One further point in the action of the antitoxines must be
noticed. As we have seen, a recovery from an attack of most germ
diseases renders the individual for a time immune against a second
attack. This applies less, however, to a recovery after the
artificial inoculation with antitoxine than when the individual
recovers without such aid. If the individual recovers quite
independently of the artificial antitoxine, he does so in part
because he has developed the antitoxines for counteracting the
poison by his own powers. His cellular activities have, in other
words, been for a moment at least turned in the direction of
production of antitoxines. It is to be expected, therefore, that
after the recovery they will still have this power, and so long as
they possess it the individual will have protection from a second
attack. When, however, the recovery results from the artificial
inoculation of antitoxine the body cells have not actively
produced antitoxine. The neutralization of the poisons has been a
passive one, and after recovery the body cells are no more engaged
in producing antitoxine than before. The antitoxine which was
inoculated is soon eliminated by secretion, and the body is left
with practically the same liability to attack as before. Its
immunity is decidedly fleeting, since it was dependent not upon
any activity on the part of the body, but upon an artificial
inoculation of a material which is rapidly eliminated by
secretion.

CONCLUSION.

It is hoped that the outline which has been given of the bacterial
life of Nature may serve to give some adequate idea of these
organisms and correct the erroneous impressions in regard to them
which are widely prevalent. It will be seen that, as our friends,
bacteria play a vastly more important part in Nature than they do
as our enemies. These plants are minute and extraordinarily
simple, but, nevertheless, there exists a large number of
different species. The number of described forms already runs far
into the hundreds, and we do not yet appear to be approaching the
end of them. They are everywhere in Nature, and their numbers are
vast beyond conception. Their powers of multiplication are
inconceivable, and their ability to produce profound chemical
changes is therefore unlimited. This vast host of living beings
thus constitutes a force or series of forces of tremendous
significance. Most of the vast multitude we must regard as our
friends. Upon them the farmer is dependent for the fertility of
his soil and the possibility of continued life in his crops. Upon
them the dairyman is dependent for his flavours. Upon them
important fermentative industries are dependent, and their
universal powers come into action upon a commercial scale in many
a place where we have little thought of them in past years. We
must look upon them as agents ever at work, by means of which the
surface of Nature is enabled to remain fresh and green. Their
power is fundamental, and their activities are necessary for the
continuance of life. A small number of the vast host, a score or
two of species, unfortunately for us, find their most favourable
living place in the human body, and thus become human parasites.
By their growth they develop poisons and produce disease. This
small class of parasites are then decidedly our enemies. But,
taken all together, we must regard the bacteria as friends and
allies. Without them we should not have our epidemics, but without
them we should not exist. Without them it might be that some
individuals would live a little longer, if indeed we could live at
all. It is true that bacteria, by producing disease, once in a
while cause the premature death of an individual; once in a while,
indeed, they may sweep off a hundred or a thousand individuals;
but it is equally true that without them plant and animal life
would be impossible on the face of the earth.