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.