parasitic micro-organism. Pasteur later devoted several
years' study to an exhaustive investigation into the
same subject ; and in like manner Tulasse, in 1864,
and Kiihne, in 1855, showed that certain specific affec-
tions in grains, the potato, etc., were due to the inva-
sion of parasites.
Very soon after this it was demonstrated that micro-
organisms were the cause of certain infectious diseases
in man and the higher animals. Bacteriological re-
search has always been of special interest to physicians.
Many of the most distinguished physicians of the day,
in the earlier history of the science, concerned them-
selves in these investigations, and the progress made
during the past fifteen or twenty years has been largely
due to their work. Davaine, a famous French physi-
cian, has the honor of having first demonstrated the
causal relation of a micro-organism to a specific infec-
tious disease in man and animals. The anthrax bacil-
lus was discovered in the blood of animals dying from
this disease by Pollender, in 1849, and by Davaine,
in 1850 ; but it was not until 1863 that the last-named
observer demonstrated by inoculation experiments that
the bacillus was the cause of anthrax. These experi-
ments were subsequently confirmed by Pasteur, Koch,
The next discoveries made were those relating to
wounds and the infections to which they are liable.
Rindfleisch, in 1866, and Waldeyer and von Reckling-
hausen, in 1871, were the first to draw attention to the
minute organisms occurring in the pysemic processes
resulting from infected wounds, and occasionally fol-
lowing typhoid fever. Further operations were made
in erysipelatous inflammations secondary to injury by
Wilde, Orth, von Recklinghausen, Luthomsky, Bill-
roth, Ehrlich, Fehleisen, and others, agreeing that in
these conditions micro-organisms could always be de-
tected in the lymph-channels of the subcutaneous
tissues ; and through numerous experiments on ani-
mals the pathogenic character of the micro-organisms
found in erysipelas, suppuration from wounds, diph-
theria, puerperal fever, etc., was established by Oertel,
Huester, Birsch-Hirschfeld, Narsiloff, Classen, Letz-
erich, Leber, Frisch, Eberth, Klebs and others.
The brilliant results obtained by Lister, in 1863
1870, in the antiseptic treatment of wounds, to prevent
or inhibit the action of infective organisms, exerted a
powerful influence on the doctrine of bacterial infec-
tion, causing it to be recognized far and wide and
gradually lessening the number of its opponents.
The next important discovery was that of Ober-
meier, a German physician, who, in 1873, announced
having found in the blood of patients suffering from
relapsing fever a minute spiral, actively motile micro-
organism the spirochcete Obermeieri which is now
generally recognized as the specific infectious agent in
In 1877, Weigert and Ehrlich recommended the use
of the aniline dyes as staining agents in the micro-
scopical examination of micro-organisms in cover-glass
In 1878, Koch published his important work on
traumatic infectious diseases.
Hausen, in 1879, reported the discovery of bacilli
in the cells of leprous tubercles, which, from subsequent
researches, are believed to be the cause of leprosy.
Neisser, in the same year (1879), discovered the
" gonococcus " in gonorrhoeal discharges.
In 1880, Eberth and Koch independently observed
the typhoid bacillus, but it was not until 1884 that
Gaffky published his important researches, and proved
the etiological relation of this bacillus to typhoid fever.
In the same year (1880) several important communi-
cations in bacteriological research appeared. Pasteur
published his discovery of the bacillus of fowl cholera
and his investigations upon the attenuation of the virus
of anthrax and of fowl cholera, and upon protective
inoculation against these diseases.
Sternberg and Pasteur independently observed (1880)
a pathogenic micrococcus in saliva, which was subse-
quently proved by Fraenkel and others (1885) to be
the organism most commonly associated with acute
croupous pneumonia the " diplococcus pneumoniae"
and now recognized as the usual cause of that disease.
In 1881, Koch made his fundamental researches upon
pathogenic bacteria, the result of which was the estab-
lishment of a foundation upon which bacteriology of the
future was to rest. He introduced solid culture media
and the " plate method 7 ' for obtaining pure cultures,
and showed how different organisms could be isolated,
cultivated independently, and by inoculation of pure
cultures into susceptible animals made, in many cases,
to reproduce the specific disease of which they were the
cause ; and he laid down the laws by which it may be
proved that a micro-organism is the specific cause of
a disease. It was in the course of this work that the
Abbe system of substage condensing apparatus was first
used in bacteriology and that Weigert's method of
staining was generally employed.
30 BACTEEIOLOG T.
In 1882, Koch published his discovery of the tubercle
The same year (1882) Pasteur published his in-
vestigations upon "rouget" or hog erysipelas. In
this year, also, his first communication upon rabies
In 1882, also, Loemer and Schiitz discovered the
bacillus of glanders.
The cholera spirillum, or " comma bacillus/' was
discovered by Koch in 1884.
The diphtheria bacillus was discovered during the
same year (1884) by Loemer, though it had been ob-
served by Klebs the year before (1883).
Rosenbach, in 1884, by the application of Koch's
methods, fixed definitely the characters of the various
micro-organisms found in the pus from acute abscesses,
The tetanus bacillus was also discovered in 1884 by
Nicolaier. Carle and Rattone showed that tetanus is
an infectious disease communicable to man by inocula-
tion. Kitasato, in 1889, obtained the bacillus in pure
In 1892, Pfeiffer and Canon independently discov-
ered a bacillus which is believed to be the specific
cause of influenza.
In 1894, Kitasato, the Japanese bacteriologist, during
a visit to China, discovered the bacillus of the bubonic
These include all the most important pathogenic
bacteria, the discovery of which is of special interest
to medical students and physicians. We cannot close
this brief historical review, however, of the progress of
our knowledge in this department of science, without
INTR OD UCTION. 31
referring to the recent discovery of the antitoxins of
diphtheria and tetanus, the protective inoculations
against rabies, the plague, cholera, etc., and the pecu-
liar characteristics of the serum of those ill with infec-
tious diseases. These discoveries, in which the names
of Pasteur, Koch, Behring, Kitasato, Roux, Pfeiffer,
and Widal are among the most prominent, mark an
epoch in the history of bacteriology and scientific medi-
cine. Lately, attention has also been given to the
smaller group of the animal parasites, the protozoa,
which may prove to be the source of infection in many
diseases, such as the exanthemata, in one of which
smallpox they have already been apparently found.
THE GENERAL CHARACTERISTICS OF BACTERIA
THEIR MORPHOLOGY AND CHEMICAL COMPOSITION.
BACTERIA are among the smallest of all known liv-
ing organisms, the largest of them having a diameter
of only a few micromillimetres, while the smallest do
not measure more than a fraction of a micromillimetre.
Structurally and morphologically they are extremely
simple, though biologically very variable. Through
their ability to derive their carbon from tartrates and
their nitrogen from ammonia or its salts, they are
ranked in the vegetable kingdom. They obtain their
food entirely through the surface absorption of soluble
nutritious substances. They are reproduced by trans-
verse division, and in some respects resemble the fungi ;
hence called by Nageli fission-fungi, or schizomycetes.
They are also closely allied to certain kinds of algae,
though they must receive their nourishment from living
or dead organic material, since they are without chloro-
phyll, the green coloring matter possessed by the higher
plants, by means of which they are enabled, in the
presence of sunlight, to decompose CO 2 , NH 3 , and H 2 S
into their elementary constituents. A few varieties of
unicellular organisms resemble bacteria in all their
known characteristics, except that they possess chloro-
phyll or substances similar to it. Others, still, which
have no chlorophyll, are able in the absence of light to
build up organic substances synthetically. Bacteria,
especially the motile forms, are also closely allied to
some of the micro-organisms which belong to the ani-
mal kingdom. If we exclude the micro-organisms
containing chlorophyll, bacteria may be defined as
extremely minute vegetable organisms; without chloro-
phyll, consisting of single spherical, rod-shaped, or
corkscrew-like cells or aggregates of such cells, between
whose protoplasm and nucleus it has been as yet im-
possible to differentiate with certainty.
Bacteria occur as saprophytes or refuse-eaters and as
parasites. Saprophytic bacteria are such as commonly
exist independently of a living host, obtaining their
supply of nutriment from soluble food-stuffs in dead
organic matter. Parasitic bacteria, on the other hand,
live on or in some other organism, from which they
derive their nourishment for the whole or a part of
their existence. Those bacteria which depend entirely
upon a living host for their existence are known as
strict parasites ; those which can lead a saprophytic
existence, but which can also thrive within the body
of a living animal, are called facultative parasites.
The strict saprophytes, which represent the large
majority of all bacteria, while they destroy refuse, are
not only harmless to living organisms but perform
many important functions in nature without which
existence would be impossible, such as the destruction
of dead organic material through decomposition, putre-
faction, and fermentation. The parasites, on the con-
trary, though some of them may multiply in the secre-
tions or on the surface of the body without injury to
the animal upon which they depend for their exist-
ence, are usually harmful invaders, giving rise through
tHe lesions brought about in the body tissues by their
GENERAL CHARACTERISTICS OF BACTERIA. 35
growth and products to derangements which are known
as acute or chronic infectious diseases.
Numerous attempts have been made by various
authors to classify bacteria systematically, but usually
with the proviso that the system was only a temporary
one. The classification of the older naturalists and
botanists was based generally upon purely morphological
peculiarities. As this depended, at times, upon slight
variations that were seen to occur in the size and shape
of one and the same species, it naturally resulted in a
more or less complicated arrangement. In this place
the morphological character of the bacteria will alone
be given, their classification being left until the general
characteristics of bacteria have been considered.
The basic forms of the single bacterial cells are
threefold the sphere, the rod, and the segment of a
spiral. Although under different conditions, the form
of any one species may vary considerably, yet these
three main divisions under similar conditions are per-
manent; and, so far as we know, it is never possible
by any means to bring about changes in the organisms
that will result in the conversion of the morphology of
the members of one group into that of another that
is, micrococci always, under suitable conditions, produce
micrococci, bacilli produce bacilli, and spirilla produce
The form of the bacterial cells at their stage of com-
plete development must be distinguished from that
which they possess just after or just before they have
divided. As the spherical cell develops preparatory to
its division into two cells it becomes elongated and ap-
pears as a short oval rod; at the moment of its division,
on the contrary, the transverse diameter of each of its
two halves is greater than their long diameter. A
short rod becomes in the same way, at the moment
of its division, two cells, the long diameter of each of
which may be even a trifle less than its short diameter,
and thus they appear on superficial examination as
As bacteria multiply the cells produced from the
parent cell have a greater or less tendency to remain
attached. In some varieties this tendency is extremely
slight, in others it is marked. This union may appear
simply as an aggregation of separate bacteria or so close
that the group appears as a single cell. According to
the method of the cell division and the tenacity with
which the cells hold together, we get different group-
ings of bacteria, which aid us in their differentiation and
identification. Thus whether the bacterial cell divides
in one, two, or three planes, we get forms built in
one, two, or three dimensions. If we group bacteria
according to the characteristic form of the cells, and
then subdivide them according to the manner of their
division in reproduction and the tenacity with which
the newly developed cells cling to one another, we will
have the following varieties :
1. Spherical Form, or Coccus (Figs. 1 to 4). The size
varies from about Q.3/J. as minimum diameter to 3// as
maximum. The single elements are at the moment of
their complete development, so far as we can deter-
mine, absolutely spherical; but when seen in the process
of multiplication through division the form is seldom
that of a true sphere. Here we have elongated or lancet-
GENERAL CHARACTERISTICS OF BACTERIA. 37
shaped forms, as frequently seen in the diplococcus of
pneumonia, or the opposite, as in the diplococcus of
gonorrhoea, where the cocci appear to be flattened
against one another. Those cells which divide in one
Single coccus, grouped irregularly. Diplococcus of pneumonia, with sur-
Staphylococcus. rounding capsule.
direction only and remain attached are found in pairs
(diplococci) or in shorter or longer chains (streptococci).
Those which divide in two directions, the one at right
angles to the other, form bunches of four (tetrads).
Those which divide in three directions and cling to-
gether form packets in cubes (sarcinse). Those which
apparently divide irregularly in any axis form irregu-
larly shaped, grape-like bunches (staphylococci).
There are a considerable number of bacteria which
appear to frequently assume spherical forms, or at least
forms so like spheres that they cannot be differentiated
from them, and yet under other conditions they generate
rod-like forms. These apparently spherical bacteria
we can properly regard as short forms of bacilli, which,
owing to the rapidity of division, are for the time being
of the same size in both diameters. Under suitable
conditions, however, the true rod- shape is always de-
2. Rod Form, or Bacillus. The type of this group is
the cylinder. The length of the fully developed cell
is always longer than its breadth. The size of the
cells of different varieties varies enormously, from a
length of 30// and a breadth of 4// to a length of 0.2//
and a breadth of 0.1 //. The largest bacilli met with
in disease do not, however, average over 3/*. In des-
cribing their forms bacilli are roughly classed as slender
when the ratio of the long to the transverse diameter is
from 1 : 4 to 1 : 10, and as thick when the proportions
of the long to the short diameter is approximately 1 : 2.
The characteristic form of the bacillus is one with a
straight axis, uniform thickness throughout, and flat
ends (Fig. 13, page 47); but there are many exceptions
to this typical form. Thus frequently the motile bac-
teria have rounded ends (Fig. 10, page 43); many of
the more slender forms have the long axis bent; some
few species, such as the diphtheria bacilli (Fig. 5), in-
variably produce many cells whose thickness is very
GENERAL CHARACTERISTICS OF BACTERIA. 39
unequal at different portions. Spore formation also
causes an irregularity of the cell outline (Fig. 12).
The bacilli divide only in the plane perpendicular to
their long axis. A classification, therefore, of bacilli
i, single and in threads.
\& j3fc > * > 7 . \ /*V&~ OF T
$Sg\ ( UNWEF
Small bacilli, mostly in pairs.
according to their manner of grouping is much simpler
than in the case of the cocci. We may thus have bacilli
as isolated cells, as pairs, or as longer or shorter chains.
3. Spiral Form, or Spirillum. The members of the
third morphological group are spiral in shape, or rather
segments of a spiral. Here, too, we have large and
small, slender and thick spirals. The twisting of the
long axis, which here lies in two planes, is the chief
characteristic of this group of bacteria. Under normal
conditions the twisting is equal throughout the entire
length of the cell. The spirilla, like the bacilli, divide
only in one direction. A single cell, a pair, or the
union of two or more elements may thus present the
appearance of a short segment of a spiral or a comma-
shaped form, an S-shaped form, or a complete spiral
or corkscrew-like form.
Among uncommon morphological peculiarities in
true bacteria may be mentioned dichotomy, or branch
formation that is, a side growth projecting from the
bacterial cell. True dichotomous branching has occa-
sionally been observed in the bacilli viz., the bacilli
of tuberculosis, diphtheria, and glanders.
Summary of Morphological Forms of Bacteria. 1.
Coccus, or micrococctis. Spherical or subspherical
GENERAL CHARACTERISTICS OF BACTERIA. 41
a. Single coccus, grouped irregularly.
b. Diplococcus, forming pairs.
c. Streptococcus, forming chains, often showing
d. Tetracoccus, forming fours by division through
two planes of space.
e. Sarcina, forming packets of eight members by
division through three planes of space.
2. Bacillus. Oblong or cylindrical forms, having
one dimension greater than any other, more or less
straight, and never forming spirals.
a. Single bacillus.
b. Diplobacillus and streptobacillus, forming twos
or longer chains, the bacilli attached end to
c. Filaments, or thread-like growths, in which
divisions into bacilli of the normal length
are not apparent, or occur irregularly and
transversely, to the long axis of the growth.
3. Spirillum. Cylindrical and curved forms, con-
stituting complete spirals or portions of spirals.
The determination of morphological characters for
the description of bacteria should always be made from
fully developed cultures; those which are too young
may present, as already noted, immature forms, due to
rapid multiplication, while in old cultures altered or
degenerated forms may be observed.
When growth is obtained upon different media, varia-
tions, especially in size, may sometimes be observed.
These differences should always be described, together
with a note of the media upon which they were devel-
oped and a statement as to whether such variation is a
marked feature of the species under consideration.
The conditions of temperature and of nutrition which
favor growth are very various for different species, so
that no fixed temperature, medium, or age of growth
can be determined upon as applicable to all species.
Morphological descriptions should always be accom-
panied by a definite statement of the age of the growth,
the medium from which it was obtained, and the tem-
perature at which it was developed.
It is further advisable that the appearance observed
in growths developed upon a solid and in a liquid
medium should be recorded.
The structure of bacterial cells has recently attracted
considerable attention among naturalists. According to
Fischer and Migula, the bacterial cells consist of a cell-
membrane, a protoplasmic layer, and a central fluid; no
nucleus was observed by them. In salt solutions and
when dried upon a cover-glass a shrinkage of the pro-
toplasmic layer with partial dissolution of the cell-wall
occurs, due to the abstraction of water. This process
is known as plasmolysis, and it explains the occurrence
of the clear, unstained spaces so frequently seen in
the stained cover-glass preparations which have erro-
neously been taken for spores. In water, or by
the continued action of salt solution, this shrink-
age does not take place. In many species of bacteria,
such as the diphtheria bacilli, there is observed in the
interior of the cells, on suitable staining, a peculiar
granulation, to which Bab6s has given the name of
metachromatic bodies, but which Ernst on more careful
study has termed sporagenous granules.
With regard to the cell membrane, it should be
noticed that it is frequently not sharply defined and
often difficult to demonstrate. In many species of
GENERAL CHARACTERISTICS OF BACTERIA. 43
bacteria, however, commonly known as capsule bacteria,
as shown in Fig. 2, the cell membrane or the outer layers
of the membrane are so much thickened that the bacteria
seem to be surrounded by a gelatinous envelope or cap-
sule, which is distinguished by a diminished power of
staining with the ordinary aniline dyes. The demonstra-
tion of this capsule may be of help in differentiating
between certain bacteria e. g., some forms of the
streptococcus and pneumococcus. A peculiarity of
Faintly stained flagella attached to heavily stained bacilli.
the capsule bacteria is that, except very rarely, they
exhibit this envelope only when grown in the animal
body or in special' culture media, such as liquid blood
serum, bronchial mucus, etc. ; grown on nutrient
gelatin, agar, or potato the capsule is only visible under
very exceptional conditions, and then not distinctly.
The outer surface of bacteria when occurring in the
form of spheres and short rods is almost always smooth
and devoid of appendages ; but the longer rods and
spirals are usually provided with fine hair-like append-
ages or flagella, which are their organs of motility.
These flagella, either singly or in numbers, are some-
times distributed over the entire body of the cell, or
they may form a tuft at one end of the rod, or only
one polar flagellum is found. The polar flagella
appear on the bacteria shortly before division. The
nature of flagella is little understood; they are believed
by some to be formed of protoplasmic material which
penetrates the cell membrane, and probably have the
property of protrusion and retraction. So far as we
know, the flagella are the only means of locomotion
possessed by the bacteria. They are not readily
stained, special staining agents being required for
this purpose. The envelope of the bacteria, which
usually remains unstained with the ordinary dyes,
then becomes colored and more distinctly visible than
is commonly the case. Occasionally, however, some
portion of the envelope remains unstained, when the
flagella present the appearance of being detached from
the body of the bacteria by a narrow zone. Unfor-
tunately, many of the methods employed for staining
flagella cause them to become degenerated, so that their
perfect demonstration is often very difficult. In cul-
tures of richly flagellated bacteria peculiar pleated
masses sometimes are observed, consisting of flagella
which have been detached and then matted together.
Bacteria may lose their power of producing flagella for
a series of generations. Whether their power be per-
manently lost or not we do not know.
The vegetative reproduction of bacteria takes place by
division. When development is in progress a single
GENERAL CHARACTERISTICS OF BACTERIA. 45
cell will be seen to elongate, in the case of spherical
bacteria only slightly, and in the rod-shaped organ-
isms considerably in one direction. Over the centre
of the long axis thus formed will appear a slight in-
dentation in the outer envelope of the cell ; this inden-
tation increases in extent until there exists eventually
two individuals. As a rule, the cells separate from