The Bacteria (also known as Schizomycetes, and Fission-Fungi) are the
smallest known organisms, and form a parallel group to the Blue-green
Algæ, but separated from these Algæ by the absence of their colouring
material; chlorophyll is only found in a few Bacteria.
The various forms under which the vegetative condition of the Bacteria
appear, are termed as follows:
1. GLOBULAR FORMS, COCCI (Figs. 27, 30 _c_): spherical or ellipsoidal,
single cells, which, however, are usually loosely massed together and
generally termed “_Micrococci_.”
2. ROD-LIKE FORMS: more or less elongated bodies; the shorter forms
have been styled “_Bacterium_” (in the narrower sense of the word),
and the term “_Bacillus_” has been applied to longer forms which are
straight and cylindrical (Figs. 28, 29, 30 _E_).
[Illustration: FIG. 23.--_Spirillum sanguineum._ Four specimens.
One has two cilia at the same end, the sulphur grains are seen
internally.]
3. THREAD-LIKE FORMS: unbranched, long, round filaments, resembling
those of _Oscillaria_, are possessed by _Leptothrix_ (very thin,
non-granular filaments; Fig. 30 _A_, the small filaments) and
_Beggiatoa_ (thicker filaments, with strong, refractile grains or drops
of sulphur (Fig. 31); often self-motile). Branched filaments, with
false branching like many _Scytonemaceæ_, are found in _Cladothrix_
(Fig. 30 _B_, _G_).
4. SPIRAL FORMS: Rod-like or filamentous bodies, which more or less
strongly resemble a corkscrew with a spiral rising to the left. In
general these are termed _Spirilla_ (Fig. 23); very attenuated spirals,
_Vibriones_ (standing next to Fig. 30 _M_); if the filaments are
slender and flexible with a closely wound spiral, _Spirochætæ_ (Fig.
24).
5. The MERISMOPEDIUM-FORM, consisting of rounded cells arranged in
one plane, generally in groups of four, and produced by divisions
perpendicular to each other.
6. The SARCINA-FORM, consisting of roundish cells which are produced
by cellular division in all the three directions of space, united into
globular or ovoid masses (“parcels”) _e.g. Sarcina ventriculi_ (Figs.
25, 26).
[Illustration: FIG. 24.--_Spirochæte obermeieri_, in active
motion (_b_) and shortly before the termination of the fever
(_c_); a blood corpuscles.]
All Bacteria are unicellular. In the case of the micrococci this is
self-evident, but in the “rod,” “thread,” and “spiral” Bacteria, very
often numerous cells remain united together and their individual
elements can only be recognised by the use of special reagents.
[Illustration: FIG. 25.--_Sarcina ventriculi._ One surface only
is generally seen. Those cells which are drawn with double
contour are seen with the correct focus, and more distinctly than
those cells lying deeper drawn with single contour.]
[Illustration: FIG. 26.--_Sarcina minuta_: _a-d_ successive
stages of one individual (from 4–10 p.m.); _f_ an individual of
32 cells.]
The condition termed “Zooglœa,” which reminds us of _Nostoc_, is
produced by the cells becoming strongly mucilaginous. A number of
individuals in active division are found embedded in a mass of
mucilage, which either contains only one, or sometimes more, of the
above-named forms. The individuals may eventually swarm out and
continue their development in an isolated condition. Such mucilaginous
masses occur especially upon moist vegetables (potatoes, etc.), on the
surface of fluids with decaying raw or cooked materials, etc. The
mucilaginous envelope is thrown into folds when the Bacteria, with
their mucilaginous cell-walls, multiply so rapidly that there is no
more room on the surface of the fluid.
The cells of the Bacteria are constructed like other plant-cells in
so far as their diminutive size has allowed us to observe them. The
cell-wall only exceptionally shows the reactions of cellulose (in
_Sarcina_, _Leuconostoc_; also in a Vinegar-bacterium, _Bacterium
xylinum_); a mucilaginous external layer is always present. The body
of the cell mostly appears to be an uniform or finely granulated
protoplasm. Very few species (_e.g. Bacillus virens_) contain
chlorophyll; others are coloured red (purple sulphur Bacteria); the
majority are colourless. _Bacillus amylobacter_ shows a reaction
of a starch-like material when treated with iodine before the
spore-formation. Some Bacteria contain sulphur (see p. 37). The body,
which has been described as a _cell-nucleus_, is still of a doubtful
nature.
Artificial colourings with aniline dyes (especially methyl-violet,
gentian-violet, methylene-blue, fuchsin, Bismarck-brown and Vesuvin)
play an important part in the investigations of Bacteria.
MOVEMENT. Many Bacteria are self-motile; the long filaments of
_Beggiatoa_ exhibit movements resembling those of _Oscillaria_. In many
motile forms the presence of cilia or flagella has been proved by the
use of stains; many forms have one, others several cilia attached at
one or both ends (Fig. 23) or distributed irregularly over the whole
body; the cilia are apparently elongations of the mucilaginous covering
and not, as in the other Algæ of the protoplasm. In _Spirochæte_ the
movement is produced by the flexibility of the cell itself. Generally
speaking, the motion resembles that of swarm-cells (_i.e._ rotation
round the long axis and movement in irregular paths); but either end
has an equal power of proceeding forwards.
The swarming motion must not be confounded with the hopping
motion of the very minute particles under the microscope
(Brownian movement).
VEGETATIVE REPRODUCTION takes place by continued transverse division;
hence the name “Fission-Fungi” or “Fission-Algæ,” has been applied to
the Bacteria.
SPORES. The spores are probably developed in two ways. In the
ENDOSPOROUS species (Figs. 28, 29), the spore arises as a new cell
inside the mother-cell. The spores are strongly refractile, smaller
than the mother-cell, and may be compared to the aplanospores of other
Algæ. In addition to these there are the ARTHROSPOROUS species in
which the cells, just as in _Nostoc_ and other Blue-green Algæ, assume
the properties of spores without previously undergoing an endogenous
new construction, and are able to germinate and form new vegetative
generations (Fig. 27). The formation of spores very often commences
when the vegetative development begins to be restricted.
[Illustration: FIG. 27.--_Leuconostoc mesenterioides_: _a_
a zooglœa, natural size; _b_ cross section of zooglœa; _c_
filaments with spores; _d_ mature spores; _e-i_ successive stages
of germination; in _e_ portions of the ruptured spore-wall are
seen on the external side of the mucilaginous covering. (_b-i_
magnified 520.)]
The spores germinate as in _Nostoc_ by the bursting of the external
layer of the cell-wall, either by a transverse or longitudinal cleft,
but always in the same way, in the same species (Fig. 28, example of
transverse cleft).
DISTRIBUTION. Bacteria and their germs capable of development, are
found everywhere, in the air (dust), in surface water, and in the
superficial layers of the soil. The number varies very much in
accordance with the nature of the place, season, etc. They enter,
together with air and food, into healthy animals and occur always in
their alimentary tract.
GROWTH AND REPRODUCTION depend upon the conditions of temperature.
There is a certain minimum, optimum and maximum for each species; for
instance (in degrees Centigrade)--
Minim. Opt. Maxim.
_Bacillus subtilis_ + 6 c. 30 + 50
_B. anthracis_ 15 20–25 43
_Spirillum choleræ asiaticæ_ 8 37 40 (but grows
only feebly
if under
16°).
_Bacterium tuberculosis_ 28 37–38 42
[Illustration: FIG. 28.--_Bacillus megaterium_: _a_ outline of a
living, vegetative cell-rod; _b_ a living, motile, pair of rods;
_p_ a similar 4-celled rod after the effects of iodine alcohol;
_c_ a 5-celled rod in the first stages of spore-formation; _d-f_
successive stages of spore-formation in one and the same pair
of rods (in the course of an afternoon); _r_ a rod with mature
spores; _g^1–g^3_ three stages of a 5-celled rod, with spores
sown in nutritive solution; _h^1–h^2_, _i_, _k_, _l_ stages of
germination; _m_ a rod in the act of transverse division, grown
out from a spore which had been sown eight hours previously.
(After de Bary; _a_ mag. 250, the other figures 600 times).]
[Illustration: FIG. 29.--_Bacillus amylobacter._ Motile rods,
partly cylindrical and without spores, partly swollen into
various special shapes and with spore-formation in the swelling.
_s_ Mature spore, with thick mucilaginous envelope. (After de
Bary; mag. 600 times, with the exception of _s_, which is more
highly magnified.)]
The functions of life cease on a slight excess of the maximum or
minimum temperature, numbness setting in when either of these limits
is passed. _Crenothrix_-threads provided with mucilaginous envelopes
may, according to Zopf, sustain a temperature of-10°. Some Bacteria are
said to be able to resist the exposure to as low a temperature as-110°
for a short time. It is not known at what degree of cold the death of
the Bacteria occurs: the greatest degree of heat which the vegetative
cells can withstand is about the same as that for other vegetative
plant-cells, namely, about 50–60° C. Certain Bacteria, _e.g. B.
thermophilus_, grow and thrive vigorously at 70° C. Many spores, on the
contrary, are able to bear far higher temperatures (in several species
a temperature for some duration of above 100°, those of _Bacillus
subtilis_, for instance, can withstand for hours a temperature of 100°
in nutrient solutions; the spores remain capable of development after
exposure to a dry heat of 123° C.).
The _Desiccation_ of the air, if prolonged, kills many forms when in
the vegetative condition. The spores however can bear a much longer
period of dryness, some even several years.
OXYGEN. Some species cannot live without a supply of free oxygen
(_Aerobic_), _e.g._ the Vinegar-bacteria, the Hay-bacilli, the
Anthrax-bacilli, the Cholera-_Microspira_. Other species again thrive
vigorously without supply of free oxygen, and are even checked in their
development by the admission of air (_Anaerobic_), _e.g._ the butyric
acid Bacterium (_Clostridium butyricium_ = _Bacillus amylobacter_).
A distinction may be drawn between obligate and facultative aerobics
and obligate and facultative anaerobics. Several Bacteria, producing
fermentation, may grow without the aid of oxygen when they are living
in a solution in which they can produce fermentation; but, if this is
not the case, they can only grow when a supply of oxygen is available.
A great number of the pathogenic Bacteria belong to the facultative
anaerobics.
A luminous Bacterium (_Bacillus phosphorescens_) which in the presence
of a supply of oxygen gives a bluish-white light, has been found in
sea-water. Phosphorescent Bacteria have frequently been observed
upon decaying sea-fish, as well as on the flesh of other animals; by
transferring the Bacteria from cod fish to beef, etc., the latter may
be made luminous.
_Organic carbon compounds_ are indispensable for all Bacteria, (except,
as it appears, for the nitrifying organisms), as they can only obtain
the necessary supplies of _carbon_ from this source. The supplies
of _nitrogen_, which also they cannot do without, can be obtained
equally as well from organic compounds as from inorganic salts, such as
saltpetre or ammonia-compounds. The various “ash-constituents” are also
essential for their nourishment.
While Moulds and Yeast-Fungi grow best in an acid substratum, the
_Bacteria_, on the other hand, generally thrive _best_ in a _neutral_
or slightly _alkaline_ one.
In _sterilization_, _disinfection_, and _antisepsis_, means are
employed by which the Bacteria are killed, or checked in their
development, for instance, by heat (ignition, cooking, hot vapours,
hot air, etc.), or poisons (acids, corrosive sublimate). The process
of preserving articles of food, in which they are boiled and then
hermetically sealed, aims at destroying the Bacteria, or the spores of
those which already may be present in them, and excluding all others.
As the Bacteria are unable to assimilate carbon from the carbonic
acid of the air, but must obtain it from the carbon-compounds already
in existence in the organic world, they are either _saprophytes_
or _parasites_. Some are exclusively either the one or the other,
_obligate_ saprophytes or parasites. But there are transitional
forms among them, some of which are at ordinary times saprophytes,
but may, when occasion offers, complete their development wholly or
partly as parasites--_facultative parasites_; others are generally
parasitic, but may also pass certain stages of development as
saprophytes--_facultative saprophytes_.
All chlorophyll-free organisms act in a transforming and disturbing
manner on the organic compounds from which they obtain their
nourishment, and while they themselves grow and multiply, they produce,
each after its kind, compounds of a less degree of complexity, _i.e._
they produce _fermentation_, _putrefaction_, sometimes the formation of
_poisons_, and in living beings often _disease_.
Those organisms which produce fermentation are called _ferments_;
this word, however, is also employed for similar transformations in
purely chemical materials (inorganic ferments or enzymes). Many organic
(“living”) ferments, among which are Yeast-cells and Bacteria, give
off during their development certain inorganic and soluble ferments
(enzymes) which may produce other transformations without themselves
being changed. Different organisms may produce in the same substratum
different kinds of transformation; alcoholic fermentation may for
instance be produced by different species of Fungi, but in different
proportions, and the same species produces in different substrata,
different transformations (_e.g._ the Vinegar-bacteria oxydize diluted
alcohol to vinegar, and eventually to carbonic acid and water).
In the study of Bacteria it is absolutely necessary to sterilize
the vessels employed in cultivation, the apparatus, and nutrient
solutions, _i.e._ to free them from Bacteria germs and
also to preserve the cultures from the intrusion of any foreign
germs (“pure-cultures”). A firm, transparent, nutritive medium
is frequently employed. This may be prepared by adding to
the nutrient solutions (broth) either gelatine, or--when the
Bacteria are to be cultivated at blood-heat--serum of sheep’s or
calf’s blood, agar-agar or carragen; serum alone may in itself
serve as a nutrient medium. The so-called “plate-cultures” are
frequently employed, _i.e._ the germs are isolated by
shaking them with the melted liquid nutrient gelatine, which
is then spread on a glass plate and allowed to coagulate; when
later on the individual germs grow into colonies, these remain
separate in the solid substratum and it is easy to pursue
their further development. Similar plate-cultures may also be
cultivated in test-tubes and on microscopic slides. The slides
and glass plates must be placed in “moist chambers” free from
Bacteria. By sowing a few cells (if possible one) using a fine
platinum wire, pure cultures for further investigation may be
obtained.
In order to prove the relationship between pathogenic Bacteria
and certain diseases, the experimental production of pathogenic
Bacteria by the inoculation of Bacteria from pure cultures into
healthy animals, is very important.
It has not so far been possible to establish a _classification_
of the Bacteria, as the life-history of many species, has not yet
been sufficiently investigated.[7] The opinions of botanists are at
variance, in many cases, about the forms of growth of a particular
kind. Some species are pleomorphic (many-formed) while others possess
only one form.