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Page 29
The bacteria (Fig. 17) are unicellular organisms and vary greatly in
size, shape and capacity of growth. The smallest of the pathogenic or
disease-producing bacteria is the influenza bacillus, 1/51000 of an
inch in length and 1/102000 of an inch in thickness; and among the
largest is a bacillus causing an animal disease which is 1/2000 of an
inch in length and 1/25000 of an inch in diameter. Among the
free-living non-pathogenic forms much larger examples are found. In
shape bacteria are round, or rod-shaped, or spiral; the round forms
are called micrococci, the rod-shaped bacilli and the spiral forms are
called spirilli. A clearer idea of the size is possibly given by the
calculation that a drop of water would contain one billion micrococci
of the usual size. Their structure in a general way conforms with that
of other cells. On the outside is a cell membrane which encloses
cytoplasm and nucleus; the latter, however, is not in a single mass,
but the nuclear material is distributed through the cell. Many of the
bacteria have the power of motion, this being effected by small
hair-like appendages or flagell� which may be numerous, projecting
from all parts of the organisms or from one or both ends, the movement
being produced by rapid lashing of these hairs. A bacterium grows
until it attains the size of the species, when it divides by simple
cleavage at right angles to the long axis forming two individuals. In
some of the spherical forms division takes place alternately in two
planes, and not infrequently the single individuals adhere, forming
figures of long threads or chains or double forms. The rate of growth
varies with the species and with the environment, and under the best
conditions may be very rapid. A generation, that is, the interval
between divisions, has been seen to take place in twenty minutes. At
this rate of growth from a single cholera bacillus sixteen quadrillion
might arise in a single day. Such a rate of growth is extremely
improbable under either natural or artificial conditions, both from
lack of food and from the accumulation in the fluid of waste products
which check growth. Many species of bacteria in addition to this
simple mode of multiplication form spores which are in a way analogous
to the seeds of higher plants and are much more resistant than the
simple or vegetative forms; they endure boiling water and even higher
degrees of dry heat for a considerable time before they are destroyed.
When these spores are placed in conditions favorable for bacterial
life, the bacterial cells grow out from them and the usual mode of
multiplication continues. This capacity for spore formation is of
great importance, and until it was discovered by Cohn in 1876, many of
the conditions of disease and putrefaction could not be explained.
Spores, as the seeds of plants, often seem to be produced when the
conditions are unfavorable; the bacterium then changes into this form,
which under natural conditions is almost indestructible and awaits
better days.
The bacteria are divided into species, the classification being based
on their forms, on the mode of growth, the various substances which
they produce and their capacity for producing disease. The
differentiation of species in bacteria is based chiefly upon their
properties, there being too little difference in form and size to
distinguish species. The introduction of methods of culture was
followed by an immediate advance of our knowledge concerning them.
This method consists in the use of fluid and solid substances which
contain the necessary salts and other ingredients for their food, and
in or on which they are planted. The use of a solid or gelatinous
medium for growth has greatly facilitated the separation of single
species from a mixture of bacteria; a culture fluid containing
sufficient gelatine to render it solid when cooled is sown with the
bacteria to be tested by placing in it while warm and fluid, a small
portion of material containing the bacteria, and after being
thoroughly mixed the fluid is poured on a glass plate and allowed to
cool. The bacteria are in this way separated, and each by its growth
forms a single colony which can be further tested. It is self-evident
that all culture material must be sterilized by heat before using, and
in the manipulations care must be exercised to avoid contamination
from the air. The refraction index of the bacterial cell is so slight
that the microscopic study is facilitated or made possible by staining
them with various aniline dyes. Owing to differences in the cell
material the different species of bacteria show differences in the
facility with which they take the color and the tenacity with which
they retain it, and this also forms a means of species differentiation.
The interrelation of science is well shown in this, for it was the
discovery of the aniline dyes in the latter half of the nineteenth
century which made the fruitful study of bacteria possible.
From the simplicity of structure it is not improbable that the
bacteria are among the oldest forms of life, and all life has become
adapted to their presence. They are of universal distribution; they
play such an important part in the inter-relations of living things
that it is probable life could not continue without them, at least not
in the present way. They form important food for other unicellular
organisms which are important links in the chain; they are the agents
of decomposition, by which the complex substances of living things are
reduced to elementary substances and made available for use; without
them plant life would be impossible, for it is by their
instrumentality that material in the soil is so changed as to be
available as plant food; by their action many of the important foods
of man, often those especially delectable, are produced; they are
constantly with us on all the surfaces of the body; masses live on the
intestinal surfaces and the excrement is largely composed of bacteria.
It has been said that life would be impossible without bacteria, for
the accumulation of the carcasses of all animals which have died would
so encumber the earth as to prevent its use; but the folly of such
speculation is shown by the fact that animals would not have been
there without bacteria. It has been shown, however, that the presence
of bacteria in the intestine of the higher animals is not essential
for life. The coldest parts of the ocean are free from those forms
which live in the intestines, and fish and birds inhabiting these
regions have been found free from bacteria; it has also been found
possible to remove small animals from their mother by C�sarian section
and to rear them for a few weeks on sterilized food, showing that
digestion and nutrition may go on without bacteria.
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