he
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.