ous
sugars--lactose, glucose, saccharose, &c.--are added to test the
fermentative action of the bacterium on these substances; litmus is added
to show changes in reaction, specially standardized media being used for
estimating such changes; peptone solution is commonly employed for testing
whether or not the bacterium forms indol; sterilized milk is used as a
culture medium to determine whether or not it is curdled by the growth.
Sometimes a bacterium can be readily recognized from one or two characters,
but not infrequently a whole series of tests must be made before the
species is determined. As our knowledge has advanced it has become
abundantly evident that the so-called pathogenic bacteria are not organisms
with special features, but that each is a member of a group of organisms
possessing closely allied characters. From the point of view of evolution
we may suppose that certain races of a group of bacteria have gradually
acquired the power of invading the tissues of the body and producing
disease. In the acquisition of pathogenic properties some of their original
characters have become changed, but in many instances this has taken place
only to a slight degree, and, furthermore, some of these changes are not of
a permanent character. It is to be noted that in the case of bacteria we
can only judge of organisms being of different species by the stability of
the characters which distinguish them, and numerous examples might be given
where their characters become modified by comparatively slight change in
their environment. The cultural as well as the microscopical [v.03 p.0173]
characters of a pathogenic organism may be closely similar to other
non-pathogenic members of the same group, and it thus comes to be a matter
of extreme difficulty in certain cases to state what criterion should be
used in differentiating varieties. The tests which are applied for this
purpose at present are chiefly of two kinds. In the first place, such
organisms may be differentiated by the chemical change produced by them in
various culture media, _e.g._ by their fermentative action on various
sugars, &c., though in this case such properties may become modified in the
course of time. And in the second place, the various serum reactions to be
described below have been called into requisition. It may be stated that
the introduction of a particular bacterium into the tissues of the body
leads to certain properties appearing in the serum, which