CHAPTER VII.

CULTURES, AND THEIR STUDY.

THE objects which we have had before us in the prep-
aration of the culture-media were numerous. We have
prepared them so as to allow us to separate—or, rather,
to isolate-—bacteria, to keep them in healthy growth for
considerable lengths of time, to enable us to observe their
biologic peculiarities, and to introduce them without dif-
ficulty into the bodies of animals.

The isolation of bacteria was impossible until the fluid
culture-media of the early observers were replaced by the
solid media, and was exceedingly crude until Koch gave
us the solid, transparent media and the well-known
<c plate-cultures."

A growth of artificially-planted micro-organisms in
which an immense number are massed together is called
a ctilture. If such a growth contains but one kind of
organism, it is known as a pure culture.

It has become the habit at present to use the term c c cul-
ture n rather loosely, so that it does not always signify a
growth of micro-organisms artificially planted, but may
signify a growth taking place under natural conditions;
thus, typhoid bacilli are said to exist in the spleens of
patients dead of that disease "in pure culture," because
no other bacteria are there ; and sometimes, when in ex-
pectorated fragments of cheesy matter from tuberculosis
pulmonalis the tubercle bacilli are very numerous and
unmixed with other bacteria, the term "pure culture''
is again used to describe the condition.

Three principal methods are at present employed to
enable us to secure pure cultures of bacteria, biit before
beginning a description of them it is well to observe that

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