dia from the store-flasks are placed in glass test-tubes or small
flasks, protected from contamination by cotton-wool plugs, and are
sterilized by heat. For most purposes the solid media are to be preferred,
since bacterial growth appears as a discrete mass and accidental
contamination can be readily recognized. Cultures are made by transferring
by means of a sterile platinum wire a little of the material containing the
bacteria to the medium. The tubes, after being thus inoculated, are kept at
suitable temperatures, usually either at 37deg C., the temperature of the
body, or at about 20deg C., a warm summer temperature, until growth
appears. For maintaining a constant temperature incubators with regulating
apparatus are used. Subsequent cultures or, as they are called,
"subcultures," may be made by inoculating fresh tubes, and in this way
growth may be maintained often for an indefinite period. The simplest case
is that in which only one variety of bacterium is present, and a "pure
culture" may then be obtained at once. When, however, several species are
present together, means must be adopted for separating them. For this
purpose various methods have been devised, the most important being the
_plate-method_ of Koch. In this method the bacteria are distributed in a
gelatine or agar medium liquefied by heat, and the medium is then poured
out on sterile glass plates or in shallow glass dishes, and allowed to
solidify. Each bacterium capable of growth gives rise to a colony visible
to the naked eye, and if the colonies are sufficiently apart, an
inoculation can be made from any one to a tube of culture-medium and a pure
culture obtained. Of course, in applying the method means must be adopted
for suitably diluting the bacterial mixture. Another important method
consists in inoculating an animal with some fluid containing the various
bacteria. A pathogenic bacterium present may invade the body, and may be
obtained in pure culture from the internal organs. This method applies
especially to pathogenic bacteria whose growth on culture media is slow,
_e.g._ the tubercle bacillus.

The full description of a particular bacterium implies an account not only
of its microscopical characters, but also of its growth characters in
various culture media, its biological properties, and the effects produced
in animals by inoculation. To demonstrate readily its action on various
substances, certain media have been devised. For example, vari