.

Besides these solid media, different liquid substances are extensively
used, such as beef broth, milk, and infusions of various vegetable and
animal tissues. Skim-milk is of especial value in studying the milk
bacteria and may be used in its natural condition, or a few drops of
litmus solution may be added in order to detect any change in its
chemical reaction due to the bacteria.

[Illustration: FIG. 2. A gelatin plate culture showing appearance of
different organisms in a sample of milk. Each mass represents a
bacterial growth (colony) derived from a single cell. Different forms
react differently toward the gelatin, some liquefying the same, others
growing in a restricted mass. _a_, represents a colony of the ordinary
bread mold; _b_, a liquefying bacterium; _c_, and _d_, solid forms.]

~Methods of isolation.~ Suppose for instance one wishes to isolate the
different varieties of bacteria found in milk. The method of procedure
is as follows: Sterile gelatin in glass tubes is melted and cooled down
so as to be barely warm. To this gelatin which is germ-free a drop of
milk is added. The gelatin is then gently shaken so as to thoroughly
distribute the milk particles, and poured out into a sterile flat glass
dish and quickly covered. This is allowed to stand on a cool surface
until the gelatin hardens. After the culture plate has been left for
twenty-four to thirty-six hours at the proper temperature, tiny spots
will begin to appear on the surface, or in the depth of the culture
medium. These patches are called _colonies_ and are composed of an
almost infinite number of individual germs, the result of the continued
growth of a single organism that was in the drop of milk which was
firmly held in place when the gelatin solidified. The number of these
colonies represents approximately the number of germs that were present
in the milk drop. If the plate is not too thickly sown with these germs,
the colonies will continue to grow and increase in size, and as they do,
minute differences will begin to appear. These differences may be in the
color, the contour and the texture of the colony, or the manner in
which it acts toward gelatin. In order to make sure that the seeding in
not too copious so as to interfere with continued study, an
_attenuation_ is usually made. This consists in taking a drop of the
infected gelatin in the first tube, and transferring it to another tube
of sterile media. Usually this operation is repeat