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46                   PA THOGENIC BA CTERIA.

in distilled water to which the smallest amount of organic
matter has been added; others require so concentrated a
medium that only blood-serum can be used for their
cultivation. Sometimes a species with a preference for a
particular culture-medium can gradually be accustomed"
to another, though immediate transplantation causes the
death of the transplanted organism. Sometimes the ad-
dition of such substances as glucose and glycerin has a
peculiarly favorable influence upon bacteria, causing, for
example, the tubercle bacillus to grow upon agar-agar.

(e) Moisture.—A certain amount of water is always
necessary for the growth of bacteria. The amount can
be exceedingly small, however, so that the Bacillus pro-
digiosus is able to develop successfully upon crackers and
dried bread. Materials used as culture-media should not
be too concentrated; at least 80 per cent of water should
be present. Most bacteria grow best in liquid media;
that is, they form the longest threads, and diffuse them-
selves throughout the liquid so as to be present in far
greater numbers than when on solid media.

The statement that certain forms of bacteria can flour-
ish in clean distilled water seems to be untrue. When
transferred to such a medium the organisms soon die and
undergo a granular degeneration of their substance. If,
however, in their introduction a good-sized drop of cul-
ture-material is carried with them, the distilled water
ceases to be such, and becomes a dilute bouillon fitted to
support life for a time.

(ff) Reaction.—Should the pabulum supplied to bacte-
ria contain an excess of either alkali or acid, the growth
of the organisms is inhibited. Most true bacteria grow
best in a neutral or feebly alkaline medium. There are
exceptions to this rule, for the Bacillus butyricus and the
Sarcina ventriculi can grow well in strong acids, and the
Micrococcus urea can tolerate excessive alkalinity. Acid
media are excellent for the cultivation of moulds.

(e) Light.—Most species of bacteria are not influenced
in their growth by the presence or absence of light. The