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48                  PATHOGENIC BACTERIA.

Rarely, the presence of one species of microorganism
entirely eradicates another species. Hankin found that
the Micrococcus Ghadialli destroyed the typhoid and colon
bacilli, and suggested the use of this coccus to purify
waters polluted with typhoid.1

(z) Temperattire.—The question of temperature is of
importance from its bearing upon sterilization. Accord-
ing to Frankel, bacteria will scarcely grow at all below
16° and above 40° C.

• The researches of Fliigge show that the Bacillus sub-
tilis will grow very slowly at 6° C., and as the tempera-
ture is elevated it is said that until 12.5° C. is reached
fission does not occur oftener than every four or five
hours. When 25° C. is reached the fission occurs every
three-quarters of an hour, and at 30° C. about every half

Most bacteria die at a higher temperature than 60-
75° C. The spores can resist boiling water, but are
killed by dry heat if exposed to 150° C. for an hour or to
175° C. for five to ten minutes. Freezing kills many, but
not all bacteria, but does not affect the spores at all.

Most bacteria grow best at the ordinary temperature of
a comfortably heated room, and are not affected by its
occasional slight changes. Some, chiefly the pathogenic
forms, are not cultivable except at the temperature of
the animal body (37° C.); others, like the tubercle bacil-
lus, grow best at a temperature a little above that of the
body—40° C.

Variations in the amount of oxygen, temperature, moist-
ure, etc., beyond what have been described, are prej-
udicial to the growth and development of bacteria, first
inhibiting their growth, thus tending toward their de-
struction. In the practical application of our knowledge
of the biology of the bacteria we constantly make use of
such precautions as removing from surgical dressings,
sponges, etc., every substance that can possibly afford
nutriment to bacteria, and heating such materials, as well

1 Brit. Med. Jour., Aug. 14, 1897, p. 418.