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MORPHOLOGY AXD BIOLOGY OF BACTERL4. 11

governing this principle, or to say just what degree of concen-
tration is best adapted to each. So much, however, is certain,
that whereas no degree of dilution protects culture media against
bacteria, a too large proportion of solid matter or a too small
proportion of water impedes, or entirely prevents, their develop-
ment.
The chief condition for the formation of spores appears to be
an at least partial exhaustion of the available nourishment in
the solution, Avhile a certain influence is also exerted by its tem-
perature and composition. The first condition is presented in
most perfect form by the necrotic tooth-pulp, a fact which, on
account of its great importance, will be referred to at more
length in a later chapter.

6. Influence of Various Coxditioxs ox the Growth op
Bacteria.
a. Action of Temperature.
Temperature naturally exerts an unlimited influence U[)on the
vegetation of bacteria. That temperature which permits the
most rapid increase (designated as the optimum) varies consid-
erably for the different kinds, for the most, however, ranging
between 25° and 40° C. Above 40° the development rapidly
diminishes, and ceases almost or entirely at 41° ; below 5°,
on the other hand, proliferation takes place very slowly, if at all.
Many bacteria will not grow at a temperature below 20° ; some
even not under 30° C.
The temperature most favorable for the majority of bacteria,
particularly for the pathogenic varieties, is 37° or 38° C. In this
respect, again, the oral cavity presents an excellent culture me-
dium.
b. Action of Oxygen.
The access of atmospheric air exerts special influence upon
the vegetation of bacteria. In accordance with this fact, we
distinguish three groups of bacteria. For the development of
the first, oxygen is absolutely necessary ; they are called aerobic.
Bacteria of the second group thrive better without oxygen, or