ng the oxygen which they need from organic
compounds such as sugar, etc., and are therefore able to live and grow
under conditions where the atmospheric air is excluded. These are known
as _anaerobic_. While some species grow strictly under one condition or
the other, and hence are _obligate_ aerobes or anaerobes, others possess
the ability of growing under either condition and are known as
_facultative_ or optional forms. The great majority of milk bacteria are
either obligate or facultative aerobes.

~Rate of growth.~ The rate of bacterial development is naturally very much
affected by external conditions, food supply and temperature exerting
the most influence. In the neighborhood of the freezing point but little
growth occurs. The rate increases with a rise in temperature until at
the _optimum_ point, which is generally near the blood heat or slightly
below (90 deg.-98 deg. F.), a single cell will form two cells in 20 to 30
minutes. If temperature rises much above blood heat rate of growth is
lessened and finally ceases. Under ideal conditions, rapidity of growth
is astounding, but this initially rapid rate of development cannot be
maintained indefinitely, for growth is soon limited by the accumulation
of by-products of cell activity. Thus, milk sours rapidly at ordinary
temperatures until the accumulation of acid checks its development.

~Detrimental effect of external conditions.~ Environmental influences of a
detrimental character are constantly at work on bacteria, tending to
repress their development or destroy them. These act much more readily
on the vegetating cell than on the more resistant spore. A thorough
knowledge of the effect of these antagonistic forces is essential, for
it is often by their means that undesirable bacteria may be killed out.

~Effect of cold.~ While it is true that chilling largely prevents
fermentative action, and actual freezing stops all growth processes,
still it does not follow that exposure to low temperatures will
effectually destroy the vitality of bacteria, even in the vegetative
condition. Numerous non-spore-bearing species remain alive in ice for a
prolonged period, and recent experiments with liquid air show that even
a temperature of -310 deg. F. for hours does not effectually kill all
exposed cells.

~Effect of heat.~ High temperatures, on the other hand, will destroy any
form of life, whether in the vegetative or latent stage. The temperature
at which the vitality of