It may be accomplished in a number of
ways, but most often is done by the use of heat. For culture
material, which is always organic in character, moist heat is
employed. The various culture media, in appropriate containers, are
subjected to a thorough steaming in a steam cooker. This destroys
all of the vegetating cells but not the resistant spores that may be
present. The media are then stored, for twenty-four hours, at
temperatures favorable for the germination of the spores and are
then again heated. Three such applications on successive days are
usually sufficient to free the media from all living germs, since
between the heating periods the spores germinate and the resulting
vegetative cells are more easily destroyed. The sterile media will
keep for an indefinite period in a moist place.

The media are usually placed in glass containers which may be
sterilized before use by heating them in an oven, it being possible
to thus secure a much higher temperature than with streaming steam.
All glass or metal articles may be sterilized by the use of dry heat
but for organic media, to avoid burning, moist heat must be used.

All kinds of materials may be sterilized by treatment with steam
under pressure. An exposure for a few moments at 250 deg. F., a
temperature attained with 15 pounds steam pressure, will destroy all
kinds of bacteria and their spores. This method of sterilization is
used in the canning of meats and vegetables and in the preparation
of evaporated milk. To avoid contamination of the media after
sterilization, the flasks and tubes are, after being filled,
stoppered with plugs of cotton-wool, which effectually filter out
all bacteria and mold spores from the air, and yet allow the air to
pass freely in and out of the containers.

=Methods of determining the number of bacteria.= The method of
determining the number and kinds of bacteria in any substance can be
illustrated by the process as applied to milk. For this purpose the
method of procedure is as follows: Sterile gelatin in glass tubes is
melted and then cooled until it is barely warm. To this melted
gelatin a definite quantity of milk is added. The medium is gently
shaken, so as to thoroughly mix the milk and gelatine, and the
mixture then poured into a sterile, flat, glass dish, and quickly
covered, where it is allowed to cool until the gelatin hardens.
After the culture plate has been left for twenty-four to thirty-six
hours at the proper tempe