GENERAL CHARACTERS OF BACTERIA l7 
retain their vitality. They may be heated very much hotter than 
the active forms without injury; indeed, some of these spores may 
be in boiling water for many minutes—an hour or longer—without 
having their vitality destroyed, since, if the spores are subse- 
quently cooled, they are capable of germinating and growing into 
new bacteria. As a result of this it will follow that, while it is 
very easy to kill ordinary bacteria by heat, it is far more difficult 
to destroy spores. Many species of bacteria produce such spores 
(Fig. 11, f); others do not, and hence some are much more easily 
killed by heat than others. Milk, for example, contains many 
kinds of bacteria. By the simple boiling or, indeed, the heating 
of the milk to a temperature of 160°F., a vast majority of the 
bacteria are killed; but the few spores that may chance to be in 
the milk are not thus killed, and subsequently these will be able 
to develop. If milk contains spore-bearing bacteria, it cannot 
be sterilized by boiling; and, since it almost always does contain 
them, boiling is not sufficient to sterilize it. This phenomenon of 
the high resisting powers of spores must always be borne in mind 
in all problems of sterilizing. 
Relations to Conditions.—Temperature.—The rate of multipli- 
cation of bacteria, yeasts, and molds depends upon the tempera- 
ture. At freezing they do not grow at all. As the temperature 
rises above freezing they begin to multiply, and their rate of 
multiplication increases as the temperature rises, up to a certain 
point which is the optimum temperature. If the temperature 
rises still higher, the rate declines and finally growth stops. If 
heated still more, the organisms are killed. The lowest tempera- 
ture at which they will grow, the minimum temperature, varies 
with different species. Some will grow at a temperature only 
just above freezing, at 33°F., while, at the other extreme, some 
will not grow at temperatures lower than 120° to 140°F. The 
optimum temperature also varies. Some species grow best at 
moderately low temperatures, 60° or as low as 50°F., while others 
flourish best. at a temperature from 90° to roo F. When the 
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