1 6 THE GENERAL CHARACTERS OF MICROORGANISMS. 
far more difficult to destroy spores. Many species of bacteria 
produce such spores (Fig. u, /) ; 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 temperature. 
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 temperature 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 100 F. When the temperature is above 100, most bacteria 
grow less rapidly than when it is a little lower, while at a slightly 
higher temperature they cease growing. A few species, however, 
grow best at unexpectedly high temperatures, some having been 
found flourishing at 140 or even higher. These peculiar bacteria 
are called thermophiles. How they can find conditions in nature 
warm enough for their growth is a question. 
The death temperature is a factor of great importance, since it is 
so closely associated with the matter of sterilization by heat. Most 
bacteria, when in an active condition, are killed by a temperature 
