1 6 THE GENERAL CHARACTERS OF MICROORGANISMS. 



far more difficult to destroy spores. Many species of bacteria 

 produce such spores (Fig. n, /) ; 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 



