PHYSIOLOGY OF MICROORGANISMS 51 



of the animal body. The great majority of microorganisms 

 have their optimum-growth temperature between 20 and 

 40 C. (68-104 P.). The term nxsophilic is applied to 

 ihe>e. For otliers the optimum is lower; these are called 

 pgychrophUiCf while those that grow at high temperatures 

 are called thernwphilic. The minimum temperature for the 

 l^\ ehrophilic organisms is usually below C. (32 F.). 

 The maximum temperature for the thermophilic may reach 

 70 C. (158 F.), a temperature at which most vegetative 

 bacteria are quickly destroyed. It is thus seen that the 

 temperature zone in which the growth of bacteria is pos- 

 sible is an extremely wide one, extending from below the 

 fn-f/iu.ir-point of water to what are ordinarily termed scald- 

 ing temperatures. No one organism is able to grow through 

 this entire range. Organisms that are able to grow at high 

 and others that are able to grow at low temperatures are 

 common in soil and in the feces of animals. 



Organisms can grow below the freezing-point of water 

 only when the freezing-point is depressed by the presence 

 of soluble material in the water of the food. 



The temperature zone of existence is far wider than that 

 of growth. Freezing does not normally destroy either vege- 

 tative bacteria or the spores. If the organisms are kept in 

 a frozen condition for long periods, they gradually die. 

 The death rate is so slow that food removed from cold stor- 

 age may spoil as quickly as it would have done at the same 

 temperature before being frozen. Indeed, the spoiling is 

 often more rapid in certain foods that are so changed by 

 freezing as to adapt them better for the invasion and growth 

 of bacteria therein. Repeated freezing and thawing is 

 more harmful than being kept continuously in a frozen con- 

 dition. Still lower temperatures do not seem to have much 

 additional effect. 



As the temperature is increased above the maximum for 



