RELATION TO ENVIRONMENT 135 



are destroyed. The thermal death point will vary, depending, 

 among other things, upon the time the bacteria are sub- 

 jected to this temperature. The thermal death point is usu- 

 ally denned as the degree of heat required to kill the bacteria 

 when it is applied for ten minutes in a moist condition. The 

 effect of any given temperature upon a bacterium depends 

 upon its condition, that is, whether it is in a vegetative or 

 a latent stage. When the bacterium is in an actively growing 

 condition it is much more susceptible to external influences 

 than when it is in a latent condition, and attention has already 

 been called to the fact that in the spore state the bacteria are 

 unusually resistant. Another factor of a good deal of importance 

 is the kind of heat that is employed ; that is, whether it is moist 

 or dry. Moist heat is much more effective than dry heat at 

 the same temperature. 



According as bacteria are affected by heat they are divided 

 into three classes. Those which have a low optimum tem- 

 perature are known as the psychrophilic bacteria. These 

 grow at a temperature only a few degrees above zero. The 

 bacteria which have a medium optimum temperature are 

 known as the mesophilic bacteria and are more abundant than 

 any other class. This group may well be divided into two 

 subgroups : one in which the optimum temperature is about 

 that of a summer's day (25 to 30 C.) and includes particularly 

 those bacteria which live on dead organic matter, the sapro- 

 phytes; and the disease-producing bacteria, or parasites, which 

 have as their optimum temperature that of the body of their 



