48 METHODS OF STUDYING MICROORGANISMS 



While this sufficiently indicates the uses of sterili- 

 zation for the preparation of food for bacteria, a few 

 words upon sterilization in general are necessary. 

 This term is usually reserved for the killing of bacteria 

 by means of heat, either dry or moist. For the killing 

 of bacteria by other means see Chapter V. 



The most widely applicable and efficient physical 

 agent for sterilization is heat. A certain amount of 



FIG. 19. Arnold steam sterilizer. 



heat is necessary for the life of bacteria, but there are 

 certain temperatures beyond which they cease to live. 

 While 38 C. or 98.5 F. is their optimum or most suit- 

 able temperature, they find it increasingly difficult to live 

 as the temperature rises to 50 C. or 122 F. Beginning 

 there and extending to 62 C. or 144 F. the commoner 

 pathogenic organisms are killed by ten minutes' expo- 

 sure. For example, the typhoid bacillus dies when 



