STERILIZATION 45 



The most widely applicable and efficient physical agent 

 for sterilization is heat. A certain amount of 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 suitable 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' exposure. For example, the typhoid bacillus dies 



1 



FIG. 19. Arnold steam sterilizer. 



when heated to 56 C. or 133 F. for ten minutes, and the 

 pneumonia coccus at 52 C. or 126 F. for ten minutes. The 

 tubercle bacillus is much more resistant and requires from 

 ten to twenty minutes' exposure at 70 C. or 158 F., varying 

 directly with the density of the material in which it happens 

 to be exposed. The spore-forming organisms are character- 

 ized by a vastly greater resistance. This is due to the 

 peculiar property of spores of resisting deleterious agencies. 

 Low temperatures are much less destructive than high 



