STERILIZATION BY HEAT 79 



ing or growing stage i. e., the stage at which they are most 

 susceptible to detrimental influences. In order to accom- 

 plish this it is necessary that there should exist conditions 

 of temperature, nutrition, and moisture which favor the 

 vegetation of the bacilli and the germination of any spores 

 that may be present. When, as in freshly prepared nutrient 

 media, this combination is found, the spore-forming organ- 

 isms are not only less likely to enter the spore-stage than 

 when their environment is less favorable to their vegetation, 

 but spores which may already exist develop very quickly 

 into mature cells. 



It is plain, then, that with the first application of steam 

 to the substance to be sterilized the mature vegetative forms 

 are destroyed; while certain spores that may be present 

 resist this treatment, providing the sterilization is not con- 

 tinued for too long a time. If now the sterilization be 

 discontinued, and the material which presents conditions 

 favorable to the germination of the spores be allowed to 

 stand for a time, usually for about twenty-four hours, at 

 a temperature of from 20 to 22 C., those spores which 

 resisted the action of the steam will, in the course of this 

 interval, germinate into the less resistant vegetative cells. 

 A second short exposure to the steam kills these forms in 

 turn, and by a repetition of this process all bacteria that 

 were present may be destroyed without the application of 

 the steam having been of long duration at any time. It 

 should be remembered that while spores which may be 

 present are not directly killed by such an exposure to heat 

 as they experience in the intermittent method of sterili- 

 zation, still their power of germination is somewhat inhibited 

 by this treatment. In this method, therefore, if the tem- 

 perature of 100 C. be employed for too long a time, it is 



