STERILIZATION BY HEAT. 69 



taken advantage of in the process of sterilization by 

 steam known as the discontinuous, fractional, or inter- 

 mittent method, and are the essential feature of the 

 principles on which the method is based. 



As culture-media are dependent for their usefulness 

 upon the presence of more or less unstable organic 

 compounds, the object aimed at in this method is to 

 destroy the organisms in the shortest time and with 

 the least amount of heat. It is accomplished by sub- 

 jecting them to the elevated temperature at a time 

 when the bacteria are in the vegetating or growing stage 

 i. e., the stage at which they are most susceptible to 

 detrimental influences. In order to accomplish this it is 

 necessary that there should exist conditions of tempera- 

 ture, 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 or- 

 ganisms are not only less likely to enter the spore-stage 

 than when their environments are 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 vege- 

 tative forms are destroyed ; while certain spores that 

 may be present resist this treatment, providing the 

 sterilization is not continued for too long a tinie. 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 



