STERILIZATION BY HEAT. 53 



steam known aw the fractional or intermittent method, 

 and are the essential feature of the principles on which 

 the method is based. 



As the culture media to be sterilized 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 subjecting 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, these surroundings are found, the spore-forming 

 organisms 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 time. If 

 now the sterilization is discontinued, and the material 

 which presents conditions favorable to the germination 

 of the spores is allowed to stand for a time, usually 

 for about twenty-four hours, at a temperature of from 

 20°-30° 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 



