52 BACTERIOLOGY. 



already been made. When in the vegetative or grow- 

 ing stage, most bacteria are killed in a short time by a 

 relatively low temperature, whereas, when conditions 

 have arisen which favor the production of spores the 

 spores are seen to be capable of resisting very much 

 higher temperatures for an appreciably longer time. 

 These differences in resistance toward heat which the 

 spore-forming organisms are seen to possess at their 

 different stages of development are taken advantage of 

 in that process of sterilization by steam known as the 

 fractional or intermitlent method, and form the principle 

 on which the method is based. 



As the culture media to be sterilized are of a more 

 or less unstable nature, being dependent for their value 

 upon the presence of more or less unstable organic com- 

 pounds, 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 they are 

 in the vegetating or growing stage — i. e., the stage at 

 which they are least resistant. In order to accomplish 

 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 pre- 

 pared 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 the 

 steam to the substance to be sterilized the mature vege- 

 tative forms of these organisms are destroyed, while cer- 



