52 BACTERIOLOGY. 



to destroy all living organisms which may be upon them. 

 With the use of steam, on the other hand, the objects to 

 be sterilized are frequently of such a nature that a 

 prolonged application of heat might materially injure 

 them. For this and other reasons steam is usually ap- 

 plied intermittently and for short periods of time. The 

 principles involved in this method of sterilization depend 

 upon differences of resistance toward heat which the 

 organisms to be destroyed are seen to possess at different 

 stages of their development. During the life cycle of 

 many of the bacilli there is a time in which the resist- 

 ance of the organism toward the action of both chemical 

 and thermal agents is much higher than at other stages 

 of their development. This increased power of resistance 

 appears when these organisms are in the spore or resting 

 stage, to which reference has already been made. When 

 in the vegetative or growing 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 appre- 

 ciably longer time, indeed spores of certain bacilli have 

 been encountered that retain their power of germinating 

 after an exposure of from five to six hours to the tem- 

 perature of boiling water. Such powers of resistance 

 have never been observed in the vegetating stage of 

 development. These differences in resistance toward 

 heat which the spore-forming organisms possess at 

 their different stages of development are taken advan- 

 tage of in that process of sterilization by steam known 

 as the fractional or intermittent method, and form the 

 principle on which the method is based. 



As the culture media to be sterilized are dependent 



