STERILIZATION BY HEAT 81 



mittently and for short periods of time. The principles 

 involved in the intermittent method of sterilization depend 

 upon differences of resistance to heat which the organisms 

 to be destroyed are known to possess at different stages 

 of their development. During the life cycle of many of the 

 bacilli there is a stage in which the resistance of the organism 

 to the action of both chemical and thermal agents is much 

 greater 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, under conditions which favor the 

 production of spores, the spores are seen to be capable of 

 resisting very much higher temperatures for an appreciably 

 longer time; indeed, spores of certain bacilli have been 

 encountered that retain the power of germinating after an 

 exposure of from five to six hours to the temperature of 

 boiling water. Such powers of resistance have never been 

 observed in the vegetative stage of development. These 

 differences in resistance to heat which the spore-forming 

 organisms possess at their different stages of development is 

 taken advantage of in the process of sterilization by steam 

 known as the discontinuous, fractional, or intermittent 

 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 organ- 

 isms 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 vegetat- 

 6 



