68 MICROBIOLOGY 



grow through them.* Few if any of the other domestic fil- 

 ters are capable of removing bacteria from liquids. 



Fractional or discontinuous sterilization. It has been 

 stated that spores are not destroyed promptly in all cases 

 by the application of heat even at 100 C. It has also been 

 stated that heating a substance at a lower temperature for a 

 longer time will, in the absence of spores, sterilize, as well 

 as heating to a higher temperature for a shorter time. If 

 spores are presenter organisms more resistant to heat, it is 

 not sufficient to heat a substance once. Fractional steriliza- 

 tion consists in heating substances in a water bath or 

 steamer at a temperature of from 60 to 100C. according 

 to the requirements of the substance for from 10 to 30 min- 

 utes each day for three or more consecutive days. The rea- 

 son for this is that the first heating destroys all the bacteria 

 in the vegetative stage; by allowing the substance to stand 

 over night the spores of the more resistant bacteria w^ill begin 

 to germinate or multiply, when the second heating will de- 

 stroy them. The same holds for the third heating. 



The practical application of sterilization. There is 

 perhaps a no more important lesson for the laboratory 

 worker who is to become a practitioner of medicine to learn 

 in bacteriology than the one regarding sterilization. The 

 necessity of a knowledge of sterilization and the means by 

 which it can be produced has many practical bearings. 



1. In the laboratory. In bacteriological work it is of 

 the greatest importance that all culture media and instru- 

 ments used in making cultures should be sterile. In order 

 to have these sterile it is necessary to understand the meth- 

 ods by which this condition can be brought about without 

 injury to the various substances. Likewise, all jars and bot- 

 tles used for collecting material for a bacteriological exami- 

 nation should be sterile. If one understands that the prin- 

 ciple of sterilization consists simply in the application of 



* Smith and Moore. Centralbl. f. Bakteriology, Bd. XII (1892) 

 p. 628. 



