208 



Sterilization and Disinfection 



Sterilization by Filtration. Liquids that cannot be 

 subjected to heat without the loss of their most important 

 qualities may be sterilized by nitration i. e., by passing 

 them through unglazed porcelain or some other material 

 whose interstices are sufficiently fine to resist the passage 

 of bacteria. This method is largely employed for the 

 sterilization of the unstable bacterial toxins that are de- 

 stroyed by heat. Various substances have been used for 

 nitration, as diatomaceous earth (Berkefeld filters), stone, 



Fig. 42. Different types of bacteriologic filters : a, Kitasato ; b, 

 Berkefeld; c, Chamberland ; d, Reichel. 



sand, powdered glass, etc., but experimentation has shown 

 unglazed porcelain to be the only reliable filtering material 

 by which to remove bacteria. Even this material, whose 

 interstices are so small as to allow the liquid to pass through 

 with great slowness, is only certain in its action for a time, 

 for after it has been repeatedly used the bacteria seem 

 able to work their way through. To be certain of the efficacy 

 of any filter, the fluid first passed through must be tested 

 by cultivation methods to prove that all the bacteria have 

 been removed. The complicated Pasteur-Chamberland and 

 the simple Kitasato and Reichel filters are shown in figures 

 41 and 42. 



