STERILIZATION 



87 



Sterilization at Temperatures Lower than Boiling-point. 



It is sometimes necessary to sterilize media, particularly blood- 

 serum, at temperatures lower than the boiling-point of water. 

 This is accomplished by placing the material to be sterilized in an 

 apparatus where it may be heated to the desired temperature, 

 usually 70-80 for one to two hours on each of five or more 

 successive days. If large numbers of spores of certain organisms, 

 such as Bacillus subtilis, are present, it is almost impossible to 

 sterilize efficiently by this method. However, if care is used in 

 securing the blood-serum to prevent the introduction of such organ- 

 isms, sterilization may be easily accomplished at this tempera- 

 ture. 



Sterilization by Addition of Chemicals. It is only under excep- 

 tional conditions that chemicals are used to sterilize media. 



Fig. 51. Apparatus for sterilization by filtration (McFarland). 



It has been found that the addition of soluble materials, such 

 as lactose, in considerable quantities to media containing pure 

 cultures of certain bacteria will destroy the organisms so that 

 they may be used as a vaccine. This method does away with 

 the destruction of any of the characteristic metabolic products 

 by heat. 



Sterilization by Filtration. Bacteria may be removed from a 

 liquid by passing it through a filter with pores so fine that the 

 organism cannot penetrate. Such filters are made up in a great 

 variety of shapes and densities. Among the many used are the 

 Berkefeld, the Pasteur, and the Chamberland. These are made of 

 unglazed porcelain. In filtration through these it is necessary, of 

 course, that all the apparatus used, particularly the vessel into 

 which the filtrate runs and the filter itself, be sterilized before use. 



