7 8 METHODS USED IN THE STUDY OF BACTERIA 



Sterilization by Physical Argents 



Filtration. It is possible to sterilize fluids by means of 

 nitration. By this means the bacteria are actually removed. 

 Practically the only means of producing a germ-free filtrate 

 is by means of unglazed porcelain. This has been used espe- 

 cially where it is desired to sterilize substances that would be 

 changed when subjected to the action of heat or chemicals. 

 Filtration is used largely to free the bacteria from their soluble 

 products of growth, e.g. their toxins. There are two systems 

 of porcelain filters, one is the Pasteur-Chamberlain, the other 

 the Berkefeld system. The Pasteur system is the most 

 reliable, but it is very slow. The Berkefeld is much more 

 rapid, but it is, consequently, not so reliable. With either 

 system great care is necessary with their use and frequent 

 sterilization of them is necessary since the bacteria frequently 

 grow through their walls after they have been in use for a 

 short time. 



Sunlight has good disinfecting or sterilizing properties 

 which were first called attention to by Downs and Blunt in 

 1877, and since then many experiments have been performed. 

 While it has the power of destroying bacteria when they are 

 exposed to it for a few minutes or a few hours, its power 

 of destroying them cannot be depended upon, and it is of 

 no practical importance in this connection. 



Cold. Cold is ordinarily thought to be very effective in 

 destroying bacteria, but in reality it is of very little value. 



