24 



Bacteria in Relation to Country Life 



non-formation of spores is utilized in our methods of 

 sterilization in the laboratory, the dairy, and the can- 

 ning industries. A distinction 

 is drawn between pasteuriza- 

 tion, in which the tempera^ 

 ture is raised sufficiently high 

 to kill all of the bacterial cells 

 except the spores, and steriliza- 

 tion, in which the temperature 

 is considerably higher, — enough 

 to insure the destruction of all 

 the bacterial cells. In the case 

 of milk, pasteurization is ad- 

 equate for the complete elimi- 

 nation of the disease germs 

 enumerated above. 



Intermittent sterilization. — 

 The method of intermittent 

 sterilization is employed in 

 bacteriological laboratories. It 

 consists in heating the material 

 to be sterilized for one-half 

 hour at a time on three succes- 

 sive days, at the temperature of boiling water. The 

 first heating destroys' all of the cells except the spores. 

 The material is then allowed to cool, the spores ger- 

 minate, and the new vegetative cells thus formed are 

 destroyed by the second boiling on the following day. 

 If any of the organisms escape the second heating, 

 they are destroyed on the third day. Intermittent 

 sterilization is not, however, always effective in practice. 



Fig. 9. Autoclave. 



