36 AGRICULTURAL BACTERIOLOGY 



ing liquid. The agar is not liquefied by any of the ordi- 

 nary bacteria; it serves simply as a solidifying material, 

 while gelatin may serve as food for many bacteria. Gelatin 

 is a protein ; agar, a carbohydrate. 



The introduction of gelatin in 1882, by Robert Koch, 

 made possible the great developments in bacteriology 1liat 

 took place in the last two decades of the nineteenth century. 



Sterilization of culture media. The media as prepared 

 will contain many microorganisms that were in the ingredi- 

 ents or the vessels used. These organisms must be de- 

 stroyed if the media are to be of any value. This is most 

 commonly accomplished by heating the media until they 

 are free from living forms. They are then said to be 

 sterile. The process is termed sterilization. The media 

 ordinarily will contain bacteria, yeasts, and molds, and the 

 spores of each. A short exposure to the boiling-point will 

 destroy all except the bacterial spores. These can be de- 

 stroyed with certainty only by prolonging the period of 

 exposure to 212 F. for a number of hours. This is not 

 practical, and therefore other methods must be used. 



The culture medium in appropriate containers is placed 

 in a steamer, so that the containers will be surrounded by 

 streaming steam. An exposure for fifteen minutes after the 

 temperature of the medium has reached that of the steam 

 will suffice to kill everything except the bacterial spores. 

 If the medium is now placed at ordinary room temperature, 

 the resistant spores will germinate within a few hours, and 

 the resulting vegetative cells may then be readily destroyed 

 by a second heating. In practice, three exposures are made 

 on successive days. The process is called intermittent 

 sterilization.. 



If the temperature of the steam can be raised, it will, of 

 course, have a more destructive effect on the spores. This 

 is done by placing the media in their containers in a steam- 



