Methods of Studying Bacteria. 21 



ical research, for their use permits the separation of the 

 different forms of bacteria that may happen to be in 

 any mixture. Gelatin is advantageously used, because 

 the majority of bacteria present wider differences, due 

 to growth upon this medium, than upon any other. It 

 remains solid at ordinary temperatures, becoming liquid 

 at about 80 F. Agar, a gelatinous product derived 

 from a Japanese seaweed, has a much higher melting 

 point, and is used especially with those organisms whose 

 optimum temperature for growth is above the melting 

 point of gelatin. 



Besides these solid culture media, different liquid 

 substances are extensively used, such as beef broth, milk 

 and infusions of various vegetable and animal tissues. 

 Skim milk is of especial value in studying the milk bac- 

 teria, and may be used in its natural condition, or a few 

 drops of litmus solution may be added, in order to de- 

 tect any change in its chemical reaction due to the bac- 

 teria. 



Sterilization. The various ingredients that are used 

 in the preparation of culture media are not free from 

 micro-organisms, hence the media would soon spoil if 

 they were not destroyed, and the media subsequently 

 protected from contamination from the air, etc. The 

 process of rendering the media free from living micro- 

 organisms is known as sterilization. It may be accomp- 

 lished in a number of ways, but most often is done by 

 the use of heat. For culture material, which is always 

 organic in character, moist heat is employed. The var- 

 ious culture media, in appropriate containers, are sub- 

 jected to a thorough steaming in a steam cooker. This 

 destroys all of the vegetating cells but not the resistant 

 spores that may be present. The media are then stored, 



