14: Dairy Bacteriology. 



Solid media may be either permanently solid like pota- 

 toes, or they may retain their solid properties only at cer- 

 tain temperatures like gelatin or agar. The latter two are 

 of utmost importance in bacteriological research, for their 

 use, which was introduced by Koch, permits the separation 

 of the different forms that may happen to be in any mix- 

 ture. Gelatin is used advantageously because the majority 

 of bacteria present wider differences due to growth upon 

 this medium than upon any other. It remains solid at 

 ordinary temperatures, bacoming liquid at about 70 F. 

 Agar, a gelatinous product derived from a Japanese sea- 

 weed, has a much higher melting point, and can be suc- 

 cessfully used, especially with those organisms whose 

 optimum growth point is above the melting point of gel- 

 atin. 



Besides these solid 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 bacteria and may be 

 used in its natural condition, or a few drops of litmus solu- 

 tion may be added in order to detect any change in its 

 chemical reaction due to the bacteria. 



Methods of isolation. Suppose for instance one wishes 

 to isolate the different varieties of bacteria found in milk. 

 The method of procedure is as follows : Sterile gelatin in 

 glass tubes is melted and cooled down so as to be barely 

 warm. To this gelatin which is germ-free a drop of milk 

 is added. The gelatin is then gently shaken so as to thor- 

 oughly distribute the milk particles, and poured out into a 

 sterile flat glass dish and quickly covered. This is allowed 

 to stand on a cool surface until the gelatin hardens. After 

 the culture plate has been left for twenty-four to thirty- 



