18 Dairy Bacteriology. 



forms of organic life are thoroughly eliminated. Germ- 

 free culture material is kept in sterilized glass tubes and 

 flasks, and is protected from outside infection by plugs 

 of sterile cotton. Material thus prepared, if protected 

 from evaporation, will keep indefinitely, as the cotton 

 acts as an effectual filter against the passage of any par- 

 ticles of matter. 



26. Culture media. For culture media, many differ- 

 ent substances are employed. In fact, bacteria will grow 

 on almost any organic substance whether it is solid or 

 fluid, provided the essential conditions of growth are fur- 

 nished. The food substances that are used for culture 

 purposes are divided into two classes; solids and liquids. 



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. These last are of 

 utmost importance in bacteriological research, for their 

 use, which was introduced b(y Koch, permits the separa- 

 tion of the different forms that may happen to be in any 

 mixture. Gelatin is used advantageously because the 

 majority of bacteria present wider differences in their ap- 

 pearance upon this medium than upon any other. It re- 

 mains solid at ordinary temperatures, becoming liquid in 

 the neighborhood of 70 F. Agar, a gelatinous product 

 derived from a Japanese sea- weed, has a much higher 

 melting point, and can be successfully used, especially 

 with those organisms whose optimum growth point is 

 above the melting point of gelatin. 



Besides these solid media, different liquid substances 

 are extensively used, such as beef broth, milk, and in- 

 fusions 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 solution may be added in order to detect any 

 change in its chemical reaction due to the bacteria. 



