BOUILLON, AGAR AND MILK. 233 



fore, is extremely useful in the study of pathogenic organ- 

 isms. Unlike gelatin, it is not liquefied by bacteria except 

 perhaps in a very few rare cases, and hence, it can not 

 supplant gelatin in the study and differentiation of bac~ 

 teria. 



In addition to the solid media, it is desirable to have a 

 liquid medium. The beef-tea or bouillon, prepared accord- 

 ing to the following directions, is extremely useful for this 

 purpose. Indeed, before the introduction of solid media, it 

 was the chief medium employed. The growths in beef -tea 

 are frequently very characteristic, and materially assist in 

 the recognition of various bacteria. 



The meat extract employed in the preparation of gela- 

 tin, agar and bouillon, is usually made out of beef. The 

 Pasteur school, almost invariably, use veal for the prepara- 

 tion of these media. The reason for this lies in the greater 

 amount of extractive substances contained in veal. For 

 ^special purposes, bouillon etc., is made out of chicken, 

 pork, or other flesh. The commercial meat extracts can 

 be employed for making bouillon and the other media 

 (see p. 243). 



Milk constitutes another important liquid medium. On 

 account of the presence of lactose and casein, milk is espe- 

 cially valuable in differentiating certain bacteria. Some 

 bacteria will decompose the milk-sugar and cause the evo- 

 lution of gas-bubbles, whereas others will not do so. Again, 

 some will cause coagulation, as is seen in sour milk, while 

 other bacteria produce no appreciable change. 



Consequently, before taking up the study of pathogenic 

 bacteria, the student will prepare, in addition to the gela- 

 tin, and potato media, already on hand, the necessary bouil- 

 lon, agar, and milk media (see p. 243). 



