240 BACTERIOLOGY. 



milk; or, if that cannot be obtained, the milk should be 

 skimmed. In order to avoid fermentative changes, the 

 milk should be steamed for at least 15 minutes. It can 

 then be set aside in a cool place for the cream to rise. By 

 means of a 100 c.c. bulk pipette the thin milk can be drawn 

 off from the bottom and filled into tubes. 



Milk is used in order to ascertain whether an organism 

 can coagulate casein, and whether it can decompose the 

 lactose thus giving rise to gas bubbles, or to acid pro- 

 ducts. The first two changes can be observed in the ordi- 

 nary sterile milk. To detect the presence of acid products it 

 is necessary to color the milk with blue litmus. This can 

 be done by adding a concentrated litmus solution to the 

 milk before sterilization; or, by adding the sterile litmus so- 

 lution to the sterilized milk, by means of a drawn out 

 pipette (Fig. 61). 



Modified Media. 



The ordinary nutrient agar, bouillon, or gelatin, pre- 

 pared according to the directions heretofore given, may be 

 greatly improved as nutrient media for certain organisms 

 by the addition of glycerin, glucose, lactose, litmus, etc. 



Peptonless agar. — This is ordinary agar minus the pep- 

 ton. It is especially useful when it is desired to obtain 

 spores, as in the case of anthrax. 



Glycerin media. — Usually 5 percent, of glycerin is added 

 to bouillon, or to agar. In the case of Roux potato tubes 

 (Pig. 34 b, p. 184), the lower bulb may be filled with a 5 per 

 cent, aqueous solution of glycerin. The addition of glycerin 

 renders these media especially useful for the cultivation of 

 the tubercle bacillus. The streptococci, and the bacteria of 

 diphtheria, glanders and pneumonia likewise thrive exceed- 



