46 STUDY AND IDENTIFICATION OF BACTERIA 



Liquefaction of gelatin is a very important means of differentiating. When a 

 room-temperature incubator is not at hand (20 to 22 C.), it is better to put the 

 inoculated gelatin tube in the body-temperature incubator, and from day to day test 

 the power of solidifying with ice-water. If the organism digests the gelatin (a 

 liquefier), the medium will remain fluid when placed in ice- water; if the organism 

 is a nonliquefier, the medium in the tube becomes solid. Of course we lose the 

 information to be obtained from the shape of the area of liquefaction. 



FIG. 10. Series of stab cultures in gelatin, showing modes of growth of different 

 species of bacteria. (Abbott.) 



For routine work the only sugar media used are the glucose and the 

 lactose bouillon. These are of the utmost importance in differentiating 

 organisms of the typhoid and colon group. Following Ford, these 

 intestinal bacteria have primarily been separated by their action on 

 litmus milk whether turning it pink or only slightly changing or not 

 changing at all the original color. 



Examine the colonies on Petri plate at first with the unaided eye, 

 then with a hand magnifying glass or low-power objective, using re- 



