CULTURAL TECHNIQUE 29 



Qualitative analysis of milk. It is frequently desirable 

 to determine the number of different kinds of bacteria in a 

 sample of milk, as, for instance,* acid-producing organisms, 

 organisms liquefying or not liquefying the gelatin. For these 

 purposes media must be used which will permit of the de- 

 velopment of the desired species. 



In order to determine the number of acid-producing and 

 liquefying organisms, litmus lactose gelatin may be used, pro- 

 ceeding exactly as in the case of the quantitative examination 

 of milk, except that to each tube of lactose gelatin, after melt- 

 ing, is added with a sterile pipette sufficient sterile litmus 

 solution to give the medium a decidedly blue color. The acid- 

 producing bacteria are easily differentiated on this medium 

 by their red color unless the plates are so thickly seeded with 

 acid colonies that the entire mass of the medium is changed. 

 For qualitative analysis thickly seeded plates are of little value. 



This medium also divides the bacteria which develop into 

 two classes, namely: (1) liquefiers, those which develop fer- 

 ments capable of permanently rendering the gelatin medium 

 liquid; (2) those unable to produce this change. The lique- 

 fiers are further differentiated into those which perform this 

 change very rapidly and those which produce a restricted 

 and often deepened pit of liquefied material closely surround- 

 ing the colony. 



The differentiation of the bacteria can be carried further 

 by a study of individual colonies. These vaiy in size and 

 appearance with different organisms, and the differences are 

 more or less constant. Especial use is made of these colony 

 characteristics in the detailed study and identification of the 

 various kinds of organisms. 



Exercise. Prepare litrnus-lactose-gelatin plates from the samples 

 of milk furnished. Incubate at 20 C. Count the number of acid 



