278 BACTERIOLOGICAL ANALYSIS OF MILK. 



Study of the Gelatin Plates. After the plates have grown 

 for about six days, they are to be carefully examined with a 

 lens, and if necessary with a low-power microscope. In this 

 study the following points are to be determined : 



1. The total number of bacteria. These are to be counted 

 by the ordinary methods. 



2. The number of acid-producing bacteria. These can 

 easily be detected from the fact that every acid colony will be 

 red, and commonly surrounded by a red halo where the acid 

 has turned the blue litmus. This detection of acid bacteria 

 is also possible upon agar. It must always be borne in mind 

 that an alkaline colony in the vicinity of an acid colony may 

 obscure the production of acid, and some of the lactic bac- 

 teria will not show signs of acidity if lying close to a colony 

 developing an alkaline reaction. It is for this reason that it 

 is necessary to have several dilutions, so that in one of them 

 the colonies may be far enough apart to have no such in- 

 fluence upon each other. 



3. The number of liquefying colonies is to be counted. 

 This is especially important, inasmuch as the liquefying 

 colonies commonly represent the putrefactive organisms, and 

 their relative abundance in milk is a matter of importance. 



4. The number of bacteria producing no reaction in the 

 gelatin, or an alkaline reaction, but which do not liquefy, are 

 to be counted. This is very easy to do, provided the plates 

 have been properly diluted and have grown sufficiently. 

 Where it is necessary to study the plates early because of the 

 abundance of liquefiers, the distinction between the acid bac- 

 teria and those producing no reaction is less sharp and not 

 always satisfactory. 



5. The number of surface-growing acid bacteria of the 

 aerobic type, see page 275. 



Calculation of Results. The method of calculating the re- 

 sults is as follows : 



