280 PRACTICAL DAIRY BACTERIOLOGY 



to make a successful plate with 'gelatin, it is necessary to know 

 something of the age of the milk, in order that the proper dilu- 

 tion may be obtained. If milk is not more than 24 hours old, 

 and has been kept fairly cool, a dilution of 1,000 will ordinarily 

 be satisfactory; sometimes a higher dilution is necessary. This 

 difficulty of determining the amount of dilution is the difficult 

 point of the gelatin method. For this reason, when gelatin is 

 used for estimating bacteria in milk, it is desirable to make 

 several plates of different dilutions in order that the proper 

 one may be reached in one at least. 



Procure some milk from 12 to 24 hours old. Dilute it as 

 above described so that one vial has a dilution of 1,000, another 

 of 10,000, and another of 100,000. Add I c.c. of each dilution 

 to separate tubes of the melted gelatin, thoroughly mix and 

 pour into the Petri dishes, replacing the cover. It is best to 

 make at least two such Petri dishes from each solution. Place 

 at first in a cool place to harden. The plates must be kept in a 

 temperature of not much over 70 for the gelatin will melt. 

 For this reason it is difficult to use gelatin in summer. The 

 bacteria at 70 do not grow as rapidly as they do at 370, and the 

 colonies do not begin to appear, usually, for 2 days. The ap- 

 pearance of the colonies in the gelatin plates is much different 

 from that in agar plates, as will be noticed later. 



*No. 11. Litmus Gelatin Plates. This requires only a slight 

 modification of the process described in the last paragraph. 

 Dilute the milk as above. To several tubes of gelatin prepared 

 in 8, a, add 2 c.c. of the litmus solution prepared in 8, b to e. 

 Melt and mix thoroughly. Place in each tube i c.c. of the 

 diluted milk, then pour into the Petri dishes, after hardening, 

 keep at about 70. After the colonies have grown for two or 

 three days, those which produce acid will be found to be red, 

 or surrounded by a little red halo where the blue litmus has 

 been turned to red. It is, therefore, easy by examination to 

 determine the number of acid producing bacteria in such milk. 



