BACTERIA OF MILK 85 



other bacteria cease to develop and the lactic-acid organisms 

 increase rapidly, causing the milk to become sour. 



Make a study of the characters of the organism on gelatin, 

 agar slope, litmus-lactose agar, milk, and litmus milk. Stain 

 with carbol-fuchsin and Gram's stam. 



Demonstrate the presence of acid-forming bacteria by means 

 of calcium-carbonate agar. Proceed as follows : 



1. Sterilize in the dry oven three Petri dishes and a test tube 

 half full of calcium carbonate. 



2. When cool introduce about 1/10 cc. of calcium carbonate 

 into each Petri dish. 



3. Melt three tubes of whey agar, and when cooled to 45° C 

 inoculate with a very dilute culture of Bact. lactis-acidi or with 

 a diluted sample of milk. 



4. Pour the liquid agar into the Petri dishes. The plates 

 should appear turbid, but not white, if the right amount of 

 calcium carbonate was added. 



5. Set the plates in the incubator at 37° C. As the acid-form- 

 ing colonies develop, they dissolve the calcium carbonate in their 

 vicinity and appear surrounded by a clear zone in the turbid agar. 



Exercise 122. Study of Bacillus lactis aerogenes 



, This organism forms acid and gas in milk. It is sometimes 

 the cause of " gassy " cheese and also injures the cheese by 

 producing undesirable flavors. 



Make a study of its cultural characters on agar slope, gelatin, 

 htmus-lactose agar, dextrose broth in fermentation tubes, and milk. 



Exercise 123. Study of Bacillus subtilis and the Type of 

 Fermentation it Produces 



It has already been shown in Exercise 120 that the spore- 

 forming organisms may survive the heat of pasteurization. 

 Although such milk may not turn sour, it generally spoils as 

 the result of the growth of spore-forming bacteria. In normal 

 souring of milk most of this class of bacteria are prevented 

 from growing by the lactic acid produced by other bacteria. 



