ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 55 
EXERCISE 81 
STUDY OF CASEIN DIGESTING BACTERIA 
Many bacteria are proteolytic in their metabolism rather than fermentative. 
Such organisms when grown in milk will peptonize or digest the casein instead 
of fermenting the lactose with the production of acid. Many different species of 
bacteria having this property are always present in milk but under normal con- 
ditions their activities are inhibited by the lactic acid bacteria. Hence, milk usu- 
ally sours instead of undergoing a proteolytic decomposition. 
From cultures of Bacillus subtilis, Pseudomonas pyocyaneus and Proteus vul- 
garis inoculate: (1) plain agar slopes, (2) gelatin stabs, (3) Durham tubes of 
brom cresol purple lactose broth, (4) plain milk, and (5) brom cresol purple 
milk. 
After incubation make a study of their morphological and cultural charac- 
teristics. Which of these organisms produce spores? What was their action in 
the brom cresol purple lactose broth? Do they liquefy gelatin? From the results 
obtained with lactose broth and gelatin, what action would you expect these or- 
ganisms to have on milk? What were the results obtained with the cultures 
grown in plain milk and brom cresol purple milk? 
EXERCISE 82 
PASTEURIZATION OF MILK 
Obtain a sample of raw market milk and plate it out on lactose agar using 
dilutions of 1/10,000 and 1/100,000. 
Pasteurize the milk by heating in water to 62.8° C. (145° F.) for 30 minutes. 
Plate out the pasteurized milk using dilutions of 1/100 and 1/1,000. Incubate all 
the plates at 37° C. for 48 hours. 
Hold tubes of pasteurized and raw milks at laboratory temperature. Ob- 
serve, the length of time each sample remains sweet, and the type of fermenta- 
tion which occurs in each. From the results obtained, is it your opinion that all 
of the lactic acid producing bacteria are destroyed at the temperatures used in 
pasteurization? Of what significance is this fact? 
