15^ LABORATORY GUIDE IN BACTERIOLOGY 



set in an ice chest, one set in the locker, and the third 

 set in a thermostat at 37° C. 



3. Prepare plates in dextrose litmus agar from the 

 original three samples and incubate these at 37° C. 



4. Remove every day, with a sterile 5 c.c. pipette, 

 S c.c. of the milk from all samples, determine the 

 acidity by titration with 1/20 N'NaOH and phenol- 

 phthalein as indicator. When the acidity becomes 

 constant titrations may be omitted. 



5. Prepare plates from all samples every day for 

 one week, or until the numbers do not increase materi- 

 ally. It will be necessary to carry the dilutions up to 

 a milUon after a day or two of market milk, after three 



— or- four days of pasteurized milk, and toward the end of 

 the week of certified milk. 



6. Differential counts of acid-forming and non-acid- 

 forming bacteria should be made when this is possible. 



7. After the week has passed, allow the samples 

 to stand for two weeks longer and make titrations and 

 plates every three days until the acidity and the num- 

 ber of colonies are constant. 



The results of this study should be tabulated, as 

 they will illustrate the process of "natural souring" 

 of milk. 



If milk is kept at 37° C. for three weeks or longer the 

 acidity often reaches more than 2 per cent and even 

 up to 3 per cent. This is due to the activity of a group 

 of bacteria wholly different from the ordinary lactic 

 acid bacteria. These bacteria grow poorly on ordi- 

 nary media. Their presence may be demonstrated by 

 plating the sour milk in beerwort agar, or better, in 

 whey agar (see p. 36). 



