METHODS OF EXAMINING MILK 275 



EXAMINATION FOR COLI 



The presumptive tests for Bacillus coli do not have 

 the same value in milk examination as in water analysis, 

 because it is practically impossible to obtain milk from 

 the cow, even under good dairy conditions, without it 

 being infected to some extent with colon bacilli from the 

 intestines of the animal. The detection of this organism 

 in milk is therefore of little value, except that when it is 

 present in large numbers it is an evidence of uncleanli- 

 ness and usually an indication of gross fecal contamina- 

 tion. 



One of the presumptive tests for Bacillus coli in milk 

 approved by the Laboratory Section of the American 

 Public Health Association is made as follows: "1 c.c. of 

 a 1 to 1,000 dilution of milk is placed in ordinary bile 

 containing 1 per cent, lactose in a fermentation tube and 

 allowed to stand at 37 C. for 72 hours, at the end of 

 which time, if there is more than 15 per cent, gas, plates 

 are made, colonies isolated and run through species tests." 



The fermentation test described on page 278 is a 

 simple and convenient method of detecting the presence 

 of an excessive number of coli or other gas-forming or- 

 ganisms in milk. 



EXAMINATION FOR TUBERCLE BACILLI 



The method recommended by the Laboratory Section 

 of the American Public Health Association is as follows : 



"Pint or quart samples of milk should be obtained, 

 kept well iced, and delivered to the laboratory as soon as 

 possible. The milk and cream should be well mixed by 

 shaking vigorously. 50 c.c. of the mixed milk are then 

 transferred to a large centrifuge flask and 100 c.c. of 

 sterile water added. Centrifuge for one hour at 2,000 



