THE BACTERIOLOGIC EXAMINATION OF MILK 427 



It is clear that a test for Bacillus coli in milk requires a thorough 

 knowledge of conditions for proper interpretation. It cannot, at 

 the present stage of our knowledge, serve as a measure of manurial 

 pollution, but does indicate that the milk has probably been kept 

 at a temperature high enough to permit growth. And since it 

 appears that B. coli is really of manurial origin, while B. aerogenes 

 comes chiefly from a different source, such tests must remain of 

 doubtful value as long as a rapid method of distinguishing the 

 two types is not known. And furthermore, while high counts 

 of bacteria of the coli-aerogenes group are due to growth, 

 initial pollution is largely due to poorly cared for vessels in 

 which growth has taken place before the bacteria have entered 

 the milk. 



The presumptive colon test has been thought to give proof 

 whether milk has been properly pasteurized, on the assumption 

 that the bacillus is destroyed by pasteurization temperature. 

 This subject has been studied by Ayers and Johnson, who have 

 stated that at 60 C. (140 F.) for thirty minutes 54.59 per cent, 

 of the test cultures survived; at 62.8 C. (145 F.) 6.89 per cent, 

 survived, and at 65.6 C. (150 F.) one culture survived on the 

 first heating, but in repeated experiments was always destroyed. 

 This work shows that the finding of colon bacilli in pasteurized 

 milk, even in relatively large numbers due to growth, is not a 

 reliable test for efficient pasteurization. Furthermore, in an- 

 other publication the same authors have shown that some anaero- 

 bic bacteria occurring in milk produce gas in the closed arm of 

 the fermentation tube. This might become a serious source of 

 error when gas formation is the only criterion for determining 

 the presence of Bacillus coli. 



Anaerobes are constantly present in milk either in the vege- 

 tative form or as spores, or in both forms. Recently Weinzirl 

 and Veldee have worked out a method of numerical determina- 

 tion of Bacillus sporogenes in milk. The object of the test is a 

 quantitative estimate of manurial pollution and is made by plac- 

 ing 10 c.c. of the milk in one tube and suitable dilutions in other 

 tubes, covering the milk with melted paraffin and heating to 80 C. 

 for ten minutes. Only spores will survive this treatment. The tubes 

 are incubated and the presence of Bacillus sporogenes is indicated 

 by gas formation which pushes the paraffin plug up. If proteus 

 is present there will be digestion of the casein, but no gas. The 

 real value of the test must be determined by comprehensive 

 investigations. As a matter of fact, the same argument that 

 holds in the interpretation of total counts and colon bacilli 

 holds in that of anaerobes. It is probably true that strict an- 

 aerobes do not multiply in milk under normal conditions, while 



