THE BACTERIOLOGIC EXAMINATION OF MILK 425 



chains. When long chains of streptococci are observed the as- 

 sumption is sometimes made that a pathologic condition of the 

 udder is indicated. This important subject will be considered in 

 another connection. 



The presumptive Bacillus coli test has been successful in de- 

 tecting polluted drinking-water, and in the opinion of some 

 sanitarians the presence of B. coli in milk assumes similar im- 

 portance'. The test is carried out in the same manner as the pre- 

 sumptive test for water pollution. It must be confessed, how- 

 ever, that the presence of B. coli in milk and that of B. coli in 

 water receive different interpretations. When B. coli is present in 

 water it is, as a rule, derived from human excreta, although a 

 few cells may enter from another source. The presence of B. 

 coli in 1 c.c. or less of a surface water is the usually accepted stand- 

 ard, and is considered an indication of pollution with human 

 excreta. In such polluted water organisms causing intestinal dis- 

 eases in man, chiefly typhoid fever, may be present. In milk, 

 however, when B. coli is present and it is invariably present in 

 raw market milk it is usually derived from bovine feces. Cows 

 are not susceptible to typhoid fever and do not carry the typhoid 

 bacilli in their digestive tract, hence the presence of colon bacilli 

 in milk does not indicate the probability of infectiousness for man. 

 Since the majority of bacteria in milk are of manurial origin a 

 positive qualitative test for B. coli adds little to our knowledge. 



There is considerable evidence to the effect that market milk 

 is never entirely devoid of bacteria of the coli-aerogenes group. 

 Prescott found Bacillus coli in 77 per cent, of 118 samples of cer- 

 tified milk, and stated his conviction that if a sufficiently large 

 volume of certified milk were examined the organism could be 

 detected in all samples. Race substantially agrees with Pres- 

 cott. Avers and Clemmens found that commercial milk produced 

 under the best of conditions always contained bacteria of the 

 coli-aerogenes group. 



It is necessary to know the origin of coli-aerogenes bacteria in 

 milk if significance is to be attached to their presence. The 

 methods generally practised for their detection do not differentiate 

 the Bacillus coli and B. aerogenes types. In market milk the 

 two types are present in nearly equal proportion, according to 

 Rogers, Clark, and Evans. These authors state that in bovine 

 feces B. aerogenes is rare, while B. coli is the predominating type. 

 Ayers and Clemmens found only 4 cultures of B. aerogenes in 

 1160 cultures obtained from samples of cow excreta. It follows 

 that B. aerogenes does not represent fecal pollution in milk, no 

 matter how large the number present, although Ayers and Clem- 

 mens found that "as feces dries the proportion of B. aerogenes to 



