608 BACTERIOLOGY OF MILK 



the casein; in symbiosis with Bacillus coli not only are the protein 

 constituents of the milk thoroughly decomposed a large volume of 

 gas is formed as well and the milk-sugar is converted into carbon 

 dioxide, hydrogen and lactic acid. 1 The alkaline products of putre- 

 faction formed by Bacillus mesentericus neutralize, to a large degree, 

 the acid products formed by Bacillus coli and the net change in the 

 chemical composition of the milk is much greater than the sum of 

 their separate activities. 



Abnormal bacterial fermentations of milk are occasionally sources 

 of great trouble to dairymen. One of the more common of these is 

 known as ropy or shiny milk, in consequence of the viscidity which 

 develops. Several kinds of bacteria cause ropiness, but of these 

 Bacillus lactis viscosus appears to be more frequently concerned. A 

 bitter flavor may be imparted to milk either from the feed of the cow 

 or by the growth of bacteria. The latter is usually due to the partial 

 digestion of the milk proteins resulting in an accumulation of pep- 

 tones. The gas bacillus Bacillus aerogenes capsulatus produces 

 an energetic fermentation of milk-sugar and eventually a rather deep- 

 seated digestion of the casein if its activity is not restricted. The spores 

 of the organism are very resistant to physical agents and are often 

 found in commercial grades of lactose, which is prepared from milk. 

 There is evidence that this organism, transmitted through milk, may 

 incite mild or severe diarrhea in children, less frequently in adults. 

 Pasteurized milk, particularly that originating in unclean dairies, 

 occasionally contains considerable numbers of gas bacilli and the 

 absence of lactic-acid-forming bacteria in such milk (which normally 

 restrain their activity) may be a factor in its ability to develop rapidly. 



Proteolytic bacteria, particularly spore-forming varieties of the 

 Subtilis-Mesentericus Group, decompose milk proteins with the forma- 

 tion of casein peptones or even polypeptids. They occasionally mul- 

 tiply rapidly in pasteurized milk, when the degree of heat applied has 

 been sufficient to kill the lactic-acid-producing bacteria; ordinarily 

 lactic acid restrains the growth of proteolytic bacteria. 



Pathogenic bacteria, as a rule, produce very little change in the 

 appearance of milk and the chemical composition also is not greatly 

 altered during their development. 2 Ordinarily it is impracticable to 

 search for pathogenic bacteria in this medium, for the chances of 

 success are minimal. 



1 Kendall, Boston Med. and Surg. Jour., 1910, clxiii, 322. 



^ Kendall, Day, and Walker, Jour. Am. Chem. Soc., 1914, xxxvi, 1937-1966. 



