IDENTIFICATION OF BACTERIA IN MILK 607 



garget are suspected as sources of infection in man, both human and 

 bovine types should be looked for." 



The most numerous of the saprophytic bacteria commonly found 

 in raw milk belong to the group of organisms which form lactic acid, 

 but no gas, from lactose. They are frequently referred to as lactic 

 acid bacteria, but this name is not wholly appropriate nor is it dis- 

 tinctive; many unlike organisms possess this property in common. 

 The best known and most widely distributed of these lactic acid 

 bacteria is a streptococcus, Streptococcus lacticus, 1 an organism which 

 is present not only in moderate numbers in the feces of the cow, but 

 also upon the udder and flanks of the animal as well if cleanliness is 

 not strictly observed. The initial infection of milk with Streptococcus 

 lacticus is usually not extensive, but milk appears to be a particularly 

 favorable medium for its development and even after a few hours 

 the organism may have increased greatly in numbers if the tempera- 

 ture conditions are favorable. The most noteworthy chemical change 

 associated with the growth of Streptococcus lacticus is a rapid accu- 

 mulation of acid, principally lactic acid, which soon results in an acid 

 coagulation of the casein. The degree of acidity is usually sufficient 

 to inhibit the development of proteolytic bacteria and also a majority 

 of pathogenic bacteria as well. Occasionally other types of fecal 

 bacteria may be isolated from milk. Of these Bacillus coli has 

 received much attention, chiefly through its constant association with 

 human as well as with bovine excrement. Papasotirin and Prescott 2 

 have isolated bacteria indistinguishable from Bacillus coli by cultural 

 methods from hay and dried grains and the organism is very frequently 

 present in flour, consequently the identification of it in milk does not 

 furnish conclusive evidence of contamination either from human or 

 bovine sources. Bacillus coli does not produce more than minimal 

 amounts of gas in milk, although its aerogenic activity in dextrose and 

 lactose broth is one of its noteworthy cultural cMaracters. It does, 

 however, form sufficient acid from lactose to cause an acid coagulation 

 of the casein. In this respect it does not differ markedly from other 

 lactic acid bacteria. Occasionally, in association with a strongly 

 proteolytic bacterium, as certain strains of Bacillus mesentericus, a 

 deep-seated change is brought about in milk by the combined action 

 of the two organisms. Bacillus mesentericus acting alone liquefies 



1 Kruse, Centralbl. f. Bakt., 1903, I. Abt., xxxiv, 737; Heinemann, Jour. Inf. Dis., 

 1906, iii, 173. 



2 Centralbl. f. Bakt., Ref., 1903, xxxiii, 279. 



