THE MILK CONTRACTOR 275 



spore bearers or with B. troilii it assumes the power of growth under 

 aerobic conditions and its reactions overwhelm those of the bacteria 

 with which it grows. When the organisms are heated to 140F. St. 

 lacticus inhibits the reactions produced by B. welchii upon milk in 

 the presence of B. troilii and the aerobic spore bearers, unless these 

 spore bearers are present in vast numbers, when it fails to do so, as it does 

 when no heat is applied. The reactions observed in milk heated at 

 temperatures ranging from 140 to 185F. may be simulated in the pres- 

 ence of B. welchii by the growth of the aerobes commonly surviving 

 the temperatures considered after the application of the desired degree 

 of heat. 



H. R. Brown made a study of the anaerobic bacteria in the market 

 milk of Boston, and the results of his studies appear in the reports of the 

 Massachusetts State Board of Health. 



Since Fliigge's time painstaking investigations have been made to de- 

 termine the bacteria that survive pasteurization. Inquiry has been 

 focussed on the lactic bacteria, the coli, the streptococci and the spore 

 formers. 



Other Bacteria that Survive Pasteurization. Ayres and Johnson 

 working under laboratory conditions that precluded recontamination of 

 the milk, studied the bacteria that survive pasteurization by the holder 

 process at several temperatures. They classify the bacteria in raw milk 

 in four groups, namely: (1) the acid; (2) the inert; (3) the alkali; and (4) 

 the peptonizing groups. The acid group is divided into two subgroups, 

 to wit: (a) the acid-coagulating, which coagulates milk in less than 14 

 days; and (6) the acid group which produces acid but does not coagulate 

 milk in 14 days. In raw milk the inert group is the largest. In milk 

 pasteurized 30 min. at 145F. there is plainly a great increase in the pro- 

 portion of the acid and acid-coagulating subgroups while the alkali and 

 peptonizing groups form smaller percentages of the whole. At 160F. 

 the acid group is still the largest but the acid-coagulating subgroup is 

 composed of bacteria that coagulate very slowly. At this temperature 

 the alkali group is greatly reduced and the peptonizing group is reduced 

 to the minimum. At 170F., the acid group remains about the same, but 

 the organisms produce acid and coagulation very slowly. The alkali 

 group is practically destroyed, though an occasional sample may show 

 this group in fairly high percentage. The most important change at this 

 temperature is in the peptonizing group which, relative to the total 

 bacteria, begins to show a percentage increase. At 180F., this increase 

 of the peptonizing group is very striking for no less than 75 per cent, of 

 the surviving bacteria are peptonizers. At this temperature none of the 

 acid-coagulating subgroup and only a small percentage of the acid sub- 

 group are found. At 190F. and 200F., the bacterial groups survive in 

 about the same relative proportion as they do at 180F. From these 



