52 MILK AND THE PUBLIC HEALTH CHAP. 



a multiplication of bacteria ; but the types that develop differ so 

 markedly, that samples of the same milk kept at different tem- 

 peratures are, at later periods, very different in their bacterial 

 content, even though they contain the same number of bacteria. 



(4) The development of the ordinary lactic species, Bad. 

 lactis acidi, in practically all cases checks the growth of other 

 species of bacteria, and finally kills them, since the bacteria 

 regularly decrease in actual numbers after the lactic bacteria 

 have become very abundant. 



(5) In practically all samples of milk kept at 20 C., the 

 multiplication of the Boot, lactis acidi begins quickly, and 

 progresses with great rapidity. They grow so rapidly , that 

 they produce acid enough to curdle the milk in about 40 

 hours, the growth of other species being held in check. Milk, 

 when curdled at this temperature, shows a smooth acid curd, 

 with no gas bubbles. 



(6) Milk kept at 37 C. shows a totally different result. 

 The results are somewhat more variable than at 20 C. 

 Occasionally the Bad. lactis acidi grows vigorously at this tem- 

 perature, but the common result is a development of the B. lactis 

 aerogenes type. It forms a curd full of gas bubbles. If B. coli 

 communis is in the milk, this also grows luxuriantly at 37 C. 



(7) In milk kept at 10 C., neither of the types of lactic 

 bacteria seems to be favoured. The delay in growth lasts 

 2 or 3 days, after which all types of bacteria appear to develop 

 somewhat uniformly. Sometimes the lactic bacteria develop 

 abundantly, sometimes only slightly. The neutral bacteria 

 almost always grow rapidly, and the liquefiers in many cases 

 become abundant. In time, the milk is apt to curdle, 

 commonly with an acid reaction, but it never shows the pre- 

 dominance of Bact. lactis acidi found at 20 C. 



(8) There seems to be no difference between the effect 

 of 10 and 1 upon the bacteria, except upon the rapidity 

 of growth : 1 C. very markedly checks the growth of bacteria ; 

 but later they grow in great numbers. As at 10, the lactic 

 bacteria fail to outgrow the other species, so that all types 

 develop abundantly. A few species appear to be particularly 

 well adapted to this low temperature, and are especially 

 abundant at the end of the experiments. 



(9) The curdling point appears to be quite independent 



