252 MILK 



These figures show that the decrease, when it really exists, is 

 relatively small and hardly comparable to the germicidal action of 

 fresh blood. In some instances there is no decrease, but rather 

 the reverse. When there is an actual decrease, this may continue 

 for four to six hours at 37 C., after the lapse of which time there 

 is usually a consistent increase. 



The estimation of numbers of bacteria in practically all the 

 work reported was made by counting the colonies formed. It is 

 clear that this method has its limitations, as it is not possible to 

 determine whether each colony originated from one or more cells. 

 The method takes no account of the possible clumping of bacteria, 

 with an apparent instead of a real reduction in numbers. 



Furthermore, bactericidal substances would destroy bacteria 

 in large numbers, while the figures show merely a limited reduc- 

 tion. Agglutinins, on the other hand, would cause a smaller 

 colony count, without diminution of bacteria as a necessary con- 

 sequence. 



Rosenau and McCoy, Heinemann and Glenn, and others have 

 shown that the restraining property of fresh milk does not act 

 alike on all kinds of bacteria. In fact, some types do not seem 

 to suffer, but hold their own or even increase. This fact may 

 account for some variations in results obtained by different in- 

 vestigators since the bacterial flora of milk is a variable one. 

 When those bacteria that are sensitive to the germicidal action 

 predominate in a given sample of milk a marked reduction of 

 numbers will be noticeable; and when the bacteria that are un- 

 affected predominate the count will remain the same or increase. 

 This is graphically illustrated in Fig. 83, which shows the relative 

 effect of heating milk to 56 and 75 C. 



The milk used in these experiments was drawn fresh from the 

 cow under aseptic precautions and contained small numbers of 

 bacteria. The chart shows that even in the first two hours raw 

 milk shows a slight bacterial increase if the average number of 

 fifteen samples is taken. After four hours there is a decided 

 increase. When the milk is inoculated with large numbers of 

 bacteria growth is restrained for two hours, and then there is an 

 increase which becomes more marked after four hours. If the 

 milk is heated to 56 C. bacteria apparently multiply from the 

 start, and if heated to 75 C. this seeming increase is more pro- 

 nounced. After a lapse of two hours the increase in heated milk 

 is quite rapid. 



Rosenau and McCoy have shown that the degree of vigor 

 employed in shaking the sample of milk has some influence on 

 the number of colonies appearing. With vigorous shaking the 

 number is, as a rule, larger than with moderate shaking. This 



