IV BEHAVIOUR OF BACTERIA IN MILK 47 



the Gaertner and dysentery bacilli. Heinemann and Glenn 

 have shown that some species occurring naturally in milk 

 decrease considerably in numbers during the first 4 or 5 

 hours, some decrease slightly, some hold their own or even 

 increase. Copeland, in a single experiment, found that the 

 addition of boric acid damaged the inhibitory power, no 

 diminution of the bacteria in fresh milk resulting. 



Rosenau and McCoy have shown that the germicidal 

 power of milk is independent of its cellular contents. They 

 found that leucocyte-free milk is quite as active as the leuco- 

 cyte-rich sediment obtained by centrifugalisation. 



On the other hand, numerous investigators have shown 

 that at least part of the decrease is more apparent than real. 

 Stocking, for example, concluded that " the decrease in the 

 number of bacteria during the first few hours is not the result 

 of any germicidal condition or property possessed by the milk, 

 but simply of the natural dropping out of those species which 

 do not find the milk a suitable medium in which to develop." 



Other investigators have found that the diminution in 

 the number of bacteria is partially to be accounted for by 

 agglutination of some of the bacteria. The bacteria become 

 agglutinated into clumps, and since a small clump may grow 

 as one colony the apparent number of bacteria is diminished. 



From a general consideration of the literature it would 

 seem evident that the decrease is not due to any one cause 

 entirely. Part would appear to be due to agglutination and 

 other causes, but in addition there is evidently some action 

 exerted of a specific nature, and which is rather restraining 

 than definitely germicidal. 



B. The General Behaviour of Bacteria in Milk 

 UNDER Different Conditions 



Although milk is an excellent nutrient medium for most 

 bacteria it is not equally so for all, and it is easy to demon- 

 strate that there is a struggle for existence between the 

 different bacteria introduced, the survival of one type over 

 another depending upon a number of factors, such as the 

 initial dose, the kinds of other bacteria present, the time 

 interval, and, in particular, the temperature of the milk. It 



