48 



MILK AND THE PUBLIC HEALTH chap. 



is only in a modified sense possible to speak of " milk bacteria," 

 as if certain bacteria were typical and characteristic of milk, 

 since milk is not kept for very long periods, and any selective 

 action has rarely time to be pronounced. It will, however, 

 be found that certain types of bacteria are especially prevalent 

 in, and indeed characteristic of, stale milk. 



The rapid increase in the number of bacteria in stored 

 milk is well known. The multiplication has been shown to 

 depend mainly upon three factors : 



(a) The temperature at which the milk is kept. 



(b) The time interval since milking. 



(c) The kinds of bacteria present. 



The influence of the last factor is more variable and more 

 difficult to estimate than that of the first two causes. 



The following table from results obtained by Park^ upon 

 the development of bacteria in a pure and a contaminated 

 sample of milk shows the great influence of both time and 

 temperature. The initial content of sample A was 3000 and 

 of B 30,000 bacteria per c.c. 



The same general facts are shown in another way in the 

 following table,^ showing the development of bacteria in 

 samples incubated at different temperatures. This table is 

 introduced to show the influence of the third factor — the 

 kinds of bacteria present, — since it will be seen that the in- 

 crease was a very variable one, and only to be explained by 

 the differing initial content and kinds of bacteria present. 



1 Journ. of Hygiene, 1901, vol. i. p. 398. 



2 W. G. Savage, Appendix B, No. .4 Beport of Medical Officer, Local Go-oern- 

 ment Board, 1909-10, p. 481. 



