THE BACTERIA OF SOIL, AIR, WATER, ETC. 149 



to cool water and articles of food by surrounding the vessels 

 containing them with ice. 



Bacteria of Milk and Other Foods.*— Of the different 

 food substances, milk is probably the most important from a 

 bacteriological point of view. In the first place, most other 

 foods are cooked before eating. Furthermore, cow's milk 

 constitutes the principal fcod of young infants, who are highly 

 susceptible to certain bacteria and to substances in the milk 

 itself, after it has undergone certain alterations due to bacteria. 

 The milk of the healthy cow as it is secreted in the mammary 

 gland is sterile; however, after milking the cow a little milk 

 generally remains in the milk-ducts and in the lower part of 

 the teat in which numerous bacteria will have developed 

 before the next milking-time.* The first milk obtained at a 

 milking should therefore be discarded, as it may contain an 

 excessive number of bacteria. 



In examining milk for bacteria the number may be estimated 

 by precisely the same technic as is used for the estimation 

 of the number of bacteria in water. The milk requires to be 

 diluted usually more than water for the reason that there are 

 a great many more bacteria ordinarily present in milk than in 

 water, and consequently a very small amount of milk has to 

 be used, so small that it cannot be accurately measured except 

 by diluting. If much milk is introduced into the culture 

 medium it makes the latter opaque. 



A committee of the American Public Health Associationf 

 recommend, among other things, the following: Milk should 

 be plated within four hours after it is collected, and in the mean- 

 time should be kept below 40° C, but not frozen. Dilutions 

 recommended are i-io, i-ioo, i-iooo, 1-10,000, 1-100,000, 



*See Conn. Bacteria in Milk and its Products. 1903. Russell. Dairy 

 Bacteriology. 1906. 7th Ed. 



tPreliminary statement by the committee on standard methods of bacterial 

 milk analysis. American Journal of Public Hygiene. Vol. XVII., No. 4, 

 November, 1907. 



