194 BACTERIA IN AIR, SOIL, WATER, AND MILK 



jelly and counting the number of colonies growing 

 in forty-eight hours. It is assumed that each colony 

 grows from a single bacterium. Chemical examination 

 of water aims at the determination of the quantity of 

 organic matter indicative of sewage pollution. Stand- 

 ards have been set by sanitarians, but they are not 

 necessary here. 



BACTERIA IN MILK 



Milk in the deeper parts of the udder of the healthy 

 cow is probably wholly free from bacteria. The ducts 

 of the teats, however, are almost never free from some 

 germs, and of course the outside skin contains many. 

 In a diseased udder there may be not only the germ 

 causing the disease, but other intruders from the 

 outside. Bacteria come into milk from the cow her- 

 self or from the outside. The latter is probably the 

 more important and the factors which must be con- 

 sidered are the dirt on the skin, swishing of the soiled 

 tail, the soiled hands of the dairyman, and the cans, 

 contaminated by manure or by polluted water. 



Milk is a capital culture medium for almost all bac- 

 teria, and as it is warm when drawn, growth may begin 

 very soon. Unless the milk be cooled very soon, to a 

 temperature at which bacterial growth is retarded or 

 stopped, souring will occur. Perfectly fresh milk has a 

 very slight restraining influence upon the development 

 of some feebler bacteria, but this power is soon lost 

 and bacterial growth may be unlimited. It is best to 

 keep milk not above 40 F. or 5 C., but so low a 

 temperature is not always possible to maintain. The 



