BACTERIA IN MILK 207 



standards for its isolation. Water is carefully collected 

 and kept upon ice so that no increase of bacteria will 

 occur. In the laboratory suitable measures are taken 

 to determine the whole number of bacteria and the 

 presence of the colon bacillus. The whole number is 

 estimated by growing the water in flat plates of agar 

 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. The 

 ordinary milk bacteria are fortunately not pathogenic, 

 the dangerous varieties from the cow being only strep- 

 tococci from inflammation of the udder, and tubercle 

 bacilli. Those forms getting into milk from the sur- 



