182 BACTERIA IN AIR, SOIL, WATER AND MILK 



weeks. Relatively more cholera organisms are discharged 

 with a cholera stool than is the case of typhoid bacilli in 

 enteric fever. Dysentery bacilli live only a short time in 

 water probably, and the importance of water in the dissemi- 

 nation of dysentery is questioned by some observers. Certain 

 it is that some epidemics appear to be water-borne. 



Typhoid and colon bacilli are always present in typhoid 

 stools. It is hardly probable that the former could get into 

 water without the latter. Moreover, the colon bacillus is 

 present in all alimentary tracts. It is more easily detected 

 in any mixture or solution than any other of the intestinal 

 bacteria. Therefore, it is taken as an indication of sewage 

 pollution in water. This may not mean that typhoid bacilli 

 are present, but merely that contamination of water by 

 sewage from animal sources has occurred. Whether from 

 man or animals, it is obvious that dejecta should not come 

 into water intended for human consumption. The methods 

 of water examination now in use all aim at the detection of 

 Bacillus coli. Because of its peculiarities in the fermenta- 

 tion of sugars, certain media are adopted as 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. Standards 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 



