BACTEEIOLOGY OF WATEE AND SEWAGE 435 



tion is directed almost entirely to the recognition of organ- 

 isms which are characteristic of sewage. 



The organism usually sought to determine the potability 

 of water is the Bacterium coli, and in some cases certain of 

 its close relatives. Bacterium coli is a normal and constant 

 inhabitant of the alimentary tract of man and of most, if 

 not all species of warm-blooded animals. It is present in 

 enormous numbers in the stools of man. While certain 

 closely related forms such as Bacterium aerogenes may be 

 found not infrequently in soil, Bacterium coli itself appar- 

 ently does not live outside the human or animal body for 

 any great period of time. Its presence in water accordingly 

 is an indication of comparatively recent contamination with 

 sewage. It has come, therefore, generally to be regarded 

 as the most satisfactory indication of fecal contamination 

 of water. 



Many methods, both qualitative and quantitative, have 

 been devised for determining the presence of Bacterium coli 

 in water. 



Presumptive Tests. — In bacteriological analysis of 

 water to determine potability, it is customary first to make 

 what is termed a presumptive test. This is a test which will 

 readily differentiate waters which are quite certainly good 

 from those which are suspicious. It is not customary, how- 

 ever, to condemn a water entirely upon the basis of the 

 presumptive test. It is based upon the fact that there are 

 not many species of bacteria which are capable of producing 

 gas in fermentation tubes from the sugar lactose, quite cer- 

 tainly not in the presence of certain inhibiting agents such 

 as ox bile. In making the test, varying amounts of water 

 are added to a series of fermentation tubes containing either 

 lactose bile or lactose broth. Water which does not induce 

 gas production, even when used in quantities as great as 

 one hundred centimeters, is quite certainly free from 



