TYPES OF COLON-AEROGENES GROUP IN WATER 327 



are two types of the colon-aerogenes group which occur in fecal 

 matter in large numbers. While it is possible that they may 

 also live in the soil or other material from which they may be 

 carried to water, their presence in water is strong presumptive 

 evidence that the water was polluted with fecal matter. One 

 of these types has certain distinctive characters which render 

 identification easy; the other type is not so well marked but 

 may be identified with reasonable certainty and without great 

 difficulty. 



In one way the recent contributions to the knowledge of the 

 colon-aerogenes group has not changed the methods of water 

 bacteriology. The presence of any particular kind of bacteria 

 in water is merely an indication of the existence of certain con- 

 ditions and the bacteriologist must weigh all the available 

 evidence on the basis of his experience and make his decision 

 accordingly. However, the method which we now have of 

 separating the colon-aerogenes group into varieties which have 

 a very definite relation to habitat should be of material assist- 

 ance in forming an opinion of the potability of a water. The 

 value of this ability to separate the varieties of the colon-aero- 

 genes group is much more evident if" a sufficient number of cul- 

 tures can be isolated from each sample to establish the relative 

 numbers of the different types. This, we believe will prove to 

 be of much greater value than the mere determination of the 

 presence of colon or of any one variety of colon. 



REFERENCES 



Browne, William W. 1915 Predominance among the members of the bacillus 

 coli group in artificial!}' stored water. J. Infect. Dis. 17, 72-78. 



Browne, William W. 1917 The presence of the Bacillus coli sroup and the 

 Bacillus welchii group in the intestinal tract of fish (Stenomus chry- 

 sops). Abs. Bact. 1, 55-56. 



Clark, William Mansfield 1915 The final hydrogen ion concentrations of 

 cultures of Bacillus coli. J. Biol. Chem., 22, 87-98. 



Clemesha, W. W. 1912 Bacteriology of surface waters in the tropics. London. 



Cumming, Hugh S. 1916 Investigation of the pollution and sanitary condi- 

 tions of the Potomac watershed. U. S. Treasury Dept., Hygienic 

 Laboratory bull. 104. 



