ISOLATION OF THE COLON BACILLUS. 67 



brick- red color, and of such consistency as to be readily 

 picked up by the needle-point and the other smaller and 

 of an intense vermilion color. The colonies of the colon 

 bacillus are usually well formed, pulvinate on the sur- 

 face and fusiform when growing deeper down. 



The agar streak made from the litmus-lactose-agar plate 

 shows after twenty-four hours certain marked character- 

 istics. The most distinct types are two, the abundant, 

 first translucent, later whitish and cheesy growth, cover- 

 ing nearly the whole surface of the agar, characteristic of 

 B. coli and its allies, and a very faint growth, either con- 

 fined strictly to the streak or made up of faint isolated 

 colonies, dotted here and there over the surface. The 

 latter cultures are typical of the sewage streptococci, and 

 a microscopic examination will generally settle their status 

 at once. Of the more luxuriant growths, some of which 

 are stringy to the needle many will generally prove to be 

 atypical, and if any of the weakly fermenting forms (B. 

 mycoides) are present, a dull wrinkled growth will be 

 produced. 



Having submitted the sample of suspected water to a 

 preliminary enrichment process, and having isolated pure 

 cultures of suspicious organisms from the litmus-lactose- 

 agar plate, the third step is the examination of the specific 

 reactions of the organisms thus obtained. Just what 

 characters to use in defining the " colon bacillus" is a 

 matter of prime importance. The whole question of 

 species among the bacteria is an extremely complex one, 

 since around each definite species are grouped forms 



