lOO Elements of Water Bacteriology. 



an organism present which forms gas in dextrose but no 

 acid in lactose; ^there may be present forms which 

 individually fail to attack lactose but growing together, 

 symbiotically, produce gas in dextrose; or an organism 

 originally present and capable of fermenting both sugars 

 may have been overgrown and lost in the enrichment 

 tube. If plates are made on the first appearance of gas 

 the likelihood of the latter possibility will be reduced to 

 a minimum. Neither of the first two contingencies has 

 any sanitary significance; as we shall see later, bacteria 

 which ferment detrose and not lactose are not specially 

 characteristic of pollution. In any case, therefore, the 

 absence of red colonies on the agar plate may be con- 

 sidered a negative result. If red colonies are present 

 they must be subcultured and examined further. 



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 proye to be 

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



