134 ELEMENTS OF WATER BACTERIOLOGY 



produce gas in dextrose; B. welchii or some other 

 form which will not grow on aerobic plates may have 

 produced the gas; or an organism orginally 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 have seen, bacteria which ferment 

 dextrose and not lactose are not specially characteristic 

 of pollution. In general, therefore, the absence of red 

 colonies on the agar plate may be considered a negative 

 result. If red colonies are present they must be sub- 

 cultured and examined further. 



The agar streak made from the litmus-lactose-agar 

 plate shows after 24 hours certain marked character- 

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

 first translucent, later whitish and cheesy growth, 

 covering nearly the whole surface of the agar, character- 

 istic of B. coh and its aUies, and a very faint growth, 

 either confined 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. 

 I^The various tests which may be appHed to the cul- 

 tures after they have been isolated, the subgroups 



