204 ELEMENTS OF WATER BACTERIOLOGY 



(191 2 a ) finds that streptococci in India are present 

 in .0001 or .00001 gm. of faeces, but are rare in waters 

 unless very grossly polluted. In a series of bottle 

 experiments in the laboratory and in the study of an 

 artificially polluted tank outdoors he showed that they 

 disappear very rapidly in water, within 2 or 3 days 

 at the most. Gordon (1904) showed that certain strep- 

 tococci are abundant in normal saliva and are found 

 in air which has been exposed to human pollution but 

 not in normal air. On the whole there can be no doubt 

 of the fact that streptococci occur on the surfaces of 

 the human and animal body more commonly than 

 anywhere else in nature. 



Isolation of Sewage Streptococci. The isolation of 

 these organisms either from plates or liquid cultures is 

 easy. On the lactose-agar plate, made directly from 

 a polluted water, the colonies of the streptococci may 

 generally be distinguished from those of other acid- 

 formers by their small size, compact structure, and 

 deep-red color, which is permanent, never changing 

 to blue at a later period of incubation. Developing 

 somewhat slowly, however, they may be overlooked 

 if present only in small numbers. In the dextrose- 

 broth tube, streptococci will generally appear in abun- 

 dance after a suitable period of incubation. Prescott 

 and Baker, in the work above mentioned, found that 

 with mixtures of B. coli and streptococci in which the 

 initial ratios of the latter to the former varied from 

 i : 94 to 208 : i, the colon bacilli developed rapidly 

 during the early part of the experiment, reaching a 

 maximum after about 14 hours, and then diminishing 



