114 ELEMENTS OF WATER BACTERIOLOGY 



mentative power, many of them liquefying gelatin, 

 were present, but their numbers relatively increased 

 during the period after the original pollution had been 

 removed. 



Much valuable light is thrown upon the significance 

 of these positive dextrose broth results in the absence 

 of the colon group by the investigations of Clemesha 

 (1912^). In a careful study of 46 samples of human 

 fasces and 25 different samples of cow dung, includ- 

 ing about 3500 different colonies isolated by various 

 methods, only about 5 per cent belonged to the class 

 fermenting dextrose but not lactose. In rivers and 

 ground waters, on the other hand, this group made 

 up 24 per cent of the colon-like organisms present, 

 and in lakes with long storage, 58 per cent. The table 

 below shows the relative increase of the lactose negative 

 forms with the natural purification of rivers in four 

 dry months following rain and also their relative 

 increase in settled and filtered water as compared with 

 the raw river water. The numbers dealt with are too 

 small to give entirely uniform results, but the general 

 trend is clear. 



PERCENTAGE OF DEXTROSE-FERMENTING ORGANISMS 

 FAILING TO FERMENT LACTOSE. CALCUTTA WATER 

 SUPPLY 



(Clemesha, 191 2') 



Month 



Condition of river. 



Raw river water (80 colonies) 

 Settled water (20 colonies) . . 

 Filtered water (60 colonies) . 



Feb. 

 Clear 



79 

 99 

 95 



