59 



the colon group. The reliability of these presumptive tests therefore varies 

 directly with the degree of contamination or inversely with the remoteness 

 in time and distance from the source of pollution. The non-confirmed 

 presumptive tests are due, for the most part, to lactose fermenting spore 

 producing anaerobes which persist for a long time in water and which 

 are probably of very little sanitary significance. 



Hall and Ellefson have suggested the addition of gentian violet to 

 lactose broth to restrict the growth of these anaerobes, and recently Meur 

 and Harris indicate that brilliant green may be employed in lactose bile to 

 eliminate these spurious presumptive tests. This will be considered more 

 in detail in a subsequent chapter. 



The reliability of the presumptive test also varies with the treatment 

 which the water has received. Hauser, using lactose broth, obtained con- 

 firmations in 97.4 percent of raw water samples as compared with 86.1 

 percent of samples taken at the outlet from the clear water reservoir. 



The effect of chlorination on the reliability of the presumptive test 

 is particularly marked and is shown in the following able: 



TABLE XXVI. SHOWING CORRELATION OF RATE OF GAS PRODUCTION 



WITH CONFIRMATION OF THE PRESUMPTIVE TEST FOR COLON 



GROUP IN LACTOSE BROTH. 



The results indicated are based on a study of 1559 water analyses 

 in the Advanced Sector of the American Expeditionary Forces at Dijon, 

 France, comprising waters from both treated and raw supplies and from 

 various sources. An interesting correlation is noted between the rate of 

 gas production and the probability of confirmation. It appears evident 

 that 



1. The positive presumptive test (10 percent or more gas in 

 24 hours) is a very reliable index of the probable presence of Bact. coli 



