THE ELIMINATION OF SPURIOUS PRESUMPTIVE 



TESTS FOR B. COLI IN WATER BY THE USE OF 



GENTIAN VIOLET 



IVAN C. HALL and LILLIAN JORDAN ELLEFSON 



From the Hearst Laboratory of Pathology and Bacteriology, University of California 



Received for publication June 25, 1917 



The bacteriological criterion of potable water is the absence 

 of B. coll and its close allies, — aerobic, Gram-negative, non- 

 sporulating bacilH or coccobacilli which produce acid and usually 

 gas in media containing lactose, and which do not liquefy gela- 

 tin.^ The best test for purity of water, according to standard 

 methods (1917) is a negative presumptive test, i.e., failure of 

 gas formation in lactose broth; water, which in quantities of 

 10 cc. or more, does not produce gas, may be unreservedly ac- 

 cepted for domestic use. Even if 1 cc. of a given supply yields 

 a negative result, though colon bacilli are present in larger 

 quantities, the water should probably be regarded as merely 

 suspicious. Only the extreme difficulty of ascertaining the 

 presence of the various intestinal pathogens makes us rely upon 

 the more actively fermenting colon bacillus as a criterion of 

 potential pollution. 



While a negative result permits very definite statements as 

 to the purity of the sample, a positive result, i.e., the formation 

 of gas in the presumptive test, does not necessarily condemn 

 the water. Even if the gas produced be proven due to B. coli 

 the water may be quite safe if no pathogenic organisms have 

 entered the water along with the colon bacillus. Thus the 

 colon baciin may have had their origin in perfectly healthy 

 human beings free from disease producing micro-organisms, in 



1 In this paper the term B. coli is used to include all members of the colon 

 groups as above defined — B. acidi-lactici, B. aerogenes, B. communior, B. com- 

 munis, and their subvarieties. 



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