128 ELEMENTS OF WATER BACTERIOLOGY 



(shown to contain no gas formers) with a pure culture 

 of B. coli and stored it for different periods and under 

 different conditions, testing at intervals by various 

 presumptive tests. The colon bacilli lived for 8-10 

 days at 37°, for 38-75 days at 20°, and 77-84 days 

 at 8° C. Comparison of presumptive tests with plate 

 counts on litmus-lactose agar showed that a gas test 

 in dextrose broth corresponded to an average of 4 

 bacteria and a positive test in lactose bile to 39 bacteria. 

 In general the dextrose broth showed gas in one dilu- 

 tion higher than the lactose bile; and the difference 

 increased with the attenuation due to prolonged sojourn 

 in water. 



These results of Hale and Melia make it clear that 

 the selective action of bile salts upon the various mem- 

 bers of the colon group may perhaps be an advantage 

 rather than a disadvantage for practical sanitary 

 purposes. The Committee on Standard Methods (191 2) 

 recognizes that lactose bile does inhibit certain weaker 

 members of the colon group, but beheves that these 

 attenuated organisms indicate only remote pollution 

 and are of little significance. They say, " In the 

 interpretation of the sanitary quality of a water, 

 it is best to discount the presence of attenuated 

 B. coli and to be sure to obtain all vigorous types. 

 The lactose bile medium accomplishes both of these 

 objects." 



The advantages of the dextrose broth enrichment of 

 weak colon bacilli and of the elimination of gas-forming 

 organisms other than B. coli by bile may both be obtained 

 as pointed out above by inoculating lactose bile from 



