Isolation of the Colon Bacillus. 97 



water. If generally present in one cubic centimeter the 

 water may be considered unsafe, whether the colon bacillus 

 is found in smaller volumes or not. For special studies 

 of self -purification, etc., of course fractions of the cubic 

 centimeter must be examined. Litmus-lactose-agar plates 

 should be made from all tubes which show any gas what- 

 ever. Fuller and Ferguson (1905) have shown that B. 

 coli may be present even when gas formation in the 

 enrichment tube is quite at3rpical. Of 43 cultures isolated 

 by these observers at Indianapolis, 18 showed less than 

 20 per cent of gas after forty-eight hours in the enrich- 

 ment tube, and 11 showed less than 10 per cent. 



The procedure of the Committee on Standard Methods 

 of Water Analysis (1905) calls for a forty-eight-hour incu- 

 bation of the preliminary enrichment tube. Recent expe- 

 rience has, however, shown that a twenty-four-hour period 

 gives approximately the same results if the production of 

 gas rather than any specified amount of gas is the criterion 

 of a f)ositive test. Longley and Baton (1907) found that 

 of 109 1 enrichment tubes giving positive tests after 48 

 hours only 173 showed no gas in 24 hours; of these latter 

 only two contained B. coli. The advantage pf saving a 

 day is so great as to warrant the adoption of the shorter 

 period. 



In aU practical processes of examining water for B. coli 

 one essential step is the isolation of pure cultures upon 

 the litmus-lactose-agar plate, whether the plate be inocu- 

 lated froifl the water direct or from a preliminary enrich- 



