THE COLON GROUP OF BACILLI 107 



concluded that " where the amount of B. coli is small 

 and the colony count large, the lactose plate for plating 

 water direct is inferior to the dextrose fermentation- 

 tube." Gage came to a similar conclusion (Gage, 1902). 



The medium most commonly used in the United 

 States prior to 1906 for preliminary enrichment was 

 ordinary broth to which i.o per cent of dextrose had 

 been added, and the reaction brought to the neutral 

 point. Into each of a number of fermentation-tubes 

 of this medium a measured quantity of the water to be 

 examined is inoculated, and the culture is incubated for 

 24 hours at 37.5 C. It used to be customary to incu- 

 bate for 48 hours. Recent experience has, however, 

 shown that a 24-hour period gives approximately the 

 same results if the production of gas rather than any 

 specified amount of gas is the criterion of a positive 

 test. Longley and Baton (1907) found that of 1091 

 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 of saving a 

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

 period. At the end of 24 hours at least, the tubes 

 are examined for gas formation. If gas is found, a 

 small amount of the culture should be added, after 

 suitable dilution, to litmus lactose agar and plated. 



With polluted waters it will be found advantageous to 

 plate out on the first appearance of gas (4-8 hours). 

 It has been shown by one of us (Prescott, i902 b ) that a 

 very rapid development of B. coli takes place in the 

 first few hours after dextrose solutions are inoculated 

 with intestinal material, and a nearly pure growth of 



