116 ELEMENTS OF WATER BACTERIOLOGY 



gas in lactose broth. The five standard tests for B. 

 coli which have been most generally adopted in the 

 United States included gas production in dextrose 

 broth and coagulation of milk, but not gas production 

 in lactose broth. It was supposed that types pro- 

 ducing gas and acid in dextrose media and coagulating 

 milk, but failing to form gas in lactose broth, would 

 be rare. In the particular polluted well studied by 

 Phelps and Hammond, however, such forms were 

 very common, outnumbering true colon bacilli four 

 to one during the latter part of the investigation when 

 the pollution was less recent. Two workers following 

 the standard methods but using dextrose broth for 

 enrichment on the one hand and lactose broth on the 

 other would in 50 per cent of the samples tested have 

 reached opposite conclusions as to the presence or absence 

 of B, coli, the isolations begun with dextrose broth 

 being apparently positive and those begun with lactose 

 broth being negative. 



It is also clear on the other hand that the dextrose 

 test as ordinarily used may be negative when colon 

 bacilli are present. This is due to the interaction of 

 various bacteria in the fermentation tube and to the 

 solution and escape of gases which often prevents the 

 production of the typical gas formula. Of 43 cultures 

 isolated by Fuller and Ferguson (1905) at Indianapolis, 

 18 showed less than 20 per cent of gas after 48 hours 

 in the enrichment tube, and 11 showed less than 10 

 per cent. Hale and Melia (1910), working with a pure 

 culture of B. coli in unsterilized water (containing no 

 other gas former) report that of 818 tubes showing gas 



