THE COLON GROUP OF BACILLI 111 



It is undoubtedly true that a negative presumptive 

 test is generally obtained with unpolluted waters. For 

 example, in a study previously cited, Winslow and 

 Nibecker (1903) reported that of 775 dextrose-broth 

 tubes inoculated from 259 unpolluted sources only 41 

 showed gas. On the other hand, it is equally true 

 that in a large proportion of cases colon bacilli are 

 isolated from positive dextrose-broth tubes. Longley 

 and Baton (1907) in the examination of 3553 samples 

 of Potomac water obtained positive tests 794 times, 

 while B. coli was actually present 529 times; 67 per 

 cent of the presumptive tests were therefore correct. 

 Gage (1902), in the Massachusetts work cited above, 

 found that 70 per cent of his fermented dextrose tubes 

 contained B. coli. 



The work of recent years has made it clear, however, 

 that both the coincidence of negative presumptive tests 

 with the absence of B. coli and the general coinci- 

 dence of positive presumptive tests with the presence 

 of B. coli, are open to disastrous exceptions. 



The errors in the dextrose broth test are both positive 

 and negative ; it may lead to the inference that bacteria 

 of the colon group are present when they are not, 

 and it may fail to show them when they are really there. 

 In the first place, with some waters, positive presump- 

 tive tests may be obtained when colon bacilli are not 

 present. According to Clark and Gage (1903) there 

 are 58 well-described species of bacteria which give 

 the presumptive test in dextrose-broth, of which 23 

 are widely separated from the B. coli group. An 

 unpublished investigation by Winslow and Phelps 



