CHAPTER VIII. 



PRESUMPTIVE TESTS FOR B. COLI. 



The isolation and identification of B. coli by the 

 methods which have been described is a time-consuming 

 and laborious operation, and one sometimes impossible 

 to apply in the practical supervision of a water-supply. 



Hence it is of especial value to the engineer to have 

 certain tests which may be easily and quickly carried out 

 and which will give a probably correct idea as to whether 

 pollution does or does not exist. Such tests are spoken of 

 as "presumptive tests." If positive, the water showing 

 such a test may be regarded as suspicious, but must be 

 further examined to prove the presence of fecal bacteria. 

 If the presumptive test is negative, no further examina- 

 tion need be made. Of the cultural reactions required 

 to establish the identity of the colon bacillus, the growth 

 and gas formation in dextrose broth, as originally sug- 

 gested by Smith (i8o3 b ) is so well marked and so strongly 

 typical of B. coli that this medium may be most con- 

 veniently used as a " presumptive test." 



The details of the operation vary in individual labora- 

 tories, but the underlying principle in all is that B. coli 

 develops rapidly in dextrose broth with gas formation of 



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