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 difficult to 

 apply in the practical supervision of a water-supply. 



Hence many investigators have attempted to devise 

 tests which might be easily and quickly carried out, and 

 which would yet give a fairly correct idea as to the 

 existence of pollution. Such tests are spoken of as 

 "presumptive tests." 



The medium which was first urged for a rapid presump- 

 tive test was dextrose broth; and this method gained 

 considerable acceptance five years ago. Its underlying 

 principle is that B. coli develops rapidly in dextrose broth 

 with gas formation of from 25 to 70 per cent of the capac- 

 ity of the closed arm of the fermentation tube. Of this 

 gas approximately one-third is carbon dioxide and two- 

 thirds hydrogen, that is, as the gas formula is generally 



A H 2 



expressed, = . 



C0 2 i 



In testing a water by this method a series of samples, 

 in suitable dilution, .001, .01, .1, i.o, or ioc.c., is added 



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