Presumptive Tests for B. Coli. 



149 



of which are tabulated on page 148. It is clear that the 

 bile medium is superior to dextrose broth for the more 

 polluted waters. 



It has been pointed out in Chapter VI that the lactose- 

 bile medium is inferior to dextrose broth as a preliminary 

 enrichment medium for the full isolation of B. coli, from 

 the fact that it occasionally prevents the growth of B. 

 coli which may be isolated by the dextrose method. As 

 a presumptive test, however, it is far superior to dextrose 

 broth, giving a higher proportion of positive tests with 

 polluted waters and a lower proportion of erroneous 

 positive tests with waters of good quality. In a recent 

 examination of 176 surface waters in eastern Massachu- 

 setts, carried out under our direction, B. coli was isolated 

 70 times. The dextrose-broth test was positive 120 

 times, an error of 70 per cent; while the bile test, alone, 

 was positive 78 times, an error of only n per cent. The 

 tabulated results of these experiments indicate fairly the 

 merits of the bile medium for preliminary enrichment and 

 as a presumptive test. 



PRELIMINARY AND COMPLETE RESULTS OF DEXTROSE 

 BROTH AND BILE TESTS. 176 SURFACE WATERS. 



