92 



Elements of Water Bacteriology. 



proved, however, that with waters of fairly good quality 

 the phenol interferes with the colon bacilli themselves to 

 a serious extent. The dextrose broth furnishes a more 

 delicate test than the carbol broth when the number of 

 colon bacilli present is small, as is clearly shown by the 

 following table from Irons : 



PROPORTION OF POSITIVE RESULTS IN TESTS OF 

 POLT.UTED AND UNPOLLUTED WATERS BY DEX- 

 TROSE FERMENTATION-TUBE AND CARBOD-BROTH 

 METHODS. ' 



(IRONS, igoi.) 



Dextrose 

 Fermentation- 

 tube. 



Carbol-broth 

 Method. 



Polluted waters 



Relatively unpolluted waters 



-t- - ? 

 33 31 S 



56 38 '25 



-t- - ? 

 38 30 I 



37 61 21 



The English Committee, appointed to consider the 

 Standardization of Methods for the Bacterioscopic Exam- 

 ination of Water (1904), recommend the use of bile- 

 salt broth or glucose-formate broth for preliminary 

 enrichment, and suggest that the incubation be carried 

 out anaerobically at 42° C. 



The use of media containing bile salts, and even of 

 undiluted ox-bile to which lactose has been added (Jack- 

 son, 1906), have been urged by various American bac- 

 teriologists. With sewages and heavily polluted waters 

 the lactose-bile medium has, in fact, proved superior to 



