62 ELEMENTS OF WATER BACTERIOLOGY. 



rapid development of B. coli takes place in the first few 

 hours after dextrose solutions are inoculated with intes- 

 tinal material, and a nearly pure growth of colon bacilli 

 often results, while other bacteria multiply more slowly. 

 With highly polluted waters gas formation will probably 

 begin within twelve hours, but with fewer colon bacilli 

 present the duration must be increased. If the period 

 of incubation be too long continued, trouble in the subse- 

 quent steps of the isolation may be encountered because 

 of the overgrowth of B. coli by the sewage streptococci, 

 which are almost invariably present, and which by their 

 greater acid-producing powers may check the growth of 

 the colon bacilli. . 



As has already been stated, phenol has less inhibitory 

 action upon B. coli than upon normal water-bacteria, 

 hence a broth containing this substance may be employed 

 for preliminary enrichment; and this medium has been 

 used in place of dextrose broth for many of the studies 

 made in connection with the Chicago drainage canal 

 (Reynolds, 1902). Phenol broth consists of ordinary 

 broth to which o.i per cent phenol is added, and the 

 method of procedure is to add i c.c. of the water to 10 c.c. 

 of the sterilized phenol broth and incubate at body 

 temperature for twenty-four hours. Litmus-lactose-agar 

 plates are then made and the examination of the red 

 colonies carried on as described for the dextrose-broth 

 method. The dextrose broth furnishes, however, a much 

 more delicate test than the carbol broth when the num- 



