158 Elements of Water Bacteriology. 



The successive growth of these two intestinal groups in 

 the same dextrose-broth tube suggests the following method 

 for the detection of both B. coli and sewage streptococci: 



Inoculate the desired quantity of water, preferably i cc, 

 into dextrose broth, in a fermentation tube, and incubate 

 at 37 degrees. After a few hours' incubation examine 

 the cultures for gas. Within two or three hours after gas 

 formation is first evident, plate from the broth in litmus 

 ' lactose agar, incubating for twelve to eighteen hours at 

 37 degrees. If at the end of this time no acid-producing 

 colonies are found, it is pr(|bably safe to assume that 

 there were no colon bacilli present. On the other 

 hand, if red colonies develop, these must be further 

 examined by the regular diagnostic tests for B. coli. 

 After the first plating from the dextrose broth, replace 

 the fermentation tube in the incubator and allow it to 

 remain for twenty-foiu: to thirty-six hours, then plate 

 again in litmus lactose agar. This plating should give a 

 nearly pure culture of streptococci if these organisms were 

 originally present in the water. 



The relative relation of the streptococci and the colon 

 bacilli to sewage pollution is still somewhat uncertain. 

 Houston (Houston, 1900) held that the former microbes 

 imply "animal pollution of extremely recent and therefore 

 specially dangerous kind." Horrocks (Horrocks, 1901), 

 on the other hand, maintains, largely on the strength of 

 certain experiments with stored sewage, that the strep- 

 tococci persist after colon bacilli have disappeared and 



