OTHER INTESTINAL BACTERIA. 105 



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

 are found, it is probably safe to assume that there were no 

 colon bacilli present. On the other hand, if red colonies 

 are developed, 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-four 

 to thirty-six hours, then plate again in litmus-lactose-agar. 

 This plating should give a nearly pure culture of strep- 

 tococci if these organisms were originally present in the 

 water. If colon bacilli are not found in the first set of 

 plates the streptococci may still be isolated by this method. 

 The relative relation of the streptococci and the colon 

 bacilli to sewage pollution is still 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 

 indicate contamination with old sewage which is not 

 necessarily dangerous. On the whole, it seems probable 

 that the streptococci, like the colon bacilli, are primarily 

 parasitic organisms, the former being associated both 

 with the outer and inner surfaces of the body. Like the ' 

 colon bacilli, their presence in water indicates contact 

 with the wastes of human life, and their isolation from 

 a suspected sample furnishes valuable confirmatory evi- 

 dence of its dangerous character. 



