OTHER INTESTINAL BACTERIA 207 



I ex., into dextrose broth, in a fermentation tube, 

 and incubate at 37°. After a few hours' incubation 

 examine the cultures for gas. Within 2 or 3 hours' 

 after gas formation is first evident, plate from the 

 broth in Htmus lactose agar, incubating for 12 to 18 

 hours at 37°. If at the end of this time no acid-produc- 

 ing colonies are foimd, 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 fur- 

 ther 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 24 to 36 hours, then plate again on litmus 

 lactose agar. This plating should give a nearly pure 

 culture of streptococci if these organisms were originally 

 present in the water. 



Streptococci as Indicators of Recent Pollution. The 

 comparative 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 there- 

 fore specially dangerous kind," and Clemesha's experi- 

 ments led to the same conclusion. Horrocks (Horrocks, 

 1901), on the other hand, maintains, largely on the 

 ■ strength of certain experiments with stored sewage, 

 that the streptococci persist after colon bacilli have 

 disappeared and indicate contamination with old sewage 

 which is not necessarily dangerous. These discordant 

 results are probably to be explained by the difi^erent 

 media in which the \aabilityof the bacteria was com- 

 pared. It seems likely that in sewage where there is a 



