98 BACTERIOLOGICAL METHODS 



absorbed portion is supposedly hydrogen. The colon bacillus 

 shows a gas formation of J hydrogen. The standard time of 

 incubation is 48 hr., but if colon bacilli are abundant, gas forma- 

 tion will be observed in the tubes carrying the larger amounts of 

 the inoculated material at a much shorter time, occasionally within 

 a few hours. Small numbers of attenuated colon bacilli may 

 require 2 and 3 days before there is any gas formation 

 noticeable. In this connection it may be mentioned that the 

 attenuated colon bacilli indicate remote contamination, as all 

 B. coli of recent contamination develop readily in lactose bile. 



FIG. 26. Bacillus coli. Superficial colony on a gelatin plate 2 days old (X 21). 

 (McFarland after Heim.} 



This constitutes the usual presumptive test for the presence of 

 sewage contamination. Some investigators, however, recommend 

 that the test be supplemented as follows: Plant the suitable 

 quantities or dilutions into liver broth (in test-tubes) and in- 

 cubate at 37 C. for about 12 hr. and then transplant these cultures 

 into the lactose bile as above explained. The liver broth en- 

 richment medium is said to bring out the attenuated forms of 

 colon bacilli. In routine procedures the liver broth culturing is 

 usually omitted as the important point at issue is the determina- 

 tion of fairly recent contamination with sewage, or of sewage 



