BACTERIOLOGY OF WATER 147 



of melted agar, at 40 C., the mixture is poured in a 

 Petri dish and allowed to stand at 22 C. for forty-eight 

 hours. Then the number of colonies is counted. Since 

 the bacterial examination of water yields only approxi- 

 mate results, the American Public Health Association 

 has set up certain standards that insure uniform methods 

 and allow the results to be compared. 



If there is any reason to suspect sewage contamina- 

 tion, special tests are made to isolate the Bacillus coli. 

 The water is added to lactose broth in a fermentation 

 tube and is incubated at 37 C. for three days. The 

 production of gas in the closed arm of the tube points 

 to the presence of Bacillus coli. When litmus is added 

 to lactose-agar, and the suspected sample of water is 

 plated out, the colon bacillus will produce acid which 

 causes its colonies to be surrounded by a red halo, while 

 the rest of the culture-medium is blue. A few colon 

 bacilli may accidentally occur in water, which is safe, but 

 their presence in large numbers spells danger. It is 

 difficult to isolate the cholera vibrio and especially the 

 typhoid bacillus from contaminated drinking-water, 

 hence the presence of colon bacilli is taken as showing 

 sewage contamination, and the number of these organ- 

 isms as the index of danger. 



