BACTERIOLOGY OF SEWAGE . 231 



mann (1911) have recently pointed out, that the sampling 

 error is a very serious one with sewage. Duplicate 

 tests made at i-minute intervals for a period of 10 

 minutes in their experiments gave extreme values of 

 190,000 and 550,000 per c.c. 



The determination of the number of colon bacilli 

 in sewage and effluents should furnish an integral part 

 of bacteriological sewage analysis, since it is important 

 to know whether the decrease of intestinal bacteria in 

 the process of purification is proportional to the reduc- 

 tion of total bacteria. The State Sewerage Commis- 

 sion of New Jersey has adopted this procedure in its 

 supervision of the disposal plants in that State; and 

 the results seem amply commensurate with the labor 

 involved. As in the case of polluted waters the enumera- 

 tion of B. coli may be carried out, either by the study 

 of the red colonies which appear on litmus-lactose-agar 

 plates inoculated with the sample directly, or by the 

 use of a preliminary enrichment process. The com- 

 plete identification of B. coli seems unnecessarily 

 tedious, however, where the organisms are present 

 in such abundance. Some approximate presumptive 

 method is indicated here, if anywhere; and the 

 experience with polluted water, reviewed in Chapter 

 VI, points to the Jackson bile medium as the most 

 promising one. Experience at the Sewage Experiment 

 Station of the Massachusetts Institute of Technology 

 has shown that this presumptive test in general yields 

 good results. As pointed out above, a 48-hour incu- 

 bation period at 37 is required. All tubes showing 

 20 per cent gas at the end of this time may be con- 



