BACTERIA IN WATER 697 



has indicated a method which has frequently found successful applica- 

 tion. 



To 100 c.c. of the infected water are added one per cent of pepton 

 and one per cent of salt. This mixture is then incubated at 37.5° C, 

 and after ten, fifteen, and twenty hours, specimens from the upper 

 layers are examined microsopically and are plated upon gelatin. Upon 

 the weak pepton solution cholera spirilla increase very rapidly at in- 

 cubator temperatures, and then when plated in gelatin the detection of 

 characteristic colonies is comparatively easy. 



Because of the great difficulties outlined above in isolating specific 

 pathogenic germs from polluted waters, bacteriologists have attempted 

 to form an approximate estimate of pollution by the detection of other 

 microorganisms which form the predominating flora of sewage. Chief 

 among these is B. coli. The isolation and numerical estimation 

 of B. coli in polluted water has been for a long time one of the criteria 

 of pollution. This so-called colon test, however, should always be ap- 

 proached with conservatism and should never be carried out qualita- 

 tively only. Careful quantitative estimation should be made in every 

 case. 



B. coli in water is by no means always the result of hmnan con- 

 tamination, since this bacillus is found in great abundance in the in- 

 testines of domestic animals. According to Poujol, B. coli does not 

 even always point to fecal contamination, since this author was able 

 to find the bacillus in the water of a number of wells where no possible 

 contamination of any sort could be traced. Prescott ^ explains this, as 

 well as similar cases, by the fact that organisms of the colon group 

 may occasionally be parasitic upon plants. 



The opinions of hygienists are widely at variance as to the value 

 of the colon test. While the discovery of isolated bacilfi of the colon 

 group may therefore be of httle value, it is nevertheless safe to follow 

 the opinion of Houston,^ who states that the discovery of B. coli in 

 considerable numbers invariably points to sewage pollution, and that 

 the absolute absence of B. coli is, as a rule, reliable evidence of purity. 



Rosenau states that a ground water should be condemned even if 

 only a few colon bacilU are found, for, as he puts it, "these bacteria have 

 no business in a soil-filtered and properly protected well or spring- 

 water." Surface waters, however, may easily contain a few colon 

 bacilli without necessarily having been exposed to contamination by 



' Prescott, Science, xv, 1903. 



' Houston, Rep. Medical Officer, Local Gov. Board, London, 1900. 



