162 BACTERIA IN WATER 



known cases where a moorland water had a very high content 

 in b. coli, without there being the remotest possibility 

 that such came from man. With this proviso, we must inquire 

 as to what criteria are to be adopted in determining the 

 significance of the presence of different members of the group 

 in a water, and here reliance is chiefly to be placed on the 

 presence of the typical forms of b. coli. 



If a sufficient quantity of practically any water be taken, 

 except, perhaps, that coming from artesian wells, organisms 

 of the coli group will be found to be present. Therefore, the 

 question resolves itself into setting up some standards of 

 relative purity which may be followed in dealing with waters 

 coming from different sources. These standards are at present 

 empirical, and different bacteriologists have different views on 

 the subject. There is, however, a general agreement that deep 

 well water and the filtered water supplied to urban communities 

 should be entirely free from b. coli in quantities of 100 c.c. or 

 less. The great difficulty lies in dealing with river water and 

 water from shallow and surface wells. Here the usual view 

 is that the presence of b. coli in 10 c.c. or less is sufficient to 

 condemn the water. It may be said that under ordinary circum- 

 stances an inspection of the surroundings and an unfavourable 

 chemical analysis are sufficient to condemn such a water, for even 

 if a bacteriological examination showed the absence of b. coli 

 in large samples, yet the water ought to be condemned ; and 

 further, if in a suspicious locality the bacteriological analysis 

 yielded a bad result, the water ought to be condemned even if 

 from the chemical analysis it could be passed. The difficult 

 cases are those where the inspection of the locality is satisfactory, 

 and yet b. coli is present in large numbers. 'Here contamination 

 is often of animal origin, and the water can after careful inquiry 

 be passed. 



Great care is often necessary in interpreting bacteriological 

 analysis in consequence of the delicacy of the method. Thus 

 in examining raw waters, especially those derived from 

 moorland catchment areas to be used for urban supplies, 

 bacteriological examinations are relatively of little value, as 

 storage and filtration will completely alter the bacterial content. 

 Bacteriological methods are, however, of the greatest value 

 much more than mere chemical analysis in determining the 

 efficiency of filtration processes. 



As the b. coli is fairly widespread in nature, Klein and 

 Houston hold that valuable supporting evidence is found in 

 the presence of the b. enteritidis sporogenes and of strepto- 



