588 A MANUAL OF BACTERIOLOGY 



(e) Waters unfit for drinking. B. coli present in 1 c.c. 

 or less. 



Waters which show no B. coli in 50 c.c. are of a high degree of 

 purity, and therefore the proved absence of this organism in this 

 amount, and still better in larger quantities, is of great value. 



B. coli should be absent from at least 50 c.c. of spring or deep 

 well water, possibly from greater amounts. 



In upland surface waters the presence of B. coli in 40, 10, or even 

 2 or 1 c.c. means contamination, but not necessarily a contamination 

 which it is essential to prevent. It may be from contamination 

 with the excreta of animals grazing on the gathering areas, and is 

 by no means necessarily from sewage or other material containing 

 specific organisms of infection. If B. coli are present in numbers 

 greater than, say, 500 per litre (or even in that amount), such a 

 water is suspicious, as it is rare to get so many B. coli in a water 

 from the kind of animal contamination indicated, and further 

 investigation is desirable. In filtered samples the number of 

 B. coli is, as a rule, considerably reduced. 



In surface wells B. coli in large numbers indicate surface or other 

 'contamination, generally very undesirable if not actually dangerous. 



It must clearly be understood that the presence of the B. coli 

 in water is used as an index of pollution, just as the organic ammonia 

 is in a chemical analysis. This organism is not necessarily harmful 

 in itself ; it is what it indicates, viz. pollution, probably with human 

 excremental matters, which may contain the organisms of specific 

 disease, e.g. typhoid, dysentery, and cholera. As a routine, the 

 typhoid bacillus is never looked for, and the statement sometimes 

 seen in the report on the bacteriological examination of a sample 

 of water that " no typhoid bacilli have been detected " is of little 

 value. It is on the general results of the examination, as detailed in 

 preceding pages, that a conclusion is arrived at respecting the purity 

 or otherwise of a water. 



Bacillus Welchii. This organism being abundantly 

 present in fseces and sewage, its presence in water has been 

 suggested as an indication of pollution. Its spores, how- 

 ever, are very resistant, and it might, therefore, gain 

 access to the water in ways other than by direct pollution 

 e.g. in dust and for this reason the committee did not 

 recommend the search for this organism as a routine 



