714 A MANUAL OF BACTERIOLOGY 



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 neces- 

 sarily 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 bacterio- 

 logical 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 Group. This group of organisms being 

 abundantly present in faeces and sewage, its presence in 

 water has been suggested as an indication of pollution. 

 The spores, however, are very resistant, and might, there- 

 fore, gain access to the water in ways other than by direct 

 pollution e.g. in dust and for this reason the Com- 

 mittee did not recommend the search for this organism 

 as a routine procedure. On the other hand, Thresh x 

 lays a good deal of stress on it. If spores of B. Welchii 

 are present but no B. coli it may indicate old pollution 

 or that there is drainage from manured soil. In gross 

 pollution both B. coli and spores of B. Welchii will be 

 present. 



Streptococci. Streptococci are abundant in faeces and 

 sewage, but are extremely rare, if ever present, in unpol- 

 luted natural waters ; hence the value of their detection. 

 Streptococci as a class are delicate organisms, and it was 

 supposed that their presence indicates recent pollution. 2 

 Horrocks, on the other hand, believes that they maintain 



1 Public Health, 1004. 



2 Houston, Rep. Med. Off. Loc. Gov. Board for 1898-99. 



