BACTERIOLOGICAL STUDY OF WATER. 625 



of containing specific pathogenic bacteria i. e., waters 

 polluted with sewage and with human excreta that 

 are believed to be the source of infection of typhoid 

 fever, or, less frequently, of Asiatic cholera. In the 

 investigations of such waters there are several points of 

 which we should never lose sight, viz., unless the 

 water is under continuous study there is only a chance 

 of detecting the specific pathogenic species, for, as a rule, 

 the dangerous pollution occurs either but once or is 

 intermittent, so that even in the case of exposed streams 

 there are periods when no specifically dangerous con- 

 tamination may be in operation. As stated above, atten- 

 tion is commonly called to the water when the disease, 

 presumably caused by its use, is fully developed, and 

 this is often days or weeks after the pollution of the 

 stream really occurred. By an analysis made at this 

 time one could scarcely hope to detect the specific organ- 

 isms that had caused the disease. The organisms sought 

 for may have been present in the water and may have 

 infected the users, and yet have disappeared by the time 

 the sample taken for analysis was collected. 



When present in polluted waters pathogenic bacteria 

 are always vastly in the minority. Were they con- 

 stantly present in large numbers infection among the 

 users of such waters would be more frequent and more 

 widespread than is commonly the case. They may be 

 present in a water-supply in small numbers ; they may 

 even be in the sample supplied for analysis, and yet es- 

 cape detection if only the ordinary direct plate method 

 of isolation be used. 



From these considerations it is obvious that before 

 attempts are made to isolate the various species directly 

 from a suspicious sample of water it is advisable to 

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