646 APPLICATION OF METHODS OF BACTERIOLOGY 



once or is intermittent, so that even in the case of exposed 

 streams there are periods when no specifically dangerous 

 contamination may be in operation. As stated above 

 attention 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 

 may have occurred. By an analysis made at this time one 

 could scarcely hope to detect the specific organisms that 

 had caused the disease, especially in water from flowing 

 streams. 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 constantly 

 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 escape 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 subject 

 it to some method of treatment that will aid in separating 

 the few specific pathogenic from the numerous common 

 saprophytic species. For this purpose numerous so-called 

 methods of "enrichment" have been devised. The most 

 useful of these aim to favor the rapid multiplication of 

 pathogenic forms that may be present and to suppress or 

 check the growth of the ordinary water saprophytes. 



Attention has been called to the fact that when exposed 



