620 BACTERIOLOGY. 



weeks after the pollution probably occurred. These 

 intervals of time are ordinarily sufficient for the deli- 

 cate, non-resistant bacillus of typhoid fever to succumb 

 to the unfavorable conditions under which it finds itself 

 in water. By unfavorable conditions are meant the 

 absence of suitable nutrition ; unfavorable temperature ; 

 probably the antagonistic influence of more hardy 

 saprophytic bacteria, particularly the so-called " water- 

 bacteria," and of more highly organized water-plants ; 

 the effect of precipitation and of sedimentation ; and, 

 of great importance, the disinfecting action of direct 

 sunlight. 



Though the positive demonstration of typhoid bacilli 

 in drinking-water by bacteriological methods is of ex- 

 treme rarity, it must not be concluded that bacteriological 

 analyses of suspicious waters shed no light upon the exist- 

 ence of pollution and the suitability or non-suitability 

 of the water for drinking-purposes. 



In the normal intestinal tract of all human beings 

 and of many other mammals, as well as associated with 

 the specific disease-producing bacillus in the intes- 

 tines of typhoid-fever patients, is an organism that is 

 frequently found in polluted drinking-waters, and 

 whose presence is proof positive of pollution by either 

 normal or diseased intestinal contents ; and though 

 efforts may result in failure to detect the specific 

 bacillus of typhoid fever, the finding of the other 

 organism, bacillus coli, justifies one in expressing the 

 opinion that the water under consideration has been 

 polluted by intestinal evacuations from either human 

 beings or animals. Waters so exposed as to be liable to 

 such pollution should never be considered as other than 

 a continuous source of danger to those using them. 



