CHAPTER II. 



THE QUANTITATIVE BACTERIOLOGICAL EXAMINATION 

 OF WATER. 



THE customary methods for determining the number of 

 bacteria in water do not reveal the total bacterial content, 

 but only a very small fraction of it, as becomes apparent 

 when we consider the large number of organisms, nitrify- 

 ing bacteria, cellulose-fermenting bacteria, strict anae- 

 robes, etc., which refuse to grow, or grow only very slowly 

 in ordinary culture media, and which, therefore, escape 

 detection. On the one hand, certain obligate parasites 

 cannot thrive in the absence of the rich fluids of the 

 animal body; on the other hand, the prototrophic bacteria 

 adapted to the task of wrenching energy from nitrates 

 and ammonium compounds are unable to develop in the 

 presence of so much organic matter. Winslow (1905), 

 in the examination of sewage and sewage effluents, found 

 20-70 times as many bacteria by microscopic enumeration 

 as by the gelatin plate count. Certain special media 

 enable us to obtain much larger counts than those yielded 

 by the ordinary gelatin method. The Nahrstoff Heyden 

 agar, for example, has been strongly advocated by Hesse 

 (Hesse and Niedner, 1898) and other German bacteriol- 

 ogists upon this ground. In this country Gage and Phelps 

 (Gage and Phelps, 1902) showed that the numbers obtained 



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