CHAPTER II. 



THE QUANTITATIVE BACTERIOLOGICAL EXAMINATION 

 OF WATER. 



THAT the customary methods for determining the num- 

 ber of bacteria do not reveal the total bacterial content, 

 but only a very small fraction of it, 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 

 our notice. On the one hand certain obligate parasites 

 cannot thrive in the absence of the rich fluids of the ani- 

 mal 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. This 

 is made clear by the use of special media like the 

 NahrstofT Heyden agar (Hesse and Niedner, 1898), 

 which are particularly adapted to the needs of these 

 latter organisms; or by direct microscopic examin- 

 ation. Thus Gage and Phelps (Gage and Phelps, 



Note. In this chapter the authors have closely followed the recom- 

 mendations of the A. P. H. A., as given in the Appendix. 



19 



