438 BACTBBIOIiOGY. 



The more organic matter held in solution in a water, 

 the more suitable it is^ as a culture medium for bacteria. 

 Such a water will not only permit the multiplication of 

 common bacteria but will also favor any pathogenic germ, 

 like the typhoid bacillus, if it should chance to be intro- 

 duced. The same interpretation is to be given to a large 

 number of bacteria in a water as is given to a large amount 

 of organic matter determined by chemical analysis.. 



The counting of bacteria possesses especial - value in 

 controlling, from time to time, the purity' of a water-sup- 

 ply. A large and persistent increase in the number of 

 bacteria should lead to an investigation of the cause. 

 Where the water-supply is filtered, the daily counting of 

 the bacteria in the filtered water will give the best indica- 

 tions as to the proper working of the plant. 



The number and kind of species. — Apart from the recogni- 

 tion of specific, pathogenic bacteria very little need be said 

 regarding the common water bacteria present. "In general, 

 a water should contain but a few, different species. When, 

 for instance, 10 or more different species are present,' espe- 

 cially if each is represented by an appreciable number, it 

 would serve to indicate that the water contains consider- 

 able organic matter and is, therefore, a good medium 

 for the growth of bacteria. It cannot be said to prove 

 the existence of pollution, but it does show that there 

 are conditions favorable to bacterial growth, and hence, 

 favorable to pathogenic bacteria should they be intro- 

 duced. 



Considerable scientific interest is attached to the study 

 of the different species of water bacteria, but apart from 

 this they require no attention in the ordinary routine of 

 water analysis. The various characteristics of a given 

 species may be determined by making a microscopical and 

 cultural study of the organism in accordance with the 

 methods of study pursued heretofore. ' 



