BACTERIA 97 



frequent. Under ordinary conditions there is no reason to suppose 

 that pathogenic bacteria multiply in water or that they retain 

 their vitahty for more than a few weeks. In polluted soil, however, 

 they may Uve much longer than in water, and a river may be con- 

 tinuously polluted during a long period by bacteria that are washed 

 into it from accumulations of fecal material. Other pathogenic 

 bacteria occasionally water-borne are the dysentery bacillus and 

 the anthrax bacillus. 



Since the search for specific pathogenic bacteria in a water is 

 hardly ever likely to be crowned with success, various indirect 

 means for determining the purity of a water have been proposed. 

 The most useful of these analytical methods is the test based on 

 the determination of the relative number of Bacillus coll. This, 

 the colon bacillus, is a normal inhabitant of the healthy human 

 intestine and is found in large numbers in fresh sewage where, by 

 appropriate methods, it is usually detected in each -j o o o o ^■^- ^^~ 

 amined. Since it is also present in the droppings of many of the 

 larger domestic animals and hence occurs in garden soil and in 

 pastures, its occasional presence in water does not necessarily in- 

 dicate possible or even probable pollution with fecal matter of 

 human origin. The researches of many investigators, however, 

 have shown that the relative abundance of Bacillus coli in water 

 is a very satisfactory criterion of the sanitary quahty of such a 

 water. If, for example, it is found uniformly present in a water in 

 each I c.c. sample, the water is looked upon as distinctly suspicious. 

 In cases, however, where it is rarely found in i c.c. samples and 

 only occasionally when quantities as large as lo c.c. or even 50 c.c. 

 are examined, the water is usually considered potable. 



The bacteria in water stand in important relations to the life of 

 other aquatic plants and animals. It is a familiar fact that but for 

 bacterial activity the nitrogen and carbon in complex organic com- 

 pounds once bound would remain forever locked up and unavail- 

 able for the nutrition of other forms of Hfe. As is well known also, 

 the first steps in decomposition or the breaking down of organic 

 substances are due to bacterial agency. Ammonia and ammoni- 

 acal compounds are among the chief nitrogenous products of tliis 

 decomposition. The processes of disintegration and oxidation do 



