66 Bacteria in Relation to Country Life 



groups of bacteria find a congenial medium for their 

 development in water, and why others are crowded 

 out and disappear. The factors that influence this 

 are the supply of food, the temperature and sunlight, 

 and the presence of certain products more or less in- 

 jurious to some of the species. 



The supply of food. — The first of these factors is of 

 great significance particularly as regards the proportion 

 of organic matter in the water. It should be remembered 

 that, with very few exceptions, bacteria cannot subsist 

 on simple mineral salts. They must have for their de- 

 velopment organic matter elaborated by other organ- 

 isms — plants or animals. There is, however, a wide range 

 of adaptation as to the minimum amounts of organic 

 matter required for the growth of different species. 

 Some organisms will actually multiply in distilled water 

 containing mere traces of organic matter, others will 

 not grow unless there be present large quantities of 

 nitrogenous organic matter. Quite naturally, streams 

 flowing over sandy strata, poor in organic matter, and 

 receiving no additions of sewage, will offer but little 

 food to the latter class of organisms, while streams 

 polluted with sewage and receiving the drainage from 

 fertile soils will offer more favorable conditions for 

 their growth. The organic matter added to surface 

 waters from the drainage area does not long remain 

 unchanged. It tends to disappear, owing to the very 

 class of bacteria that uses it as food. ■ The disappearance 

 of the organic matter necessarily involves, also, the 

 disappearance of the bacteria which prey upon it. 

 Hence, the process of self-purification that occurs in 



