62 Bacteria in Relation to Country Life 



ment in the soil and, therefore, encourage larger additions 

 of bacteria to the surface waters of the region. 



Another fruitful source of bacteria for drinking-water 

 is the sewage which finds its way into it. Owing to its 

 origin, sewage is extremely rich in bacteria, frequently 

 containing tens of millions of them in every cubic centi- 

 meter. In view of the enormous volume of sewage 

 poured into some streams, the number of bacteria added 

 to the water is frequently beyond computation (page 73). 



All of these additions, made more or less constantly, 

 or at irregular intervals determined by the amount of 

 rainfall, provide an abundant supply of bacteria to 

 drinking-water. Various species are thus introduced 

 into the water, some of them capable of adapting them- 

 selves to the new conditions and of multiplying even to 

 a greater or slighter extent. 



The character of the bacteria in water. — The various 

 classes of bacteria in water exist there in different pro- 

 portions. As living organisms they are affected by con- 

 ditions favorable or unfavorable to their survival. 

 The kinds that are best adapted to their environment 

 will emerge victorious from the struggle for existence. 

 Hence, there are groups of bacteria that may be regarded 

 as normal inhabitants of water. These are encountered 

 in large numbers in all drinking-waters. 



There are others that are brought to the streams in 

 great masses from time to time, but, owing to the com- 

 petition of other bacteria, or to insufficient or improper 

 food, they die out very rapidly. They do not multiply, 

 and so disappear after a longer or shorter period. Such 

 are the numerous species of soil and sewage bacteria, 



