74 Bacteria in Relation to Country Life 



already been pointed out that enormous numbers of 

 harmless bacteria may be derived from the soil. 



To prove that a water-supply has actually been' con- 

 taminated by sewage, it is necessary to resort to methods 

 other than the mere counting of the bacteria in it. 

 Chemical methods, bacteriological methods, or both', 

 may be employed for the purpose. The chemical methods 

 are based on the detection of certain substances con- 

 tained in a considerable proportion in sewage. Unfor- 

 tunately, the chemical methods prove insufficiently 

 delicate when the sewage is strongly diluted by the 

 water to which it is added. The bacteriological methods 

 used at present, with considerable success, are more 

 delicate, and are based on the detection of certain kinds 

 of bacteria never absent from sewage, but not regarded 

 as normal inhabitants of pure water. The species of 

 bacteria most widely employed as an indicator of sewage 

 pollution is called Bacillus coli communis, or the colon 

 bacillus, a normal inhabitant of the intestinal tract of 

 man and of domestic animals. 



Frequent examinations of sewage in this country 

 and in Europe have shown that the colon bacillus is 

 always present in numbers ranging from a few thousand 

 to fifty thousand per cubic centimeter. In occasional 

 instances, this number is greatly exceeded. These results 

 are not surprising in view of the nature and origin of 

 sewage. It has been shown, also, that the colon bacillus 

 can develop and increase in numbers in sterilized sewage, 

 an indication that the sewage itself is adapted for the 

 development of this organism. Under actual conditions, 

 however, the sewage is not sterile, but is inhabitated 



