174 BACTERIOLOGY FOR NURSES 



harmless to man, unless present in extraordinary 

 numbers. 



The appearance of water cannot be a guide for its 

 purity, neither are the odor or flavor any indication of 

 its real condition. It is conceded that the only way 

 by which the purity of water may be determined is 

 by bacteriological examination. 



Surface drainage and sewage are the most common 

 sources of water pollution; both carry enormous 

 quantities of organic matter, including human ex- 

 creta, which is always particularly dangerous, into 

 wells and other sources of water supply, and both are 

 very likely to contain many pathogenic bacteria. 



Polluted water carries infection by its use as 

 drinking water, or by its use for washing dishes, or 

 by its use for cleaning foods which are eaten without 

 cooldng, such as celery, lettuce, radishes, tomatoes, 

 and many fruits. For domestic use, filtration with a 

 proper filter or boiling for ten minutes are the best 

 methods of purifying water. In large cities sand 

 filtration is employed to free the water from bacteria; 

 a well-managed sand filter will or should hold back 

 98 per cent of the bacteria. 



In Food. — The bacteria which, by their activities, 

 cause the decomposition and putrefaction of dead 

 organic matter are to be reckoned with in every 



