CHAPTER XXI 



THE BACTERIOLOGY OF WATER, AIR, AND SOIL, AND 

 THEIR BACTERIOLOGICAL EXAMINATION SEWAGE 

 BACTERIOLOGY OF MILK AND FOODS 



Some of the Commoner Organisms found in the Air, Water 

 and Soil. 



Bacterial Content of Waters and the Factors 

 influencing it. Filtration, etc. 



THE bacterial flora of natural waters is a very varied one. 

 The organisms met with in surface waters, such as streams, 

 ponds, and shallow wells, are derived from the air and 

 soil through which the water has passed, and if not con- 

 taminated from human or animal sources, from the air of 

 towns, from sewage or manure, consist mainly of non- 

 pathogenic bacilli, the majority of which are chromogenic 

 and non-liquefying, and develop best on culture media 

 at a temperature of 18 to 22 C. or thereabouts, not at 

 blood heat ; also of some sarcinse and a few micrococci ; 

 B. coli and B. Welchii are usually absent. When, however, 

 the water passes through cultivated lands, or receives 

 sewage, the number of organisms is enormously increased ; 

 a large proportion of them liquefies gelatin and develops 

 at blood heat, and B. coli and B. Welchii appear more or 

 less numerously. Whereas water from shallow wells has 

 a bacterial content nearly as great as the surrounding 

 surface water, that from deep wells, especially in the chalk, 

 is remarkably free from organisms. The following Table 



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