CHAPTER XXI 



THE BACTERIOLOGY OF WATER, AIR, AND SOIL, AND THEIR 

 BACTERIOLOGICAL EXAMINATION SEWAGE BACTERI- 

 OLOGY OF MILK AND FOODS COMMON ORGANISMS OF 



AIR, WATER AND SOIL 



Bacterial Content of Waters and the Factors 

 influencing it. Filtration, etc. 



THE bacterial flora of natural waters is a 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. Provided 

 contamination from human or animal sources, from the 

 air of towns, or from sewage or manure, be excluded, they 

 are mostly bacilli together with some sarcinse and micro - 

 cocci, many of which may be chromogenic. They are 

 generally non-pathogenic and non-liquefying, and for 

 . the most part develop best at 20-25 C. B. coli and 

 B. Welchii 1 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, 



1 This name is retained in this chapter to indicate a group of closely allied 

 organisms of which the principal members are B. perfringens and B. (enteri- 

 tidis) sporogenes (see pp. 515, 523). 



