12 2 MANUAL OF BACTERIOLOGY. 



CHAPTER III. 



THE BACTERIA OF SOIL, AIR, WATER, AND OF MILK 

 AND OTHER FOODS. 



The Bacteria of the SoiL — Bacteria are present in the soil 

 in enormous numbers — 100,000 or more in i c.c. of virgin soil, 

 according to Fliigge. Tlie depths to which they penetrate 

 depend upon the character of the soil and the character of the 

 life upon it, and whether or not it has been artificially disturbed, 

 as by cultivation. In general, at a depth of 1.25 meters (about 

 four feet) the number becomes very small, and a little deeper 

 the soil is entirely sterile. 



The bacilli of tetanus and malignant edema, and bacillus 

 aerogenes capsulatus are present in the soil of many localities. 

 According to Woodhead, certain savage tribes of Africa and 

 the East Indies use as an arrow-poison soil that is capable of 

 producing tetanus. The bacillus of anthrax may be found in 

 soil which has been infected with this organism. 



Most of the bacteria of the soil are harmless or useful sapro- 

 phytes.* The nitrifying bacteria described by Winogradsky 

 and by Jordan and Richards belong to the latter class. There 

 occur in soil organisms which have the power of converting 

 ammonia into nitrous acid which forms nitrites, and others 

 which complete the change of nitrites into nitrates. Both 

 varieties are widely distributed. These organisms will not 

 grow on ordinary culture-media, and their cultivation presents 

 great difficulties. Probably a good many bacteria have similar 

 properties to some extent. The work done by nitrifying bacteria 



* See Conn. Agricultural Bacteriology. 



