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 Flugge. The 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 dis- 

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

 (about four feet) the number becomes very small, and at 

 a depth of a few decimeters more the soil may be found en- 

 tirely 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 and some of 

 them are useful saprophytes.! To the latter class belong the 

 nitrifying bacteria described by Winogradsky and by Jordan 

 and Richards and those organisms occurring in soil which 

 have the power of converting ammonia into nitrous acid which 

 forms nitrites, and others which complete the change of nitrites 

 into nitrates. Both kinds are widely distributed. These 

 organisms will not grow on ordinary culture-media, and their 

 cultivation presents great difficulties. Probably a good many 



* Gilther, Soc. cit. p. 291. See also Voorhees and Lipman. W. S. Dept. of 

 Agr.Exp. Stat. Bulletin 194. Oct. 26, 1907. 

 t See Conn. Agricultural Bacteriology. 



