BACTERIOLOGY OF SOIL 733 



Soil 



The upper layers of soil contain large numbers of organisms, 

 chiefly bacilli. The species are very varied ; among pathogenic 

 ones may be named the bacillus of tetanus and of malignant 

 oedema. The B. mycoides is very abundant, and the varieties 

 of Proteus, the hay and potato bacilli, are common, while the 

 nitrifying forms are of course present, but do not develop on 

 ordinary media. 



Below five or six feet aerobic organisms become scanty, but 

 the anaerobic and thermophilic ones are still met with. The 

 number of organisms present in soil is variable, from 200,000 to 

 45,000,000 in ordinary earth, while in dirty and busy streets 

 there may be as many as 1,000,000,000, per gram. According 

 to Houston, uncultivated sandy soil averages 100,000, garden 

 soil, 1,500,000, and sewage-polluted soil 115.000,000 per gram. 



Houston 1 found that in virgin soils the B. coli, B. Welchii, and 

 streptococci are practically absent, but that in soils polluted with 

 animal excrement by manuring or otherwise the spores of B. 

 Welchii are present in great abundance, also B. coli and strepto- 

 cocci if the pollution be of recent date. 



The length of time pathogenic bacteria retain their vitality in 

 buried corpses has been the subject of experiment by Losener, 2 

 who injected cultures into the bodies of pigs, which were then 

 wrapped in linen, placed in wooden coffins, and buried. The 

 conclusions he arrived at were that, provided the soil has good 

 filtering properties, there is practically -no chance of the dis- 

 semination of a virus. 



Klein, 3 experimenting with the bacilli of diphtheria, cholera, 

 plague,' typhoid fever, etc., also found that the vitality and 

 infective power of these organisms passed away in a comparatively 

 short time, in most cases within a month. 



Examination of Soil 



The bacteria in the soil may be examined by adding traces of 

 the soil to sterile nutrient broth, thoroughly crushing and soaking 

 it, and then making plate or roll cultures, aerobic and anaerobic. 



1 Hep. Med. Off. Lcc. Gov. Board for 1889-1900. 



2 Centr. f. Bait. (1* Abt.), xx, 1896, p. 454. 



3 Eep. Med. Off. Loc. Gov. Board for 1898-99, p. 344. 



