BACTERIA IN THE AIR AND SOIL 1013 



purposes. Thus, bacteria are most plentiful in the neighborhood of 

 cess-pools or in manured fields and gardens. Such conditions, how- 

 ever, may be regarded as abnormal. Even in uncultivated fields 

 there is a constant bacterial flora in the soil which is of great 

 importance in its participation in the nitrogen cycle, a phase of the 

 bacteriology of soil which has been discussed in detail in another 

 section. 



There are, thus, regular and normal inhabitants of the soil which 

 fulfill a definite function and may be found wherever plant life 

 flourishes. In addition to these, innumerable varieties of sapro- 

 phytes and pathogenic germs may be present, which vary in species 

 and in number with local conditions. Numerous investigations into 

 the actual numerical contents of the soil have been made. Houston 1 

 found an average of 1,500,000 bacteria per gram in garden soil, 

 and about 100,000 bacteria per gram in the arid soil of uncultivated 

 regions. Fraenkel, 2 in studying the horizontal distribution of bac- 

 teria in the 'earth, has found that they are most numerous near the 

 surface, a gradual diminution occurring down to a depth of about 

 two yards. Beyond this, the soil may be often practically sterile. 



Pathogenic bacteria may at times be found in the surface layers, 

 and these are often of the spore-bearing varieties. Most important 

 among them from the medical standpoint are the bacillus of tetanus, 

 of malignant edema, and the Welch bacillus. If a guinea-pig is 

 inoculated subcutaneously with an emulsion of garden soil, death 

 will result almost invariably with enormous bloating and swelling 

 of the body due to gas production. This is due to the fact that 

 the spore-bearing, gas-producing anaerobic bacilli are commonly 

 present and are actively pathogenic for these animals. The frequent 

 occurrence of tetanus in persons sustaining wounds of the bare feet 

 and hands in fields and excavations, is a matter of common knowl- 

 edge. Anthrax, also, may be easily conveyed by soil in localities 

 where animals are suffering from this infection. It is not probable 

 that pathogenic germs which are not spore-bearers survive in the 

 soil for any great length of time. Unless the soil is specially pre- 

 pared by the presence of defecations or other organic material, the 

 nutrition at their disposal is not at all suitable for their needs, since 

 rapid decomposition of organic materials by saprophytes is always 



1 Houston, Report Mod. Officer, Local Govern. Bd.. London, 1897 



2 Fraenkel, Zeit. f . Hyg., ii, 1887. 



