284 BACTERIOLOGICAL EXAMINATIONS 



bles in the presence of gas under pressure; air currents may of 

 course carry germs so freed a much longer distance. 



Bacteriology of the Soil 



At least two forms of pathogenic bacteria are habitually found 

 in the soil. The tetanus bacillus, it is well known, exists in garden 

 earth, manure, and top soil generally. Dirt getting into wounds 

 is the most frequent cause of tetanus. Drinking water laden with 

 soil has been known to have in it tetanus bacilli, and if used in 

 an unsterilized condition in wounds or when a comparatively feeble 

 antiseptic, such as creolin, has been added, it may cause tetanus. 



The gaseous edema group, the bacilli of malignant edema, 

 symptomatic and parasitic anthrax are frequently found in soil. 

 The highly tilled soil of the battlefields in France was heavily 

 laden with the first, hence the great incidence of infection after 

 wounds in the late war. Streptococci and colon bacilli, too, have 

 been found in garden soil. Typhoid bacilli may contaminate soil, 

 but do not multiply in it. In sandy soil 100,000 bacteria per 

 gram have been found, in garden soil 1,500,000 bacteria per gram, 

 and in sewage-polluted soil 115,000,000 bacteria per gram have 

 been determined. The first few inches of ordinary soil contain 

 most of the bacteria, after a depth of 2 metres no bacteria at all 

 are found and the earth is sterile. 



Soil may be collected in sterile sharp-pointed iron tubes, and 

 diluted with sterile water of given quantity and plates poured 

 from it. 



Arable lands may be enriched very much by inoculating them 

 with 'certain nitrifying bacteria, some of which convert ammonia 

 into nitrous acid, which form in them nitrites; others change 

 nitrites into nitrates (nitrosomonas) . Certain of these bacteria 

 are concerned in the assimilation of nitrogen from the atmosphere 

 and adding to the nitrogen content of the soil, thus enriching it. 

 On the roots of some plants, alfalfa, beans, peas, and clover, 

 minute tubercles develop. These little growths are caused by the 



