686 BACTERIA IN AIR, SOIL, WATER, AKD MILK 



section. (See page 40.) There are, thus, regular and normal inhabitants 

 of the soil which fulfil 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 ' 

 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,^ in studying the horizontal distribution of bacteria 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 baciUi are commonly present and are actively pathogenic for 

 these animals. The frequent occurrence of tetanus in persons sustain- 

 ing wounds of the bare feet and hands in fields and excavations, is a 

 matter of common knowledge. 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 sur- 

 vive in the soil for any great length of time. Unless the soil is specially 

 prepared by the presence of defecations or other 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 

 going on in the upper layers. Furthermore, in the deeper layers the con- 

 ditions of temperature and possibly oxygen supply are not at all favorable 

 for the growth of most pathogenic bacteria. Within a short distance from 

 the surface the temperature of the soil usually sinks below 14° or 15° C. 

 An interesting series of experiments by Fraenkel ^ have demonstrated 

 this point. This investigator buried freshly inoculated agar and gelatin 

 cultures of cholera spirilla and of typhoid and anthrax baciUi at differ- 

 ent levels, and examined them for growth after two weeks had elapsed. 



' Houston, Report Med. Officer, Local Govern. Bd., London, 1897. 



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



3 Fraenkel, Zeit. f. Hyg., xi, 1887. 



