446 BACTERIOLOGY. 



The soil may be considered as the natural habitat of 

 certain pathogenic bacteria, notably, — tetanus, malignant 

 €dema, anthrax, pus-producing cocci and* symptornatic 

 anthrax. The latter has *not been isolated from the soil 

 but the others have- been repeatedly found there. This 

 is true especially of tetanus and of malignant edema 

 which seem to be distributed in the soil over the entire 

 surface of the globe. 



Owing to the enormous numbers of common saprophytic bacteria, 

 the tetanus or malignant edema bacilli cannot be isolated direct 

 from the soil, by the ordinary plate method. It is necessary to resort 

 to an animal experiment in order to effect their isolation. For this 

 purpose, an incision is made through the skin of a rabbit or guinea- 

 pig and a small pocket is made into the subcutaneous tissue (see 

 p. 262). A small amount of the suspected soil is then introduced into 

 this pouch. Most of the common bacteria are unable to grow in the 

 body and are soon destroyed. The tetanus or malignant edema germ, 

 if present, is favored by these saprophytic bacteria and is thus enabled 

 to multiply and to produce the poison which soon destroys the animal. 

 Prom the local abscess or from the tissues and serous surfaces of the 

 animal, anaerobic cultures (p. 311) can then be made and the organ- 

 ism isolated in pure culture. 



The above method is usually employed when attempting to 

 Isolate the organisms from the pus of a wound in a case of the dis- 

 ease. The direct isolation of the tetanus bacillus by the plate 

 method was accomplished once, in this laboratory, from a case which 

 originated by infection from a tooth. 



The typhoid bacillus, as mentioned on p. 427, has been isolated 

 from the soil. The cholera vibrio may at times occur in the soil, but 

 as yet it has not been found there. A non-virulent and even a viru- 

 lent pest bacillus has been found in the earth. 



Some idea of the' enormous number of bacteria present 

 may be given when it is said that 1 c.c. of the surface 

 soil usually contains several hundred thousand bacteria. 

 Indeed some observers have reported as- many as 50 to 80 

 millions of these organisms. 



The rain and snow bring down a great many bacteria 

 from the air but, as soon as the soil dries and becomes pul- 



