The Soil and They That Dwell Therein 269 



ing as saprophytes, with the possible exception of protozoa which 

 consume other microorganisms. Millions, or even billions, of or- 

 ganisms may be found per gram of soil, the numbers depending 

 upon how good a medium the particular soil happens to be at the 

 moment. In a previous chapter the various technics available for 

 determining the total and the viable bacterial counts of liquids 

 were discussed. Some of these methods, with modifications, are 

 available to soil microbiologists, but the procedures will not be 

 discussed here. 



While most soil organisms are generally considered to be non- 

 pathogenic, there are bacteria which can initiate serious disease in 

 man. When a person cuts himself, the soil microbes that gain 

 entrance to the wound are usually not as dangerous as is the intro- 

 duction into the flesh of the individual's own skin bacteria— the 

 pyogenic ( pus-producing ) cocci— which can cause wound infections 

 and septicemia (blood poisoning). 



Soil that has been fertilized with manure, particularly horse 

 manure, may contain unusually high numbers of Clostridium 

 tetani, the etiological agent in lockjaw. This is important informa- 

 tion for persons engaged in directing sports and physical educa- 

 tion. Football fields and baseball diamonds should not be fer- 

 tilized with horse manure, or persons sustaining injuries of the 

 flesh may have to be administered tetanus antitoxin to prevent the 

 development of lockjaw. It is much more desirable and it is less 

 hazardous to employ commercial preparations for maintaining good 

 o^rass in these areas. 



Another disease that can be contracted when deep wounds are 

 contaminated with heavily fertilized soil is gas gangrene, caused by 

 Clostridium perfringens. From the generic name of this organism 

 and the one causing lockjaw, it is known that these two species are 

 anaerobes; they grow in the absence of free atmospheric oxygen. 

 When an individual sustains a deep, dirty flesh wound, both 

 aerobes and anaerobes are usually forced into the injury together 

 with the soil particles, and after the aerobes have depleted the 

 supply of free oxygen in the depths of the wound, the anaerobes 

 find conditions suitable for their growth and they flourish. 



