Soil as a Transmitter of Human Pathogens

Soils are the habitat of about 25% of the Earth’s species. The majority of these organisms are not of any threat to human health, but rather function to provide numerous ecosystem services which emerge through the multitude of complex interactions between the organisms and the soil itself. These ecosystem services range from those which are vital for maintaining life on Earth, such as the formation of soil, the cycling of carbon and nutrients with the result of maintaining the global cycles of C and N, soil fertility, the filtering of water, as well as provision of useful compounds such as antibiotics, the majority of which have been isolated from soil organisms (Chap. 12). However, soils also contain microorganisms which are capable of causing diseases in humans. A wide variety of soil-related infections need to be considered, particularly in the case of wound, respiratory tract, or gastrointestinal infections. Soil-borne microbes that are pathogenic for humans include protozoa, fungi, bacteria, and also viruses which require a host for their survival. Over 400 genera of bacteria have been identified with possibly as many as 10,000 species and, with the exception of viruses, they are in most cases more abundant than any other organism in soils. Helminths, belonging to the mesofauna size class, are also important as human pathogens. The number of bacteria that can be cultured in the laboratory is probably less than 1%. Their actual diversity is thus probably much greater. Of the approximately 100,000 species of fungi currently recognized, only about 300 may cause human disease.