The discovery of streptomycin as a product of a rather obscure group of microorganisms, the actinomycetes, led to the study of these organisms as potential producers of other chemotherapeutic substances.

The soil is not a mass of dead debris, merely resulting from the physical and chemical weathering of rocks; it is a more or less homogeneous system which has resulted from the decomposition of plant and animal remains. It is teeming with life.

With the exception of autotrophic bacteria, the green, or chlorophyll-bearing, plants are the only living forms on this planet capable of synthesizing organic matter out of inorganic elements and simple compounds.

It has been recognized since the dawn of microbiology that the soil is inhabited by a living microscopic population which is responsible for the numerous reactions that take place in the soil and that affect the life and economy of man in many ways.