Soil sterilization effects on in situ indigenous microbial cells in soil

Soil was sterilized by various procedures, and then the resident microorganisms were physically separated and concentrated from the soil for viewing by transmission electron microscopy as thin sections and frozen-etched preparations. Remaining cell viability in the soil was tested by conventional plating before and after enrichment culture. The soil proved to be sterile after treatment with 60Co radiation, prolonged autoclaving, prolonged dry heat application at 200C, or glutaraldehyde (if followed by subsequent mild heating), and could be considered sterile after OsO4 treatment. Treatment with glutaraldehyde alone, or 160C dry heat for 3 h, did not sterilize the soil. Cellular fine structure was altered or destroyed by the heat treatments, but was not affected to any extent by any of the other treatments including glutaraldehyde followed by mild heating. These findings are considered in relation to the residual biological information observable by electron microscopy in soil samples which have been sterilized to eliminate possible pathogens before handling of the soil. These findings are also considered with the objective of obliterating the fine structure of the indigenous microorganisms during soil sterilization so that electron microscopy studies can be made of microorganisms inoculated into and grown in the presterilized soil.