Release of microorganisms from soil with respect to transmission electron microscopy viewing and plate counts

Abstract

Microorganisms were detached and washed from soil by various procedures involving blending and sonication of the soil in water or pyrophosphate solution, followed by successive low-speed, centrifugal-washing separations of the suspended cells from the soil debris. Some of these procedures were previously used for separating and concentrating cells from soil for transmission electron microscopy viewing. Exhaustive applications of these procedures separated up to 27% of the platable cells from the soil. Based on filterability, these cells either were no longer attached to soil particles, or were attached to very small particles. The cells fractionating with the soil debris, however, seemed to be strongly attached to it or to other cells so that they were not filterable. Laboratory-grown cultures added to sterile and non-sterile soil did not attach to the soil materials and were easily recovered from the soil even though low-speed centrifugations were being used. Electron microscopy evidence for cells released and concentrated from non-inoculated natural soil, and for cells remaining with the soil debris, suggests that the very small, probably non-platable, cells tend to release more easily than do cells in the size range of 0.3 to 0.5 μm in diameter, and that cells larger than this, including bacterial and fungal spores, are more difficult to separate from soil. Plating data for heated preparations are in agreement with this for bacterial spores. The results are considered in relation to the validity of plate counts and direct soil transmission electron microscopy for evaluating the microbial flora of soil.