Improved soil dispersion procedures for total bacterial counts, extraction of indigenous bacteria and cell survival

Abstract

The effects of different soil dispersion procedures for enumeration and extraction of indigenous soil bacteria were investigated. Increased counts and extraction efficiency were obtained with pyrophosphate instead of water as dispersion liquid. When physical dispersion was conducted in the Waring blender, the effect of container volume and number of dispersions on extraction efficiency and bacterial counts was shown. An extraction efficiency of 70–80% of soil bacteria was obtained by repeated extractions, compared to 25% with one extraction. Cell damage by different container volumes was investigated by measuring the physical cell integrity and viability of pure cultures of Escherichia coli and Bacillus subtilis when dispersed in slurries of γ-sterilized soil. Nearly identical decay rates were observed with the different Waring blender containers. Separation of soil particles and bacteria by Nycodenz density gradient centrifugation facilitated almost complete separation of structurally intact cells and soil particles. Enzymatic cleavage and chemical oxidation of soil polysaccharides by periodate treatment did not increase the extraction efficiency.