EVALUATION OF METHODS FOR EXTRACTION OF BACTERIA FROM SOIL

Several methods for dispersion of soil were tested for possible use in procedures for extraction of bacteria. Physical cell damage on cells and efficiency in extraction of indigenous cells from soil, were inves tigated. Cell damage by the dispersion methods was investigated by mea suring the physical cell integrity and viability of pure cultures of E scherichia coli and Bacillus subtilis, as well as soil bacteria extrac ted from soil, when dispersed in slurries of gamma-sterilized soil. Se paration of bacteria and soil particles on the basis of buoyant densit y was conducted with the nonionic density gradient medium Nycodenz. Wh en slurries of gamma-sterilized soil with added pure cultured cells we re centrifuged (10000 x g) over cushions of Nycodenz (1.3 g ml(-1)), p ractically all the added cells were recovered in a layer on top of the cushion. This proves that a reversible attachment and cosedimentation is not an important phenomenon in this procedure. The efficiency of t he different dispersion methods for the extraction of indigenous soil bacteria, was assessed after separation of dislodged and attached soil bacteria. This separation was done either an the basis of sedimentati on rate by low speed centrifugation, or buoyant density by Nycodenz de nsity gradient centrifugation. The physical dispersion by ultrasonic t reatment and chemical dispersion by the use of a chelating agent toget her with a detergent, were inferior to physical dispersion either by W aring blender (for large volumes) or a rotating rubber pestle treatmen t (for smaller volumes). The physical dispersion did not appear to be destructive to the cells tested.