Evaluation of a dispersion and elutriation technique for sampling microorganisms from soil

The effectiveness of a dispersion and elutriation method for sampling soil microorganisms, in which the soil was dispersed with an anionic detergent and a chelating ion-exchange resin followed by separation of the microorganisms from soil particles by elutriation in an upwardly flowing liquid column, was tested using a bacterial cell suspension, a clay soil and a sandy soil. On theoretical considerations alone, the relationship between elutriation flow rate and the sedimentation velocity of particles suggested that complete separation of microorganisms from soil particles was impracticable. This was because separation of non-filamentous microorganisms with minimum contamination by other soil particles required excessively slow elutriation flow rates. A compromise situation in which a lower elutriate purity was off-set by increased elutriation flow rate was therefore tested. A pure suspension of Bacillus subtilis cells was not elutriated at the rate predicted by Stoke's law. This was attributed to non-uniform liquid flow within the elutriator which effectively retarded the upward movement of cells in the elutriators. When tested with soil, between 70 and 80% of the microscopically-counted cells and between 50 and 60% of the ATP were recovered in 41 of clutriate. Problems in completely dissociating microorganisms from soil particles made it impossible to produce an extract containing exclusively microorganisms. Nevertheless, the ratio of biomass (direct microscopic count or ATP) to the weight of soil particles was increased, when compared to that in the soil at the outset, by factors of 2.3 and 4.3 for the clay and the sandy soils, respectively, when determined by direct microscopic counts and by factors of 1.8 and 3 for the same soils when determined by ATP contents. The differences in concentration factor between the soils were attributed to the relative ease with which the two soils were dispersed. It was concluded that the major problems that need to be overcome if this technique is to be useful in sampling microorganims from soil are: (1) complete dispersion of the soil and dissociation of microorganisms from soil particles; (2) maintaining uniform clutriation flow; and (3) concentrating the dilute elutriate sample produced.