Interaction of Bradyrhizobium sp. with an antagonistic actinomycete in culture medium and in soil

Summary

Experiments were conducted to determine the extent to which populations of Bradyrhizobium sp. strain Tal 209SR were suppressed by the antagonistic activity of a Streptomyces sp. isolated from a highly weathered tropical soil. The activity of the actinomycete was evaluated in culture medium in the presence or absence of clay minerals and in the soil from which the actinomycete was isolated after sterilization by gamma-irradiation. The presence of the actinomycete in culture medium was associated with a significant reduction in the density of Bradyrhizobium sp. (Tal 209SR). Nevertheless, the density of the Bradyrhizobium sp. surviving in the medium in the presence of the actinomycete was much higher than that normally observed when comparable densities of rhizobia are introduced into non-sterile soils. The presence of the antagonistic actinomycete in culture medium was associated with a drastic decrease in the optical density of the medium. This decrease was accompanied by a significant decrease in the insoluble exopolysaccharide content of the medium in addition to the significant decrease in bradyrhizobial cells. The actinomycete did not appear to significantly influence the growth and survival of the test Bradyrhizobium sp. in sterile soil. The inability of the actinomycete to significantly antagonize the test bacterium in sterile soil was not adequately explained by the presence of clay minerals.