Intrinsic antibiotic resistance and serological characterization of populations of indigenous Bradyrhizobium isolated from nodules of Desmodium intortum and Macroptilium atropurpureum in three soils of S.E. Queensland

Abstract

A collection of 1275 isolates of bradyrhizobia from three sites was made in the first (early) and 5th or 6th (late) growing seasons of uninoculated field plots of Desmodium intortum and Macroptilium atropurpureum. Isolates were characterized by intrinsic antibiotic resistance (IAR) and serological techniques. Non-hierarchial pattern analysis was used to group the isolates into antiobiotic inhibition response (IAR) types. Antisera were prepared for representative isolates of the most populous IAR types representing both early and late growing seasons. The data were used to test the hypothesis that under the influence of the soil root-rhizosphcre environment one or two IAR or serological types may dominate in the formation of nodules after several growing seasons. The IAR inhibition response patterns indicated a range of heterogeneity within and between sites and there was no significant increase of any IAR type in proportion of nodules formed in the late growing season samples. Most isolates at a site were accounted for by 2 or 3 of the IAR groups but the proportions of these types were different between sites. Serological grouping did not agree with the IAR groups, was more heterogeneous, and identified only 37–68% of isolates. However, there were serological types at each site that increased their proportion in the nodule population during the period of the experiment. Although the proportion of IAR types did not change between growing seasons the IAR screening provided a mechanism of selecting a representative dominant type for serological characterization and thus an avenue for selecting dominant strains that intrinsically may have the survival and competitive qualities required of a good inoculum strain.