Survival and multiplication ofRhizobium japonicum strains in silt loam

Antibiotic resistant mutants 8-0 StrR, 110 TetR and 138 KanR derived from wild typeRhizobium japonicum strains were inoculated into silt loam soil to cell concentrations greater than 2×108/g of soil. Population changes were monitored using antibiotic media and strain identification was done using immunodiffusion assay on microcores of soil. Immunodiffusion bands formed by the mutant strains with homologous antisera essentially duplicated bands formed by the parent strain. Strains 110 TetR and 8-0 StrR had cross reacting antigens whereas antigens of strain 138 KanR reacted only with the homologous antiserum. Populations ofR. japonicum strains introduced into sterile soil increased over a period of four weeks under both single and mixed culture inoculations. All populations decreased by the end of six weeks and thereafter remained constant. When theseR. japonicum strains were introduced into non-sterile soil, the population did not increase over the initial population added. Population decreased gradually for two weeks and then maintained thereafter. It was possible to recover very low populations of antibiotic resistantR. japonicum strains from both sterile and unsterile soils using media containing specific antibiotics. Detection ofR. japonicum strains by immunodiffusion was accomplished only when the population was ≥109 cells/g of soil. The method using antibiotic resistant mutants permitted an evaluation of the interactions of variousR. japonicum strains in soil with respect to their survival and multiplication.