A SELECTIVE MEDIUM FOR THE ISOLATION AND QUANTIFICATION OF BRADYRHIZOBIUM-JAPONICUM AND BRADYRHIZOBIUM-ELKANII STRAINS FROM SOILS AND INOCULANTS

The ecological examination of members of the family Rhizobiaceae has b een hampered by the lack of a selective medium for isolation of root n odule bacteria from soil. A novel non-antibiotic-containing medium has been developed which allows selective isolation of Bradyrhizobium jap onicum and B. elkanii strains from soil and inoculants. The medium, BJ SM, is based on the resistance of B. japonicum and B. elkanii strains to more than 40 mu g of the metals ions Zn2+ and Co2+ per ml. BJSM doe s not allow growth of Rhizobium sp. strains. We used BJSM to isolate b acteria from a Hubbard soil and from several commercially prepared soy bean inoculants. Ninety-eight percent of the isolates obtained from Hu bbard soil nodulated Glycine max cv. Kasota, and between 55 and 95% of the isolates from the commercial inoculants had the ability to nodula ted soybeans. Numbers of bradyrhizobia obtained by using BJSM, strain- specific fluorescent antibodies, and the most-probable-number plant in fection assay indicated that the three techniques were comparable in q uantifying B. japonicum strains in soils and inoculants, although most -probable-number counts were generally 0.5 order of magnitude greater than those obtained by using BJSM. Results of our studies indicate tha t BJSM is useful for direct isolation and quantification of B. japonic um and B. elkanii from natural soils and inoculants. This medium may p rove to be an important tool for autecological and enumeration studies of diverse populations of bradyrhizobia and as a quality control meth od for soybean inoculants.