DIRECT ISOLATION OF BRADYRHIZOBIUM-JAPONICUM FROM SOIL

Selective media for the direct isolation of unmarked Bradyrhizobium japonicum and other slow-growing rhizobia from soil are described. The method utilizes media containing various combinations of antibiotics and heavy metal compounds which are significantly more toxic to most soil bacteria, including Rhizobium species, than to strains of Bradyrhizobium. The most effective combination for isolation of B. japonicum from soil suspensions included tetracycline, rifampicin, chloramphenicol, zinc sulphate, and two antifungal antibiotics (cycloheximide and pimafucin). Depending on the soil, the proportion of B. japonicum colonies, of those selected as putative rhizobia, ranged from 6 to 100%. Whilst the technique is not proposed as a counting method there was generally good agreement between this method and the most probable number 'trap plant' method. Some potential applications and limitations of this direct isolation approach in ecological research with rhizobia are discussed.