Sensitivity of isolated strains from Far Eastern natural populations rhizobia, to antimicrobials

Abstract

Relevance. 76 rhizobia strains differing in their physiological and biochemical characteristics from B. japonicum and S. fredii were isolated from the root nodules of Vigna radiate, Vigna unguiculata and Vigna angularis, taken from the soils of the Far East of the Russian Federation, and left for further research.
Methods. The virulence of new rhizobia strains isolated into a pure culture from nodules of various leguminous crops was determined by growing bacterized seeds in test tubes (height — 200 mm, diameter — 20 mm) with a nutrient medium for plants of the following composition, g/l: K₂НРО₄ — 1.0; МgSO₄ — 1.0; CaSO₄— 0.5; FeSO₄, H₃BO₃, MnSO₄, and (NН₄)₆Мо₇О₂₄ — traces. The virulence was determined by nodule presence; intensity of nodule formation due to the strain studied was determined by the nodule number. Primary assessment of the inherent strain resistance to antibiotics was performed with the disk diffusion test. Antibiotics used for the research: nalidixic acid (30 μg), carbenicillin (100 μg), streptomycin (10 μg), erythromycin (15 μg), rifampicin (5 μg), tetracycline (30 μg). After 3 to 7 days of incubation at a temperature of +27…28 °C, the results were registered according to the diameter of the strain growth suppression zone: up to 10 mm — resistant (R); 10 to 15 mm — intermediary resistant (I); 15 to 25 mm — sensitive (S); over 25 mm — highly sensitive (HS).
Results. It was found that most strains were resistant to streptomycin, erythromycin, rifampicin, and nalidixic acid, while the highest sensitivity was noted to tetracycline and carbenicillin. Strains can be attributed to the Bradyrhizobium elkanii species.