Metal - Tolerating ability of some selected Rhizobia s trains

Heavy-metal pollution from many industrial processes is a major threat to human health and the environment resulting in loss of farming and grazing land. Many bacterial strains have been reported to tolerate metals but there is a dearth of research on the metal-tolerating ability of rhizobia strains, hence the need for this study to screen selected Rhizobial strains for their ability to tolerate varying concentrations of selected heavy metals. Ten Rhizobia strains including Bradhyrhizobium japonicum strains FA3, UDSA136, USDA 9032, USDA110, RANI 22, USDA 4675, RAUG and Bradhyrhizobium sp. strains B574, R25B and USDA 3541 obtained from the culture collection of the International Institute of Tropical Agriculture (IITA) were used for this study. They were screened for their ability to tolerate varying concentrations (10-150 μg/mL) of six selected metals (Copper, Cobalt, Cadmium, Lead, Zinc and Iron) on metal-incorporated Congo-red medium. FA3 (Bradhyrhizobium japonicum) showed the highest tolerance to iron (100 μg/mL), while FA3, USDA110 and USDA 4675 showed highest resistance to zinc with Minimum Inhibitory Concentration (MIC) of 150 μg/mL. Strains USDA 3451, USDA 4675, B547 were un