EXOPOLYSACCHARIDE PRODUCTION BY A RHIZOBIUM SP. FROM ROOT NODULES OF PHASEOLUS MUNGO (L.) IN HEAVY METAL STRESS CONDITION

摘要

Rhizobia are Gram negative bacteria that can exist either as free-living bacteria or as nitrogen fixing symbionts inside the root nodules of leguminous plants. Among rhizobial polymers, exopolysaccharides (EPS) play a key role and is indispensable for the invasion of a great majority of host leguminous plants which form root nodule. In the present investigation, initially ten bacterial strains were isolated from the root nodule of Phaseolus mungo (L.). and all were identified as a species of Rhizobium through different physiological and biochemical tests. All the strains produced EPS in yeast extract Mannitol (YEM) medium. But strain P3 produced maximum amount (470.0μg/ml) and reached its stationary phase at 24 hours. The isolate preferred pH 6.9 and sucrose (1.0%) for maximum EPS production. The effect of five heavy metals (viz. Cd, Ba, Pb, Hg and Ag) on growth and EPS production of the isolate was checked. All the tested heavy metals had inhibitory effect at higher concentration (100μg/ml), but the inhibitory effect of some heavy metals at lower concentration can be overcome by this strain. The ability of the isolated bacteria to grow and to produce EPS in heavy metal stress condition might be utilized for the production of local biofertilizer rhizobial strain which might be helpful in sustainable agriculture.