Production of extracellular polysaccharides by a Rhizobium species from the root nodules of Melilotus alba

The Rhizobium sp., isolated from the root nodules of the leguminous fodder herb Melilotus alba, produced large amounts of extracellular polysaccharide s (EPS) (963.5 mu g/ml) in a yeast extract mannitol medium. Growth and EPS production started simultaneously, but EPS production reached its maximum d uring the stationary phase of growth of the bacteria, at 20 hours. EPS prod uction was increased with all of the thirteen sugars tested. Different nitr ogen sources, such as nitrates, glutamic acid, casamino acid and L-asparagi ne, increased the EPS production although it was inhibited by glycine, nitr ite and ammonium salts. Among the vitamins and metal ions only pyridoxal ph osphate and ZnSO4 promoted EPS production. Attempts were made to optimize t he cultural requirements for growth and maximum EPS production. Maximum EPS production (1457.0 mu g/ml) was obtained when the medium was supplemented with glucose (1%), pyridoxal phosphate (2 mu g/ml), ZnSO4 x 7 H2O (10 mu g/ ml) and glutamic acid (0.1%). Under these conditions, the production was in creased by 254.3% compared to the control. The EPS contained arabinose, xyl ose and rhamnose monomers. The presence of arabinose and xylose in the EPS produced by a Rhizobium sp. was uncommon.