Variations in exopolysaccharide production by Rhizobium tropici.

Rhizobium tropici, a legume-symbiont soil bacterium, is known for its copious production of exopolysaccharide (EPS). Many aspects of this organism's growth and EPS production, however, remain uncharacterized, including the influence of environment and culturing conditions upon EPS. Here, we demonstrate that R. tropici EPS chemical composition and yield differ when grown with different substrates in a defined minimal medium in batch culture. Exopolysaccharide was quantified from R. tropici grown using arabinose, glucose, sucrose, mannitol, fructose, or glutamate as a sole carbon source. All tested substrates produced plenteous amounts of exopolysaccharide material. Variations in pH and carbon-to-nitrogen (C/N) ratio also resulted in assorted cell growth and exopolysaccharide production differences. We found that optimizing the C/N ratio has a greater impact upon R. tropici EPS production than upon R. tropici growth. A maximum EPS yield of 4.08 g/L was realized under optimized conditions, which is large even in comparison with other known rhizobia. We provide evidence that the chemical composition of R. tropici EPS can vary with changes to the growth environment. The composition of glucose-grown EPS contained rhamnose-linked residues that were not present in arabinose-grown EPS.