Identification of Environmental Factors Critical to the Production of Exopolysaccharides by Rhizobium tropici

Rhizobium tropici, a legume-symbiont soil bacterium, is known for its copious production of exopolysaccharide (EPS) material. However, many aspects of this organism's growth and EPS production remain uncharacterized, including the influence of environment and culturing conditions upon the growth of R. tropici and the chemical structure of EPS material being formed. We hypothesized that R. tropici EPS yield and chemical structure would change when R. tropici was grown with different organic carbon compounds. We characterized the EPS from R. tropici grown in batch culture in a liquid minimal salts medium using arabinose, glucose, sucrose, mannitol, fructose, or glutamate as the sole carbon source. A soluble EPS extraction procedure was developed involving centrifugal filtration of growth media followed by ethanol precipitation of the supernatant liquid. This EPS product was quantified and characterized. Changes to the carbon source were found to significantly impact EPS production, in terms of yield as well as EPS chemical composition. R. tropici's growth environment was further optimized for EPS production by examining the roles of pH and carbonitrogen ratio, both which were found to impact EPS production. A maximum EPS yield of 4.08 g/L was realized under the determined optimal conditions. Nitrogenase activity was also investigated.