Influence of growth conditions on production of capsular and extracellular polysaccharides by Rhizobium leguminosarum.

The influence of growth rate and medium composition on exopolymer production by Rhizobium leguminosarum was studied. When grown in medium containing 10 g/l mannitol and 1 g/l glutamic acid, Rhizobium leguminosarum biovar trifolii TA-1 synthesized up to 2.0 g/l of extracellular polysaccharide (EPS), and up to 1.6 g/l of capsular polysaccharide (CPS). Under non-growing cell conditions in medium without glutamic acid, CPS synthesis by strain TA-1 could proceed to 2.1 g/l, while EPS-production remained relatively low (0.8 g/l). Maximal CPS-yield was 2.9 g CPS/l medium in a medium containing 20 g/l mannitol and 2 g/l glutamic acid. The EPS-deficient strain R. leguminosarum RBL5515, exo4::Tn5 was able to produce CPS to similar levels as strain TA-1, but CPS-recovery was easier because of the low viscosity of the medium and growth of the cells in pellets. With strain TA-1 in nitrogen-limited continuous cultures with a constant biomass of 500 mg cell protein/l, EPS was the most abundant polysaccharide present at every dilution rate D (between 0.12 and 0.02 h-1). The production rates were 50-100 mg/g protein/h for EPS and 15-20 mg/g protein/h for CPS. Only low amounts of cyclic beta-(1,2)-glucans were excreted (10-30 mg/l) over the entire range of growth rates.