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 FPS-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/1) over the entire range of growth rates.