Effect of Growth Conditions on the Production , Composition and Viscosity of Xanthomonas campestris Exopolysaccharide

A streptomycin-resistant variant of Xanthomonas campestris was grown in defined nutrient-deficient media in both batch and continuous culture. The production, composition and viscosity of the extracellular polysaccharide (xanthan) synthesized by this strain were influenced by the fermentation time and nutrient exhaustion in batch culture and by the dilution rate in continuous culture. The specific rate of exopolysaccharide synthesis was maximal during exponential growth in all of the nutrient-deficient media studied although some xanthan was formed during stationary phase. The concentration of exopolysaccharide decreased at later stages of stationary phase in some cultures. Both the extent of acylation and the consistency index of xanthan isolates were low or minimal during exponential growth, maximal in polysaccharide isolated as the growth rate fell and usually lower after this time. Between dilution rates of 0.03 and 0.06 h-' in chemostat culture, the cell and exopolysaccharide dry weights were independent of dilution rate, the specific rate of xanthan synthesis decreasing at lower growth rates. Although the variation in the acyl content and consistency index was less than that observed in batch culture, exopolysaccharide isolated at higher dilution rates tended to have a higher acetyl content, lower pyruvyl content and lower consistency index.