Kinetic and metabolic effects of nitrogen, magnesium and sulphur restriction in Xanthomonas campestris batch cultures

Abstract

By reducing the concentration of nitrogen (from 5.0 to 2.5 mmol 1‐1), batch cultures of Xanthomonas campestris induced the enzyme UDP‐glucose dehydrogenase and stimulated the Entner‐Doudoroff pathway enzyme glucose‐6‐P dehydrogenase. The surplus energy generation was directed to xanthan biosynthesis resulting in a 10% polysaccharide increase. The nitrogen restriction led to a higher consumption of nitrogen (93%) whereas glucose consumption did not surpass 75% utilization. Low concentrations of both magnesium and sulphur exerted a negative effect on xanthan formation. Both restrictions reduced the phosphomannose isomerase enzyme activity by 10‐fold turning the mannose transference presumably into the rate‐limiting step for xanthan biosynthesis. Conversely, the rate of synthesis of glucuronic acid residues did not affect the rate of xanthan biosynthesis. Polysaccharide synthesis in magnesium and sulphur cultures was negatively affected in comparison with cell formation as the cell volumetric production rate increased from 0.037 to 0.091 g 1‐1 h‐1 and the xanthan volumetric production rate dropped from 0.133 g 1‐1 h‐1 to the minimum obtained at 0.083 g 1‐1 h‐1. The efficiency of the carbon substrate conversion was also greatly changed.