PRODUCTION OF XANTHAN BY IN-FLOW CULTURES OF XANTHOMONAS-CAMPESTRIS

The kinetics of xanthan formation in Xanthomonas campestris continuous and fed-batch fermentations was studied along with metabolic changes due to growth rate variation. A maximum growth rate within the range 0.11-0. 12 h-1 was obtained from the continuous culture data in defined medium, producing xanthan at rates up to 0. 36 g l-1 h-1 corresponding to a maximum 67% glucose conversion at a dilution rate (D) of 0.05 h-1. Comparatively, fed-batch cultivation was more efficient, producing maximum xanthan at 0. 75 g l-1 h-1 and 63% glucose conversion at 0. 1 h-1. When reaching D = 0. 062 h-1 in continuous cultures, a change was observed and the values of the specific rate of substrate consumption shifted, initiating an uncoupled growth region expressing a lack of balance of the catabolic and anabolic reactions. The deviation was not accompanied by a change in specific xanthan production indicating that xanthan metabolism was not affected by D. For fed-batch-grown X. campestris cells within the range D = 0.03-0.1 h-1, both metabolic parameters changed linearly with the growth rate showing a wide region coupled to growth. Outside that range, glucose accumulated and the specific xanthan production dropped, suggesting substrate inhibition.