THE EFFECT OF ACID STRESS ON KEY ENZYME-ACTIVITIES AND GROWTH-KINETICS IN CULTURES OF XANTHOMONAS-CAMPESTRIS

Extracellular polysaccharide production by Xanthomonas campestris was studied under sublethal acid stress. Chemostat cultures were obtained at a dilution rate of 0.05 h(-1) and the normal feed was then suppleme nted with 2.5 mM acetic acid, 1.96 mM pentanoic acid or 1.39 mM octano ic acid. Undissociated acid fractions led to an appreciable loss of bi omass formation expressed by the drop in the biomass. X. campestris re sponded to sublethal acid stress by overproducing xanthan, and an incr ease in the specific xanthan production and xanthan yield was observed in all cases. The corresponding specific glucose consumption rate inc reased by a maximum of 45.2% with pentanoic acid. By adding feed conta ining 2.5 mM acetic acid, the culture biomass immediately started to d ecrease, falling from 0.145 to 0.103 g litre(-1) h(-1), whereas the sp ecific xanthan production rate was stimulated from 0.282 g g(-1) cells h(-1) to a maximum 0.325 g g(-1) cells h(-1) Similar behaviour was ob served in the pentanoic acid and octanoic acid fermentations. Enhanced activities of the pentose phosphate cycle and xanthan anabolic reacti ons were observed after acid additions. (C) 1998 Elsevier Science Ltd. All rights reserved.