Sublethal acid stress and uncoupling effects on cell growth and product formation in Xanthomonas campestris cultures

The mechanism of cell adaptation to acid stress based on the assessment of proton translocating membrane ATPase titers on Xanthamonas campestris slow-growing O-2-limited cultures was studied. ATPase activity was assessed at the beginning of the exponential phase as average 29.6 mug mg(-1) min(-1). After the addition of 0.15 mM of the acid (neutral form) to the culture the ATPase activity increased steadily reaching a maximum of 40.1 mug mg(-1) min(-1) after 1 h. In parallel, the concentration of ATP decreased from 2.8 to 2.1 muM after 60 min. The specific growth rate dropped from an average of 0.34 h(-1) to an average of 0.28 h(-1) in the cultures to which acid was added. In parallel, the average rate of xanthan production increased 48% varying from 0.148 to 0.219 gl(-1) h(-1) and the glucose utilisation rate varied from 0.526 to 0.492 gl(-1) h(-1) corresponding to an average decrease of 6.5%. The excess of oxygen seems to stop the action of weak acids on X. campestris growth. In fact, pulses of these compounds at the end of the exponential phase, in presence of high aeration, do not stop growth, instead they speed up cell growth after a period of adaptation reaching stationary phase with high levels of biomass. (C) 2002 Elsevier Science B.V. All rights reserved.