Fundamental studies on the availability of oxygen from the decomposition of H2O2, in vivo, by Xanthomonas campestris, when H2O2 is used as an oxygen source are presented. It was found that the H2O2 added extracellularly (0.1-6 mM) was decomposed intracellularly. Further, when H2O2 was added, the flux of H2O2 into the cell, is regulated by the cell. The steady-state H2O2 flux into the cell was estimated to be 9.7 x 10(-8) mol m(-2) s(-1). In addition, it was proved that the regulation of H2O2 flux was coupled to the protonmotive force (PMF) using experiments with the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP), which disrupts PMF. The coupling constant between the rate of free energy availability from PMF and the rate of reduction of H2O2 flux, was found to be 46.4 mol m(-2) s(-1) J(-1) from simulations using a developed model. Also, the estimated periplasmic catalase concentration was 1.4 x 10(-9) M. (C) 2000 John Wiley & Sons, Inc.