Changes in the proton-motive force in Escherichia coli in response to external oxidoreduction potential

The pH homeostasis and proton-motive force (Delta p) of Escherichia coli are dependent on the surrounding oxidoreduction potential (ORP). Only the internal pH value and, thus, the membrane pH gradient (Delta pH) component of the Delta p is modified, while the membrane potential (Delta Psi) does not change in a significant way. Under reducing conditions (E-h < 50 mV at pH 7.0), E. coli decreases its Delta p especially in acidic media (21% decrease at pH 7.0 and 48% at pH 5.0 for a 850-mV ORP decrease). Measurements of ATPase activity and membrane proton conductance (C-m(H+)) depending on ORP and pH have shown that the internal pH decrease is due to an increase in membrane proton permeability without any modification of ATPase activity. We propose that low ORP values de-energize E. coli by modifying the thiol:disulfide balance of proteins, which leads to an increase in the membrane permeability to protons.