Escherichia coli membrane proton conductance and proton efflux depend on growth pH and are sensitive to osmotic stress

Abstract

The dependence of Escherichia coli membrane H⁺ conductance (G_m ^H+) with a steady-state pH in the presence and absence of an external source of energy (glucose) was studied, when cells were grown under anaerobic and aerobic conditions, with an assay pH of 7.0. Energy-dependent H⁺ efflux by intact cells growing at pH of 4.5–7.5 was also measured. The elevated H⁺ conductance and lowered H⁺ flux were shown for cells growing in acidic pH and under anaerobic conditions, when bacteria were fermenting glucose. The atp mutant, which is deprived of the F₀F₁-adenosine triphosphatase, had less G_m ^H+ independent of growth conditions. In contrast with wild-type or precursor strain, a remarkable difference in G_m ^H+ for atp mutant was observed between aerobic and anerobic conditions; such a difference was significant at pH 4.5. These results could indicate distinguishing pathways determining G_m ^H+ under anaerobic conditions after the fermentation of glucose at different pH and an input of the F₀F₁-adenosine triphosphatase in G_m ^H+. In addition, the effect of osmotic stress was demonstrated with grown cells. G_m ^H+ and H⁺ efflux both were increased after hyperosmotic stress at pH 7.5, and these changes were inhibited by N,N′-dicyclohexylcarbodiimide, whereas these changes were lower in atp mutant. A role of the F₀F₁-adenosine triphosphatase in osmo-sensitivity of bacteria was confirmed under fermentative conditions.