The membrane potential in a cytochrome-deficient species of Bacteroides: its magnitude and mode of generation

SUMMARY: The lipophilic cations butyltriphenylphosphonium (BTPP+), tetraphenylphosphonium (TPP+) and triphenylmethylphosphonium (TPMP+) were taken up into cells of Bacteroides amylophilus H18 under anaerobic conditions. Uptake was dependent on the presence of maltose together with both HCO- 3 and Na+; it was at a maximum at concentrations of ≥ 20 mM-HCO- 3 and ≤ 2 mM-Na+. The addition of 2-(n-heptyl)-hydroxyquinoline-N-oxide (HpHOQnO) or of an uncoupler of oxidative phosphorylation, or air, resulted in efflux of the lipophilic cations. From the binding behaviour and the physiological effects of the lipophilic cations, a membrane potential (δ ψ) of 140 mV was estimated. There was no detectable δpH at an external pH of 6.7. The cytoplasmic Na+concentration was estimated to be 0.2 mM, indicating that B. amylophilus can maintain a Na+concentration gradient equivalent to at least 150 mV. Variation in the external Na+concentration (2 to 180 mM) had little influence on δ ψ. High external concentrations of the fermentation products acetate, formate and succinate had little effect on the growth rate and the δ ψ. The cytoplasmic ATP concentration decreased rapidly on addition of HpHOQnO oxide or of an uncoupler. The maximum internal ATP concentration was only maintained at an external concentration≥2mM-Na+. The NADH: fumarate reductase activity of vesicles of B. amylophilus was associated with alkalization of the suspension medium. The amount of H+taken up was in excess of the expected amount of scalar H+and was partially sensitive to uncoupler. It is concluded that the δ ψ was generated via H+translocation driven primarily by the cytochrome-deficient NADH: fumarate reductase system. The transmembrane Na+gradient could be supported via the action of a Na+/2 H+antiporter.