Membrane H⁺ conductance of Streptococcus lactis

Membrane conductance to H⁺ was measured in the anaerobic bacterium S. lactis by a pulse technique employing a low driving force (0.1 pH unit; 6 mV). Over the pH range of 3.7 to 8.5, a constant value for passive H⁺ conductance was observed, corresponding to 0.2 μmol of H⁺/s per pH unit per g, dry weight (1.6 μS/cm² of surface area). The pH insensitivity of this low basal H⁺ conductance supports the idea that a circulation of protons can mediate highly efficient energy transductions across the membranes of bacteria.