Effects of sodium ions on the electrical and pH gradients across the membrane of Streptococcus lactis cells.

Energized cells of Streptococcus lactis conserve and transduce energy at the plasma membrane in the form of an electrochemical gradient of hydrogen ions (deltap). An increase in energy-consuming processes, such as cation transport, would be expected to result in a change in the steady state deltap. We determined the electrical gradient (deltapsi) from the fluorescence of a membrane potential-sensitive cyanine dye, and the chemical H+ gradient (deltaph) from the distribution of a weak acid. In glycolyzing cells incubated at pH5 the addition of NaCl to 200 mM partially dissipated the deltap by decreasing deltapsi, while the delta pH was constant. The deltap was also determined independently from the accumulation levels of thiomethyl-beta-galactoside. The deltap values decreased in cells fermenting glucose at pH 5 or pH 7 when NaCl was added, while the deltapH values were unaffected; cells fermenting arginine at pH 7 showed similar effects. Thus, these nongrowing cells cannot fully compensate for the energy demand of cation transport.