Proton motive force and the physiological basis of delta pH maintenance in thiobacillus acidophilus

At optimal growth pH (3.0) Thiobacillus acidophilus maintained an internal pH of 5.6 (delta pH of 2.6 units) and a membrane potential (delta psi) of some +73 mV, corresponding to a proton motive force (delta p) of -83 mV. The internal pH remained poised at this value through external pH values of 1 to 5, so that the delta pH increased with decreasing external pH. The positive delta psi increased linearly with delta pH: above a delta pH of 0.6 units, some 60% of the increase in delta pH was compensated for by an opposing increase in delta psi. The highest magnitude of delta pH occurred at an external pH of 1.0, where the cells could not respire. Inhibiting respiration by CN- or azide in cells at optimal pH decreased delta pH by only 0.4 to 0.5 units and caused a corresponding opposite increase in delta psi. Thus, a sizable delta pH could be maintained in the complete absence of respiration. Treatment of cells with thiocyanate to abolish the delta psi resulted in a time-dependent collapse of delta pH, which was augmented by protonophores. We postulate that T. acidophilus possesses unusual resistance to ionic movements. In the presence of a large delta pH (greater than 0.6 pH units), limited diffusion of H+ into the cell is permitted, which generates a positive delta psi because of resistance to compensatory ionic movements. This delta psi, by undergoing fluctuations, regulates the further entry of H+ into the cell in accordance with the metabolic state of the organism. The effect of protonophores was anomalous: the delta p was only partially collapsed, and respiration was strongly inhibited. Possible reasons for this are discussed.