Proton motive force in growing Streptococcus lactis and Staphylococcus aureus cells under aerobic and anaerobic conditions

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Abstract
Measurements of the electrochemical gradient of H+, which gives rise to the proton motive force (PMF), were carried out with growing S. lactis and S. aureus cells. The facultative anaerobe was chosen to compare the PMF of cells growning aerobically and anaerobically. During aerobic growth the cells would presumably have a higher PMF than during anerobic growth, because the H+-translocating ATPase (BF0F1) operates in the direction of H+ influx and ATP synthesis during respiration. Under anaerobic conditions, the BF0F1 hydrolyzes glycolytically generated ATP and establishes the proton gradient by extruding H+. The electrical component of the PMF, .DELTA..psi., and the chemical gradient of H+, .DELTA.pH, were measured with radiolabeled tetraphenylphosphonium and benzoate ions. In S. lactis and S. aureus cells, the PMF was constant during the exponential phase of batch growth and decreased in the stationary phase. In both species of bacteria, the exponential-phase PMF was not affected by varying the growth rate by adding different sugars to the medium. The relative contributions of .DELTA..psi. and .DELTA.pH to the PMF depended on the pH of the medium. The internal pH of S. aureus was constant at pH 7.4-7.6 under all conditions of growth tested. Under aerobic conditions, the .DELTA..psi. of exponential phase S. aureus remained fairly constant at 160-170 mV. The PMF was 250-270 mV in cells gowing aerobically in media at pH 6 and progressively lower in media of higher pH, reaching 195-205 mV at pH 7. Under anaerobic conditions, the .DELTA..psi. ranged from 100-120 mV in cells at pH 6.3-7, resulting in a PMF of 150-140 mV. The mode of energy metabolism (i.e., respiration vs. fermentation) and the pH of the medium are the 2 important factors influencing the PMF of these gram-positive cells during growth.