Membrane potential in anaerobically growing Staphylococcus aureus and its relationship to gentamicin uptake

The electrical potential (delta psi) across the cytoplasmic membranes of Staphylococcus aureus cells growing under aerobic and anaerobic conditions was determined by measuring the equilibrium distribution of [3H]tetraphenyl phosphonium. In conjunction, gentamicin uptake and killing were studied in the same cells under identical conditions. Under aerobic conditions, delta psi was -169 mV, gentamicin uptake was readily demonstrable, and the number of viable cells decreased by almost four orders of magnitude in the presence of antibiotic. In contrast, delta psi was -142 mV anaerobically, gentamicin uptake was essentially nonexistent, and the aminoglycoside had no effect on viability. Remarkably, when the ionophore nigericin was added under anaerobic conditions, delta psi increased to the level observed aerobically, gentamicin uptake tripled to about 18% of the aerobic level, and viability decreased by one order of magnitude. The results are consistent with other observations (Mates et al., Proc. Natl. Acad. Sci. U.S.A. 79:6693-6697, 1982), indicating that the relationship between delta psi and gentamicin uptake is gated, and suggest that diminution of delta psi may be an important factor in aminoglycoside resistance under anaerobic conditions.