At pH 5.0, the electrical potential (A', interior negative) across the plasma membrane of Staphylococcus aureus exhibits a minimum of -85 to -90 mV; the pH gradient (ApH, interior alkaline) across the membrane approximates a maximum of about -100 mV. Under these conditions, uptake of the aminoglycoside gentamicin is negligible, and viability of the organism is not impaired by the antibiotic. In contrast, at pH 7.5, at which At is about -130 mV and ApH is 0, gentamicin uptake is observed and the drug markedly decreases viability. Dramatically, when the ionophore nigericin is added at pH 5.0,.gentamicin uptake is induced, there is a striding decrease in viability, and. the effect is associated with an increase inAl' at the expense ofApH. Consistently, valinomycin, which dissipates A' in the presence ofpotassium, abolishes gentamicin uptake andkilling. In addition, from pH 5.0 to pH 7.5, there is a direct relationship between the magnitude ofAl and both gentamicin.uptake and its bactericidal effect. However, a threshold A' of -75 to -90 mV is apparently necessary to initiate uptake and killing. These observations provide a strong indication that Al plays a critical role in the uptake and antibacterial action of gentamicin and. suggest that nigericinlike ionophores may be clinically useful in synergy with aminoglycosides.The aminoglycoside antibiotics (e.g., streptomycin and gentamicin) are transported across the bacterial cytoplasmic membrane prior to inhibition ofprotein synthesis, and there appears to be an association between antibiotic susceptibility and the rate and magnitude of drug accumulation (1-6, f). Although aminoglycoside uptake as described in Escherichia coli (1)(2)(3)(4)(5) and Staphylococcus aureus (6, f) is a complex phenomenon, the lethal effect of these antibiotics is associated with an aminoglycoside-induced accelerated phase of uptake. Mutations affecting ribosomal binding (4, 7, ), the bacterial respiratory chain, and the H+-ATPase (5) all abolish this rapid uptake phase. In addition, strains that produce plasmid-mediated aminoglycosidemodifying enzymes (8, 9) also exhibit defective drug accumulation.According to the chemiosmotic hypothesis (10, 11), proton extrusion during respiration or ATP hydrolysis leads to the generation of a transmembrane electrochemical gradient of hydrogen ions (Aj!H+)l that is the immediate driving force for many biological processes, and recent studies (6, h) suggest that A/.H+ may be important for aminoglycoside uptake. The proton electrochemical gradient is composed ofelectrical and chemical parameters according to the following relationship: AjiH+ = AT-ZApH [1] in which AT represents the electrical potential across the plasma membrane and ApH is the transmembrane difference in H' concentration (Z is equal to 58.8 at room temperature).With regard to active transport, the chemiosmotic hypothesis predicts that transport is driven by AT (interior negative) for cationic substrates, by ApH (interior alkaline) for anionic substrates, and by A-gH+ for neutral s...

The electrical potential (g, 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, A4 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, A4 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, A* 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 A4 and gentamicin uptake is gated, and suggest that diminution of A4s may be an important factor in aminoglycoside resistance under anaerobic conditions.

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