Accumulation of gentamicin by Staphylococcus aureus: the role of the transmembrane electrical potential

Accumulation of gentamicin by gentamicin-susceptible Staphylococcus aureus was examined by using the ionophorous antibiotic, valinomycin and the protontranslocating ATPase inhibitor N,N'-dicyclohexylcarbodiimide. The effects of these inhibitors on the transmembrane electrical potential (delta psi) were determined by measuring the equilibrium distribution of the tetraphenylphosphonium ion. The results indicate a direct correlation between delta psi and the extent of gentamicin uptake. However, under conditions where a significant delta psi existed across the plasma membrane, uptake of gentamicin was negligible. A threshold delta psi may thus be required to initiate gentamicin uptake. The proposed threshold delta psi appears to vary depending upon the external concentration of gentamicin.