Effect of cell membrane-active antimicrobial agents on membrane potential and viability of mycoplasmas

Abstract

The membrane potential ofMycoplasma mycoides subsp.capri has been determined to beEM=−48 mV±10%, inside negative. In this study we investigated the influence of cell membrane-active antimicrobial agents, viz., valinomycin, gramicidin, polymyxin, and clotrimazole, on membrane potential and viability ofM. mycoides subsp.capri. Valinomycin, an ionophore with extreme potassium selectivity, induced a membrane hyperpolarization,EM=−110 mV. Valinomycin was not cidal, but static to mycoplasmas. Obviously the potassium drain induced by valinomycin can be compensated for by the organisms. Gramicidin is an antibiotic forming cation conduction channels across membranes. It induced a rapid depolarization,EM=+23 mV, of mycoplasma membranes. At low concentrations, gramicidin had a static effect, whereas at high concentrations it was cidal to mycoplasmas. The rapid permeation of cations through the stationary ion channels formed by gramicidin obviously exerts an inhibitory or even lethal effect on mycoplasma metabolism and growth. Polymyxin B induced a depolarization,EM=−35 mV, of mycoplasma membranes only when the organisms had been pretreated and hyperpolarized with valinomycin. After treatment with both valinomycin and polymyxin B, a slight inhibition of mycoplasma growth was observed. Clotrimazole, a synthetic imidazole antimycotic, hyperpolarized mycoplasma membranes (EM=−80 mV). At high concentrations clotrimazole was cidal, whereas at low concentrations it was static to mycoplasmas.