Polymyxin B induces transient permeability fluctuations in asymmetric planar lipopolysaccharide/phospholipid bilayers

The interaction of the polycationic decapeptide polymyxin B with asymmetric planar bilayers from lipopolysaccharide and phospholipid monolayers, which resemble the lipid matrix of the outer membrane of Gram-negative bacteria, was investigated. The addition of polymyxin B in micromolar amounts to the lipopolysaccharide side of the asymmetric bilayers resulted, under voltage-clamp conditions, in a fast macroscopic increase of their ionic conductance, whereas the polymyxin B nonapeptide induced no significant conductance changes. The polymyxin B induced macroscopic conductance exhibited large fluctuations and was strongly dependent on the amplitude and polarity of the transmembrane potential. The temporal pattern and amplitudes of the fluctuations were characterized by power spectra of the membrane currents and their variances, respectively. In the initial phase following peptide addition, the conductance changes appeared to be channellike discrete fluctuations. The lifetimes of the fluctuations were exponentially distributed, and the mean lifetimes were strongly voltage-dependent, ranging from approximately 30 ms at +80 mV (positive at the side opposite to peptide addition) to less than 5 ms at reverse polarity. The conductance amplitudes of the single fluctuations exhibited a broad distribution with a mean of 2 nS. A comparison of the features of the macroscopic conductance and of the discrete fluctuations showed that the former can basically be understood as a superposition of a large number of the latter. From the amplitudes of the fluctuations, the diameter of the polymyxin-induced lesions was estimated to about 3 nm. The experimental findings can be understood by assuming a detergent-like action of polymyxin B.