Antibiotics and hexagonal order in the bacterial outer membrane

Abstract

Polymyxin is a last-resort antibiotic that targets Gram-negative bacteria. It does so by binding lipopolysaccharide (LPS) in the outer leaflet of the outer membrane by an ill-defined mechanism. Recently, ref. 1 used atomic force microscopy (AFM) to image outer membrane vesicles ruptured and flattened onto mica in the presence of cations. They then added polymyxins, which resulted in the appearance of a hexagonal lattice in the membrane (Fig. 1A ). They attributed this lattice to LPS-polymyxin crystallisation and suggested that such lattices are relevant for the mechanism of action by which polymyxins induce bacterial cell death. Fig. 1: Comparison of ordered configurations in the outer membrane, as imaged by AFM. A When polymyxin was applied to purified outer membranes from MG1655 E. coli , ref. 1 saw a hexagonal lattice with a spacing of 9.8 ± 3.9 nm. B Benn et al. 7 showed that the native, untreated outer membrane of living MG1655 E. coli was dominated by trimeric OMPs arranged in an imperfect hexagonal lattice, with a nearest-neighbour distance of 9.4 ± 2.1 nm. C Jaroslawski et al. 4 observed hexagonal arrangements of trimeric porins with a lattice constant of 8.1 ± 0.3 nm in purified outer membranes from R. Denitrificans . D Oestreicher et al. 6 showed hexagonal lattices (spacing 10.5 ± 1.7 nm) on the surface of living E. coli , which they attributed to trimeric OMPs. E Comparison of lattice-spacing distributions for polymyxin-induced hexagons 1 and for OMP lattices in native (untreated) MG1655 E. coli 7 , showing no significant difference by a two-sided t test with unequal variance ( p = 0.16). Values are mean ± SD. Scale bar is 20 nm. Images A-D were reproduced from references as indicated, with permission. Full size image

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