arising from S. Manioglu et al. Nature Communications https://doi.org/10.1038/s41467-022-33838-0 (2022)
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.
Previous electron microscopy and AFM studies have shown that trimeric outer membrane proteins (OMPs) also pack together in hexagonal lattices, as observed in reconstituted 2D arrays2,3, in purified outer membranes4 and in living bacteria5,6,7 (Fig. 1B–D). These OMP arrays have lattice constants virtually identical to those seen for the polymyxin-induced hexagons reported by Manioglu et al. (Fig. 1E).
Despite this similarity, Manioglu et al. proposed that polymyxin arranges LPS into such hexagonal arrays independent of OMP content of the membrane1. To substantiate this hypothesis, they showed that polymyxin still formed hexagonal arrays in patches derived from outer membrane vesicles from BL21 (DE3) omp8 E. coli cells that did not express the primary trimeric porins OmpF, OmpC, and LamB, or the monomeric OmpA, but were enriched in other OMPs8. However, we now know that many OMPs9, not just trimeric porins, have a propensity to form heterogenous clusters in the presence of LPS. More extensive negative controls would be needed to dismiss OMPs as the basis for the hexagonal lattices that are observed following polymyxin treatment of outer membrane vesicles.
Conversely, the polymyxin-induced lattice was affected by LPS length and cation concentration, from which Manioglu et al. concluded that the lattice was determined only by polymyxin–LPS complexes1, aided by the tendency of LPS molecules to form crystalline domains. They cite LPS crystallinity seen in molecular dynamics and in model membranes10,11,12, to suggest that the polymyxin-induced hexagonal geometry is related to LPS packing. However, the hexagonal LPS arrays referenced have lattice constants of approximately one order of magnitude smaller11 than the arrays seen by ref. 1.
Based on their observations, Manioglu et al. conclude with the suggestion that “a local ordering of LPS by polymyxin must lie at the core of the [hexagonal] arrangement”1. In contrast, based on the observations above, we propose an alternative conclusion, assigning the observed hexagonal order to the symmetry of OMP networks in the untreated E. coli membrane7. The OMP–OMP interactions in such networks are mediated by LPS9, which is thereby expected to follow similar local ordering. Hence in our interpretation, the polymyxin does not order the LPS, but binds to the already ordered LPS and thereby reveals existing OMP lattices at a higher contrast than is the case for the untreated membranes.
References
Manioglu, S. et al. Antibiotic polymyxin arranges lipopolysaccharide into crystalline structures to solidify the bacterial membrane. Nat. Commun. 13, 6195 (2022).
Dorset, D. L., Engel, A., Häner, M., Massalski, A. & Rosenbusch, J. P. Two-dimensional crystal packing of matrix porin. J. Mol. Biol. 165, 701–710 (1983).
Schabert, F. A., Henn, C. & Engel, A. Native Escherichia coli OmpF porin surfaces probed by atomic force microscopy. Science 268, 92–94 (1995).
Jaroslawski, S., Duquesne, K., Sturgis, J. N. & Scheuring, S. High-resolution architecture of the outer membrane of the Gram-negative bacteria Roseobacter denitrificans. Mol. Microbiol. 74, 1211–1222 (2009).
Yamashita, H. et al. Single-molecule imaging on living bacterial cell surface by high-speed AFM. J. Mol. Biol. 422, 300–309 (2012).
Oestreicher, Z., Taoka, A. & Fukumori, Y. A comparison of the surface nanostructure from two different types of gram-negative cells: Escherichia coli and Rhodobacter sphaeroides. Micron 72, 8–14 (2015).
Benn, G. et al. Phase separation in the outer membrane of Escherichia coli. Proc. Natl Acad. Sci. 118, e2112237118 (2021).
Thoma, J. et al. Protein-enriched outer membrane vesicles as a native platform for outer membrane protein studies. Commun. Biol. 1, 23 (2018).
Webby, M. N. et al. Lipids mediate supramolecular outer membrane protein assembly in bacteria. Sci. Adv. 8, eadc9566 (2022).
Jefferies, D., Hsu, P.-C. & Khalid, S. Through the lipopolysaccharide glass: a potent antimicrobial peptide induces phase changes in membranes. Biochemistry 56, 1672–1679 (2017).
Le Brun, A. P. et al. Structural characterization of a model gram-negative bacterial surface using lipopolysaccharides from rough strains of Escherichia coli. Biomacromolecules 14, 2014–2022 (2013).
Paracini, N., Clifton, L. A., Skoda, M. W. A. & Lakey, J. H. Liquid crystalline bacterial outer membranes are critical for antibiotic susceptibility. Proc. Natl Acad. Sci. USA 115, E7587–E7594 (2018).
Acknowledgements
The authors acknowledge funding by the UK Engineering and Physical Sciences Research Council (EP/N509577/1 to G.B. and B.W.H.), the National Institute of General Medical Sciences of the NIH (R35- GM118024 to T.J.S.), the UK Biotechnology and Biological Sciences Research Council (BB/V008056/1 to C.K. and BB/R000042/1 to B.W.H.), Wellcome Trust Collaborative Award (201505/Z/16/Z to C.K.), and the European Research Council (Advanced Grant 742555, OMPorg, to C.K.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH or other funders.
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Benn, G., Silhavy, T.J., Kleanthous, C. et al. Antibiotics and hexagonal order in the bacterial outer membrane. Nat Commun 14, 4772 (2023). https://doi.org/10.1038/s41467-023-40275-0
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DOI: https://doi.org/10.1038/s41467-023-40275-0
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