Disclosure of the mycobacterial outer membrane: Cryo-electron tomography and vitreous sections reveal the lipid bilayer structure

TLDR

Mycobacterial cell walls form a permeability barrier essential for virulence, composed of extractable lipids and long‑chain mycolic acids covalently linked to peptidoglycan via an arabinogalactan network, and were long thought to be an asymmetrical bilayer whose structure had never been directly visualized. The study proposes and discusses models of the mycobacterial outer membrane structure. The authors employ cryo‑electron tomography and vitreous sectioning to investigate the outer membrane and generate these models. Cryo‑electron tomography and vitreous sectioning of M.

Abstract

The cell walls of mycobacteria form an exceptional permeability barrier, and they are essential for virulence. They contain extractable lipids and long-chain mycolic acids that are covalently linked to peptidoglycan via an arabinogalactan network. The lipids were thought to form an asymmetrical bilayer of considerable thickness, but this could never be proven directly by microscopy or other means. Cryo-electron tomography of unperturbed or detergent-treated cells of Mycobacterium smegmatis embedded in vitreous ice now reveals the native organization of the cell envelope and its delineation into several distinct layers. The 3D data and the investigation of ultrathin frozen-hydrated cryosections of M. smegmatis, Myobacterium bovis bacillus Calmette-Guérin, and Corynebacterium glutamicum identified the outermost layer as a morphologically symmetrical lipid bilayer. The structure of the mycobacterial outer membrane necessitates considerable revision of the current view of its architecture. Conceivable models are proposed and discussed. These results are crucial for the investigation and understanding of transport processes across the mycobacterial cell wall, and they are of particular medical relevance in the case of pathogenic mycobacteria.