Mapping the ultrastructural topology of the corynebacterial cell surface

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Abstract

Corynebacterium glutamicum is a diderm bacterium extensively used in the industrial-scale production of amino acids. Corynebacteria belong to the bacterial family Mycobacteriaceae , which is characterized by a highly unusual cell envelope with an outer membrane consisting of mycolic acids. Despite the occurrence of this distinctive cell envelope in several bacterial pathogens, including Corynebacterium diphtheriae, Mycobacterium tuberculosis , and Mycobacterium leprae , its ultrastructural and molecular details remain elusive.

To address this, we investigated the cell envelope of C. glutamicum using electron cryotomography and cryomicroscopy of focused ion beam-milled cells. Our high-resolution images allowed us to accurately map the different components of the cell envelope into the tomographic density. Our data reveal that C. glutamicum has a variable cell envelope, with the outermost layer comprising the surface (S-)layer, which decorates the mycomembrane in a patchy manner. We further isolated and resolved the structure of the S-layer at 3.1 Å resolution using single particle electron cryomicroscopy. Our structure shows that the S-layer of C. glutamicum is composed of a hexagonal array of the PS2 protein, which interacts directly with the mycomembrane via a coiled coil-containing anchoring segment. Bioinformatic analyses revealed that the PS2 S-layer is sparsely yet exclusively present within the Corynebacterium genus and absent in other genera of the Mycobacteriaceae family, suggesting distinct evolutionary pathways in the development of their cell envelopes.

Our structural and cellular data collectively provide a high-resolution topography of the unusual C. glutamicum cell surface, features of which are shared by many pathogenic and microbiome-associated bacteria, as well as by several industrially significant bacterial species. This study, therefore, provides a strong experimental framework for understanding cell envelopes that contain mycolic acids.

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  1. Note: This response was posted by the corresponding author to Review Commons. The content has not been altered except for formatting.

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    Reply to the reviewers

    Response to reviewers’ comments for Isbilir et al

    We thank the reviewers for their insightful comments and advice. In light of the reviewers’ constructive suggestions, we have revised our manuscript as detailed below.

    Reviewer #1 (Evidence, reproducibility and clarity (Required)):

    Summary: In this manuscript, the authors investigate the unique Mycobacteriaceae cell envelope using cryo-tomography/cryo-electron microscopy with Corynebacterium glutamicum as a model organism. Cryo-EM images of C. glutamicum cells successfully resolved previously observed densities corresponding to the MM, arabinogalactan, peptidoglycan, and inner membrane layers of the cell …

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    Referee #3

    Evidence, reproducibility and clarity

    Summary:

    In the manuscript from Isbilir et al, the authors investigate the cell envelope of Corynebacterium glutamicum, a bacterium extensively used in biotechnological applications, using state-of-the-art cryo-electron microscopy methodologies as well as bioinformatics. They convincingly demonstrate that the C. glutamicum S-layer consists of hexagonal PS2 arrays and provide the underlying structural basis of this intriguing assembly. Bioinformatic analysis further revealed conserved and divergent elements of PS2 across Corynebacteria.

    Major comments:

    • My main point of criticism relates to the first part of the results, in which the …
  3. Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

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    Referee #2

    Evidence, reproducibility and clarity

    Corynebacterium glutamicum is an organism with important industrial application, and it shares its complex cell-envelop architecture with organism of great relevance in human health such Corynebacterium diphtheriae and pathogenic mycobacteria. Using a cryo-EM and cryo-ET approaches together with phylogenetic studies, the authors provide of an in-deep structural characterization of the cell envelop of C. glutamicum. The authors map the different components of the cell envelope using high-resolution tomography, revealing unseen details of the outer wall zone, previously unsolved and attributed to the AG molecule. They provide with an …

  4. Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

    Learn more at Review Commons


    Referee #1

    Evidence, reproducibility and clarity

    Summary:

    In this manuscript, the authors investigate the unique Mycobacteriaceae cell envelope using cryo-tomography/cryo-electron microscopy with Corynebacterium glutamicum as a model organism. Cryo-EM images of C. glutamicum cells successfully resolved previously observed densities corresponding to the MM, arabinogalactan, peptidoglycan, and inner membrane layers of the cell envelope along with the S-layer. The authors found that the S-layer is patchy in a manner dependent on growth phase (i.e. liquid versus solid growth). Intriguingly, when the S-layer was present, the leaflets of the MM appeared to be disrupted. The authors …