GENERAL COMMENTARY article

Front. Chem., 20 October 2025

Sec. Chemical Biology

Volume 13 - 2025 | https://doi.org/10.3389/fchem.2025.1674350

Commentary: Electron transport across the cell envelope via multiheme c-type cytochromes in Geobacter sulfurreducens

  • 1. Electrobiomaterials Institute, Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, China

  • 2. Department of Microbiology, University of Massachusetts, Amherst, MA, United States

This article is a commentary on:

  1. Electron Transport across the Cell Envelope via Multiheme c-Type Cytochromes in Geobacter sulfurreducens

    1. Read original article

A commentary on Electron transport across the cell envelope via multiheme c-type cytochromes in Geobacter sulfurreducens by Tabari, M.Z., and Hochbaum, A.I. (2025). Front. Chem. doi: 10.3389/fchem.2025.1621274

Introduction

In their review (), Tabari and Hochbaum selectively cite a few references to make the claim that the predominant electrically conductive filaments emanating from Geobacter sulfurreducens are cytochrome filaments that are required for long-range extracellular electron transfer (EET). This commentary briefly outlines the extensive literature that Tabari and Hochbaum omitted that contradicts their claims.

Filament identification

Tabari and Hochbaum assert that the extracellular filaments originally thought () to be electrically conductive pili (e-pili) “are now widely accepted to be networks of MHCs [multi-heme c-type cytochromes]” and that “Evidence supporting this model and its reconciliation with previously published data have been extensively reviewed elsewhere.” However, all of the studies and reviews cited focus on cryo-electron microscopy. Preparing samples for cryo-electron microscopy requires many processing steps that can specifically enrich for cytochrome filaments (). Neither Tabari and Hochbaum nor the reviews they cite acknowledge published alternative experimental approaches specifically designed to avoid processing artifacts that demonstrate that there are abundant e-pili as well as cytochrome filaments emanating from cells.

For example, e-pili, not cytochrome filaments predominate when G. sulfurreducens is examined with methods that avoid sample pretreatment (; ; ) (see Figure 1A for an example). In those studies, cells were grown with fumarate as the electron acceptor, one of the same growth conditions in what Tabari and Hochbaum claim was the “landmark study…demonstrating that the conductive extracellular nanowires are, in fact, composed of polymerized OmcS.” A small drop of live culture was directly drop-cast onto a flat conductive surface (; ; ). There was no fixation. The cells remained hydrated while they were examined with high resolution atomic force microscopy (AFM).

FIGURE 1

). Inset table: abundance and conductance data for the wild-type strain and strain Aro-5, in which the wild-type pilin gene was replaced with a pilin gene modified to yield pili with lower conductivity (Data from ). (B) Data () demonstrating that strains in which the genes for one, two, or all three of the filament-forming cytochromes were deleted effectively reduced Fe(III) oxide to Fe(II). In contrast, Fe(III) oxide reduction is impaired in strains expressing a gene that yield poorly conductive pili. (C,D) Transmission electron micrographs () of immunogold labelling of cells demonstrating that when a pilin gene that encodes a his-tag at the carboxyl end of the pilin is introduced the filaments emanating from cells contain the his-tag, providing further evidence that the filaments are comprised of pilin. (E) Data from current-producing biofilms () demonstrating that deletion of genes for the filament-forming cytochromes OmcS and OmcE increased biofilm conductivity and that there was no correlation between the abundance of OmcZ and biofilm conductivity. Abbreviations: KN400, strain of G. sulfurreducens selected for superior current production; BEST, strain in which the genes for OmcB, OmcE, OmcS, and OmcT were deleted; ST, strain in which the genes for OmcS and OmcT were deleted; WT, wild-type strain; WTF, wild-type strain grown on electrodes, but with fumarate as the electron acceptor. (F) Data () comparing the conductivity of biofilms of various strains and demonstrating that a greater abundance of the pilin protein, PilA, is associated with higher biofilm conductivity. Abbreviations the same as in panel (E). Control designates system with no cells. (G) Data () demonstrating that substituting a pilin gene that yields pili with lower conductivity results in a strain, Aro-5, that produces much less current than the parental strain and that the conductivity of the Aro-5 biofilms are greatly reduced compared to the parental strain. (H) Data () demonstrating that modifying the G. sulfurreducens pilin gene heterologously expressed in E. coli to contain a short carboxyl peptide designed to bind ammonia substantially increases the electronic response of purified nanowires harvested from E. coli. This response further demonstrates that E. coli assembles the G. sulfurreducens pilin into e-pili. All images reproduced with permission.

AFM revealed (Figure 1A) that ca. 10% of filaments emanating from wild-type G. sulfurreducens had the 4 nm diameter and unique longitudinal morphology characteristic of OmcS filaments (). Conductive AFM tip analysis demonstrated conductance through the filaments to the underlying surface, consistent with the proposed conductivity of OmcS filaments. The remaining ca. 90% of the filaments (Figure 1A) had the same 3 nm diameter, morphology, and conductance () as the filaments that E. coli produces when heterologously expressing the G. sulfurreducens pilin gene, PilA (). These 3 nm diameter conductive filaments were the only filaments observed when the genes of all three of the known filament-forming cytochromes were deleted (). Thus, when processing artifacts were avoided, cytochrome filaments were a minor component of the overall G. sulfurreducens nanowire complement. The vast majority of the nanowires had the same properties as e-pili.

When the wild-type pilin gene was replaced with a modified pilin gene known to yield less conductive pili when expressed in E. coli (), ca. 90% of the filaments had the same 3 nm diameter as in the wild-type cells, but the conductance was orders of magnitude lower (). As in the parental strain, OmcS filaments accounted for the other 10% of the filaments and their conductance was unchanged. These results are consistent with the expectation that changing the amino acid composition of the pilin would change the conductance of the pili, but not the conductance of OmcS or other filaments not comprised of pilin.

Furthermore, when the PilA gene expressed in G. sulfurreducens was modified to encode short peptide sequences at the carboxyl end of the pilin protein, the predominant filaments emanating from the cells displayed those peptides () (see Figures 1C,D for examples). The expression of 3 nm diameter conductive filaments in E. coli and other microbes, as well as the ability to engineer dramatic changes in their conductivity and binding properties through targeted modifications of the pilin amino acid sequence (see Figure 1H for an example), further demonstrates that the G. sulfurreducens pilin assembles into conductive filaments (; ; ; ; ; ; ). Although 6.5 nm diameter filaments containing PilA were described in some mutant strains, as previously reviewed in detail, they have never been observed in wild-type strains and are a mutation artifact ().

Filament function

Tabari and Hochbaum critique early studies on e-pili function (), but ignore later work showing that replacing the wild-type pilin gene with variants yielding poorly conductive pili addressed concerns about cytochrome mislocalization (; ; ; ; ; ; ). Expressing poorly conductive pili impairs EET even when cytochromes are properly localized (see Figures 1B,G for examples).

Tabari and Hochbaum claim that “cleaner background strains with all cytochrome nanowires deleted for genetic studies” are needed, failing to recognize that such a strain already exists (). It effectively reduced Fe(III) oxide in the absence of all three filament-forming cytochromes. Conductive e-pili were still required ().

Tabari and Hochbaum state that “Immunolabeling indicates that OmcZ is found widely dispersed within the conductive matrix of G. sulfurreducens biofilms.” The study cited () actually reported the opposite finding: “OmcZ was highly concentrated at the biofilm–electrode interface.” This specific localization suggests that OmcZ has a specialized role in electron transfer to the electrode, not long-range electron transport through the bulk conductive biofilm, a conclusion further supported by the lack of correlation between OmcZ abundance and biofilm conductivity (). Deleting genes for OmcS and OmcE not only did not inhibit current production, it increased biofilm conductivity () (Figure 1E). In contrast, there is a positive correlation between pilin abundance and biofilm conductivity () (Figure 1F) and expressing poorly conductive pili substantially lowers biofilm conductivity and current production () (Figure 1G).

Conclusion

There is a substantial body of literature that supports the role of e-pili in G. sulfurreducens EET (; ; ; ). Cytochrome filaments are not essential for Fe(III) oxide reduction. Their role in other forms of EET requires further clarification.

Statements

Author contributions

DL: Writing – original draft, Writing – review and editing.

Funding

The author(s) declare that no financial support was received for the research and/or publication of this article.

Conflict of interest

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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References

Summary

Keywords

electromicrobiology, microbial nanowires, e-pili, cytochrome nanowires, protein nanowires, extracellular electron transfer

Citation

Lovley DR (2025) Commentary: Electron transport across the cell envelope via multiheme c-type cytochromes in Geobacter sulfurreducens. Front. Chem. 13:1674350. doi: 10.3389/fchem.2025.1674350

Received

27 July 2025

Accepted

10 October 2025

Published

20 October 2025

Volume

13 - 2025

Edited by

John D. Wade, University of Melbourne, Australia

Reviewed by

Michael Schlömann, Freiberg University of Mining and Technology, Germany

Updates

Copyright

© 2025 Lovley.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Derek R. Lovley,

Disclaimer

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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