Cyclic voltammetry of biofilms of wild type and mutant Geobacter sulfurreducens on fuel cell anodes indicates possible roles of OmcB, OmcZ, type IV pili, and protons in extracellular electron transfer

Microorganisms can transfer electrons directly to extracellular acceptors, during which organic compounds are oxidized to carbon dioxide. One of these microbes, Geobacter sulfurreducens, is well known for the “metallic-like” conductivity of its type IV pili. However, there is no consensus on what the mechanism for electron transfer along these conductive pili is. Based on the aromatic distances and orientations of our predicted models, the mechanism of electron transfer in the Geobacter sulfurreducens (GS) pili was explored by quantum chemical calculations with Marcus theory of electron transfer reactions. Three aromatic residues from the N-terminal α-helix of the GS pilin subunit are packed together, resulting in a continuous pi-pi interaction chain. The theoretical conductance (4.69 μS/3.85 μS) of the predicted models is very similar to that in the experiments reported recently (3.40 μS). These findings offer a new concept that the GS pili belongs to a new class of proteins that can transport electrons through pi-pi interaction between aromatic residues and also provide a valuable tool for guiding further researches of these conductive pili, to investigate their roles in biogeochemical cycling, and potential applications in biomaterials, bioelectronics, and bioenergy.

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Direct Observation of Electrically Conductive Pili Emanating from Geobacter sulfurreducens

10.1101/2021.07.06.451359 ◽

2021 ◽

Author(s):

Xinying Liu ◽

David Jeffrey Fraser Walker ◽

Stephen Nonnenmann ◽

Dezhi Sun ◽

Derek R. Lovley

Keyword(s):

Electron Transfer ◽

Electron Transport ◽

Long Range ◽

Geobacter Sulfurreducens ◽

Metal Corrosion ◽

Extracellular Electron Transfer ◽

Wild Type ◽

Electrically Conductive ◽

In The Wild ◽

Pilin Gene

Geobacter sulfurreducens is a model microbe for elucidating the mechanisms for extracellular electron transfer in several biogeochemical cycles, bioelectrochemical applications, and microbial metal corrosion. Multiple lines of evidence previously suggested that electrically conductive pili (e-pili) are an essential conduit for long-range extracellular electron transport in G. sulfurreducens. However, it has recently been reported that G. sulfurreducens does not express e-pili and that filaments comprised of multi-heme c-type cytochromes are responsible for long-range electron transport. This possibility was directly investigated by examining cells, rather than filament preparations, with atomic force microscopy. Approximately 90 % of the filaments emanating from wild-type cells had a diameter (3 nm) and conductance consistent with previous reports of e-pili harvested from G. sulfurreducens or heterologously expressed in E. coli from the G. sulfurreducens pilin gene. The remaining 10% of filaments had a morphology consistent with filaments comprised of the c-type cytochrome OmcS. A strain expressing a modified pilin gene designed to yield poorly conductive pili expressed 90 % filaments with a 3 nm diameter, but greatly reduced conductance, further indicating that the 3 nm diameter conductive filaments in the wild-type strain were e-pili. A strain in which genes for five of the most abundant outer-surface c-type cytochromes, including OmcS, was deleted yielded only 3 nm diameter filaments with the same conductance as in the wild-type. These results demonstrate that e-pili are the most abundant conductive filaments expressed by G. sulfurreducens, consistent with previous functional studies demonstrating the need for e-pili for long-range extracellular electron transfer.

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Improper Localization of the OmcS Cytochrome May Explain the Inability of thexapD-Deficient Mutant ofGeobacter sulfurreducensto Reduce Fe(III) Oxide

10.1101/114900 ◽

2017 ◽

Cited By ~ 8

Author(s):

Kelly A. Flanagan ◽

Ching Leang ◽

Joy E. Ward ◽

Derek R. Lovley

Keyword(s):

Electron Transfer ◽

Electron Transport ◽

Type Strain ◽

Wild Type Strain ◽

Geobacter Sulfurreducens ◽

Extracellular Electron Transfer ◽

Outer Cell ◽

Wild Type ◽

Redox Active ◽

In The Wild

AbstractExtracellular electron transfer through a redox-active exopolysaccharide matrix has been proposed as a strategy for extracellular electron transfer to Fe(III) oxide byGeobacter sulfurreducens,based on the phenotype of axapD-deficient strain. Central to this model was the assertion that thexapD-deficient strain produced pili decorated with the multi-hemec-type cytochrome OmcS in manner similar to the wild-type strain. Further examination of thexapD-deficient strain with immunogold labeling of OmcS and transmission electron microscopy revealed that OmcS was associated with the outer cell surface rather than pili. PilA, the pilus monomer, could not be detected in thexapD-deficient strain under conditions in which it was readily detected in the wild-type strain. Multiple lines of evidence in previous studies have suggested that long-range electron transport to Fe(III) oxides proceeds through electrically conductive pili and that OmcS associated with the pili is necessary for electron transfer from the pili to Fe(III) oxides. Therefore, an alternative explanation for the Fe(III) oxide reduction phenotype of thexapD-deficientstrain is that the pili-OmcS route for extracellular electron transport to Fe(III) oxide has been disrupted in thexapD-deficient strain.

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Significance of a Posttranslational Modification of the PilA Protein of Geobacter sulfurreducens for Surface Attachment, Biofilm Formation, and Growth on Insoluble Extracellular Electron Acceptors

Journal of Bacteriology ◽

10.1128/jb.00716-16 ◽

2017 ◽

Vol 199 (8) ◽

Cited By ~ 11

Author(s):

Lubna V. Richter ◽

Ashley E. Franks ◽

Robert M. Weis ◽

Steven J. Sandler

Keyword(s):

Electron Transfer ◽

Amino Acid ◽

Posttranslational Modification ◽

Biofilm Formation ◽

Electron Acceptors ◽

Type Iv Pili ◽

Geobacter Sulfurreducens ◽

Surface Attachment ◽

Content Type ◽

Type Iv

ABSTRACT Geobacter sulfurreducens, an anaerobic metal-reducing bacterium, possesses type IV pili. These pili are intrinsic structural elements in biofilm formation and, together with a number of c-type cytochromes, are thought to serve as conductive nanowires enabling long-range electron transfer (ET) to metal oxides and graphite anodes. Here, we report that a posttranslational modification of a nonconserved amino acid residue within the PilA protein, the structural subunit of the type IV pili, is crucial for growth on insoluble extracellular electron acceptors. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of the secreted PilA protein revealed a posttranslational modification of tyrosine-32 with a moiety of a mass consistent with a glycerophosphate group. Mutating this tyrosine into a phenylalanine inhibited cell growth with Fe(III) oxides as the sole electron acceptor. In addition, this amino acid substitution severely diminished biofilm formation on graphite surfaces and impaired current output in microbial fuel cells. These results demonstrate that the capability to attach to insoluble electron acceptors plays a crucial role for the cells' ability to utilize them. The work suggests that glycerophosphate modification of Y32 is a key factor contributing to the surface charge of type IV pili, influencing the adhesion of Geobacter to specific surfaces. IMPORTANCE Type IV pili are bacterial appendages that function in cell adhesion, virulence, twitching motility, and long-range electron transfer (ET) from bacterial cells to insoluble extracellular electron acceptors. The mechanism and role of type IV pili for ET in Geobacter sulfurreducens is still a subject of research. In this study, we identified a posttranslational modification of the major G. sulfurreducens type IV pilin, suggested to be a glycerophosphate moiety. We show that a mutant in which the glycerophosphate-modified tyrosine-32 is replaced with a phenylalanine has reduced abilities for ET and biofilm formation compared with those of the wild type. The results show the importance of the glycerophosphate-modified tyrosine for surface attachment and electron transfer in electrode- or Fe(III)-respiring G. sulfurreducens cells.

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Phylogenetic and structural diversity of aromatically dense pili from environmental metagenomes

10.1101/668343 ◽

2019 ◽

Author(s):

M. S. Bray ◽

J. Wu ◽

C.C. Padilla ◽

F. J. Stewart ◽

D. A. Fowle ◽

...

Keyword(s):

Amino Acids ◽

Electron Transfer ◽

Microbial Respiration ◽

Aromatic Amino Acids ◽

Structural Diversity ◽

Type Iv Pili ◽

Extracellular Electron Transfer ◽

Environmental Distribution ◽

Type Iv ◽

Genes Encoding

SummaryElectroactive type IV pili, or e-pili, are used by some microbial species for extracellular electron transfer. Recent studies suggest that e-pili may be more phylogenetically and structurally diverse than previously assumed. Here, we used updated aromatic density thresholds (≥9.8% aromatic amino acids, ≤22-aa aromatic gaps, and aromatic amino acids at residues 1, 24, 27, 50 and/or 51, and 32 and/or 57) to search for putative e-pilin genes in metagenomes from diverse ecosystems with active microbial metal cycling. Environmental putative e-pilins were diverse in length and phylogeny, and included truncated e-pilins inGeobacterspp., as well as longer putative e-pilins in Fe(II)-oxidizingBetaproteobacteriaandZetaproteobacteria.Originality and SignificanceElectroactive pili (e-pili) are used by microorganisms to respire solid metals in their environment through extracellular electron transfer. Thus, e-pili enable microbes to occupy specific environmental niches. Additionally, e-pili have important potential for biotechnological applications. Currently the repertoire of known e-pili is small, and their environmental distribution is largely unknown. Using sequence analysis, we identified numerous genes encoding putative e-pili from diverse anoxic, metal-rich ecosystems. Our results expand the diversity of putative e-pili in environments where metal oxides may be important electron acceptors for microbial respiration.

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Faculty Opinions recommendation of Biofilm formation by Pseudomonas aeruginosa wild type, flagella and type IV pili mutants.

Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature ◽

10.3410/f.1011135.192160 ◽

2003 ◽

Author(s):

Leo Eberl

Keyword(s):

Pseudomonas Aeruginosa ◽

Biofilm Formation ◽

Type Iv Pili ◽

Wild Type ◽

Type Iv

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Biofilm formation by Pseudomonas aeruginosa wild type, flagella and type IV pili mutants

Molecular Microbiology ◽

10.1046/j.1365-2958.2003.03525.x ◽

2003 ◽

Vol 48 (6) ◽

pp. 1511-1524 ◽

Cited By ~ 581

Author(s):

Mikkel Klausen ◽

Arne Heydorn ◽

Paula Ragas ◽

Lotte Lambertsen ◽

Anders Aaes-Jørgensen ◽

...

Keyword(s):

Pseudomonas Aeruginosa ◽

Biofilm Formation ◽

Type Iv Pili ◽

Wild Type ◽

Type Iv

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Lack of Electricity Production by Pelobacter carbinolicus Indicates that the Capacity for Fe(III) Oxide Reduction Does Not Necessarily Confer Electron Transfer Ability to Fuel Cell Anodes

Applied and Environmental Microbiology ◽

10.1128/aem.00804-07 ◽

2007 ◽

Vol 73 (16) ◽

pp. 5347-5353 ◽

Cited By ~ 93

Author(s):

Hanno Richter ◽

Martin Lanthier ◽

Kelly P. Nevin ◽

Derek R. Lovley

Keyword(s):

Electron Transfer ◽

Fuel Cells ◽

Fuel Cell ◽

Microbial Fuel Cell ◽

Microbial Fuel Cells ◽

Electron Donors ◽

Rrna Genes ◽

Anode Surface ◽

Oxide Reduction ◽

Fuel Cell Anodes

ABSTRACT The ability of Pelobacter carbinolicus to oxidize electron donors with electron transfer to the anodes of microbial fuel cells was evaluated because microorganisms closely related to Pelobacter species are generally abundant on the anodes of microbial fuel cells harvesting electricity from aquatic sediments. P. carbinolicus could not produce current in a microbial fuel cell with electron donors which support Fe(III) oxide reduction by this organism. Current was produced using a coculture of P. carbinolicus and Geobacter sulfurreducens with ethanol as the fuel. Ethanol consumption was associated with the transitory accumulation of acetate and hydrogen. G. sulfurreducens alone could not metabolize ethanol, suggesting that P. carbinolicus grew in the fuel cell by converting ethanol to hydrogen and acetate, which G. sulfurreducens oxidized with electron transfer to the anode. Up to 83% of the electrons available in ethanol were recovered as electricity and in the metabolic intermediate acetate. Hydrogen consumption by G. sulfurreducens was important for ethanol metabolism by P. carbinolicus. Confocal microscopy and analysis of 16S rRNA genes revealed that half of the cells growing on the anode surface were P. carbinolicus, but there was a nearly equal number of planktonic cells of P. carbinolicus. In contrast, G. sulfurreducens was primarily attached to the anode. P. carbinolicus represents the first Fe(III) oxide-reducing microorganism found to be unable to produce current in a microbial fuel cell, providing the first suggestion that the mechanisms for extracellular electron transfer to Fe(III) oxides and fuel cell anodes may be different.

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Geobacter sulfurreducens adapts to low electrode potential for extracellular electron transfer

Electrochimica Acta ◽

10.1016/j.electacta.2016.01.033 ◽

2016 ◽

Vol 191 ◽

pp. 743-749 ◽

Cited By ~ 13

Author(s):

Luo Peng ◽

Xiao-Ting Zhang ◽

Jie Yin ◽

Shuo-Yuan Xu ◽

Yong Zhang ◽

...

Keyword(s):

Electron Transfer ◽

Electrode Potential ◽

Geobacter Sulfurreducens ◽

Extracellular Electron Transfer

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Identification of different putative outer membrane electron conduits necessary for Fe(III) citrate, Fe(III) oxide, Mn(IV) oxide, or electrode reduction by Geobacter sulfurreducens

10.1101/169086 ◽

2017 ◽

Cited By ~ 1

Author(s):

Fernanda Jiménez Otero ◽

Chi Ho Chan ◽

Daniel R. Bond

Keyword(s):

Electron Transfer ◽

Outer Membrane ◽

Electron Acceptor ◽

Single Gene ◽

Gene Clusters ◽

Geobacter Sulfurreducens ◽

Transcriptional Analysis ◽

Growth Defect ◽

Redox Potentials ◽

Wild Type

AbstractAt least five gene clusters in the Geobacter sulfurreducens genome encode putative ‘electron conduits’ implicated in electron transfer across the outer membrane, each containing a periplasmic multiheme c-type cytochrome, integral outer membrane anchor, and outer membrane redox lipoprotein(s). Markerless single gene cluster deletions and all possible multiple deletion combinations were constructed and grown with soluble Fe(III) citrate, Fe(III)- and Mn(IV)-oxides, and graphite electrodes poised at +0.24 V and −0.1 V vs. SHE. Different gene clusters were necessary for reduction of each electron acceptor. During metal oxide reduction, deletion of the previously described omcBC cluster caused defects, but deletion of additional components in an ΔomcBC background, such as extEFG, were needed to produce defects greater than 50% compared to wild type. Deletion of all five gene clusters abolished all metal reduction. During electrode reduction, only the ΔextABCD mutant had a severe growth defect at both redox potentials, while this mutation did not affect Fe(III)-oxide, Mn(IV)-oxide, or Fe(III) citrate reduction. Some mutants containing only one cluster were able to reduce particular terminal electron acceptors better than wild type, suggesting routes for improvement by targeting specific electron transfer pathways. Transcriptomic comparisons between fumarate and electrode-based growth showed all of these ext clusters to be constitutive, and transcriptional analysis of the triple-deletion strain containing only extABCD detected no significant changes in expression of known redox proteins or pili components. These genetic experiments reveal new outer membrane conduit complexes necessary for growth of G. sulfurreducens, depending on the available extracellular electron acceptor.

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