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Electromicrobiology: realities, grand challenges, goals and predictions

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Microbial Biotechnology

John Wiley and Sons Inc.

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      Summary

      Electromicrobiology is a subdiscipline of microbiology that involves extracellular electron transfer (EET) to (or from) insoluble electron active redox compounds located outside the outer membrane of the cell. These interactions can often be studied using electrochemical techniques which have provided novel insights into microbial physiology in recent years. The mechanisms (and variations) of outward EET are well understood for two model systems, Shewanella and Geobacter, both of which employ multihaem cytochromes to provide an electron conduit to the cell exterior. In contrast, little is known of the intricacies of inward EET, even in these model systems. Given the number of labs now working on EET, it seems likely that most of the mechanistic details will be understood in a few years for the model systems, and the many applications of electromicrobiology will continue to move forward. But emerging work, using electrodes as electron acceptors and donors is providing an abundance of new types of microbes capable of EET inward and/or outward: microbes that are clearly different from our known systems. The extent of this very diverse, and perhaps widely distributed and biogeochemically important ability needs to be determined to understand the mechanisms, importance, and raison d'etre of EET for microbial biology.

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      Most cited references 46

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      Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms.

      Shewanella oneidensis MR-1 produced electrically conductive pilus-like appendages called bacterial nanowires in direct response to electron-acceptor limitation. Mutants deficient in genes for c-type decaheme cytochromes MtrC and OmcA, and those that lacked a functional Type II secretion pathway displayed nanowires that were poorly conductive. These mutants were also deficient in their ability to reduce hydrous ferric oxide and in their ability to generate current in a microbial fuel cell. Nanowires produced by the oxygenic phototrophic cyanobacterium Synechocystis PCC6803 and the thermophilic, fermentative bacterium Pelotomaculum thermopropionicum reveal that electrically conductive appendages are not exclusive to dissimilatory metal-reducing bacteria and may, in fact, represent a common bacterial strategy for efficient electron transfer and energy distribution.
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        Shewanella secretes flavins that mediate extracellular electron transfer.

        Bacteria able to transfer electrons to metals are key agents in biogeochemical metal cycling, subsurface bioremediation, and corrosion processes. More recently, these bacteria have gained attention as the transfer of electrons from the cell surface to conductive materials can be used in multiple applications. In this work, we adapted electrochemical techniques to probe intact biofilms of Shewanella oneidensis MR-1 and Shewanella sp. MR-4 grown by using a poised electrode as an electron acceptor. This approach detected redox-active molecules within biofilms, which were involved in electron transfer to the electrode. A combination of methods identified a mixture of riboflavin and riboflavin-5'-phosphate in supernatants from biofilm reactors, with riboflavin representing the dominant component during sustained incubations (>72 h). Removal of riboflavin from biofilms reduced the rate of electron transfer to electrodes by >70%, consistent with a role as a soluble redox shuttle carrying electrons from the cell surface to external acceptors. Differential pulse voltammetry and cyclic voltammetry revealed a layer of flavins adsorbed to electrodes, even after soluble components were removed, especially in older biofilms. Riboflavin adsorbed quickly to other surfaces of geochemical interest, such as Fe(III) and Mn(IV) oxy(hydr)oxides. This in situ demonstration of flavin production, and sequestration at surfaces, requires the paradigm of soluble redox shuttles in geochemistry to be adjusted to include binding and modification of surfaces. Moreover, the known ability of isoalloxazine rings to act as metal chelators, along with their electron shuttling capacity, suggests that extracellular respiration of minerals by Shewanella is more complex than originally conceived.
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          Bug juice: harvesting electricity with microorganisms.

           Derek Lovley (2006)
          It is well established that some reduced fermentation products or microbially reduced artificial mediators can abiotically react with electrodes to yield a small electrical current. This type of metabolism does not typically result in an efficient conversion of organic compounds to electricity because only some metabolic end products will react with electrodes, and the microorganisms only incompletely oxidize their organic fuels. A new form of microbial respiration has recently been discovered in which microorganisms conserve energy to support growth by oxidizing organic compounds to carbon dioxide with direct quantitative electron transfer to electrodes. These organisms, termed electricigens, offer the possibility of efficiently converting organic compounds into electricity in self-sustaining systems with long-term stability.
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            Author and article information

            Affiliations
            [ 1 ] Department of Earth SciencesUniversity of Southern California Los Angeles CAUSA
            Author notes
            [* ]For correspondence. E‐mail knealson@ 123456usc.edu . Tel. +1 213‐821‐2271; Fax +1 213‐740‐8801.
            Journal
            Microb Biotechnol
            Microb Biotechnol
            10.1111/(ISSN)1751-7915
            MBT2
            Microbial Biotechnology
            John Wiley and Sons Inc. (Hoboken )
            1751-7915
            10 August 2016
            September 2016
            : 9
            : 5 , Microbial Biotechnology‐2020 ( doiID: 10.1111/mbt2.2016.9.issue-5 )
            : 595-600
            27506517
            4993177
            10.1111/1751-7915.12400
            MBT212400
            © 2016 The Authors. Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology.

            This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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            2.0
            mbt212400
            September 2016
            Converter:WILEY_ML3GV2_TO_NLMPMC version:4.9.4 mode:remove_FC converted:22.08.2016

            Biotechnology

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