@article {3094, title = {Expressing the Geobacter metallireducens PilA in Geobacter sulfurreducens Yields Pili with Exceptional Conductivity.}, journal = {mBio}, volume = {8}, year = {2017}, month = {2017 Jan 17}, abstract = {

UNLABELLED: The electrically conductive pili (e-pili) of Geobacter sulfurreducens serve as a model for a novel strategy for long-range extracellular electron transfer. e-pili are also a new class of bioelectronic materials. However, the only other Geobacter pili previously studied, which were from G.~uraniireducens, were poorly conductive. In order to obtain more information on the range of pili conductivities in Geobacter species, the pili of G.~metallireducens were investigated. Heterologously expressing the PilA gene of G.~metallireducens in G.~sulfurreducens yielded a G.~sulfurreducens strain, designated strain MP, that produced abundant pili. Strain MP exhibited phenotypes consistent with the presence of e-pili, such as high rates of Fe(III) oxide reduction and high current densities on graphite anodes. Individual pili prepared at physiologically relevant pH~7 had conductivities of 277 {\textpm} 18.9 S/cm (mean {\textpm} standard deviation), which is 5,000-fold higher than the conductivity of G.~sulfurreducens pili at pH~7 and nearly 1 million-fold higher than the conductivity of G.~uraniireducens pili at the same pH. A potential explanation for the higher conductivity of the G.~metallireducens pili is their greater density of aromatic amino acids, which are known to be important components in electron transport along the length of the pilus. The G.~metallireducens pili represent the most highly conductive pili found to date and suggest strategies for designing synthetic pili with even higher conductivities.

IMPORTANCE: e-pili are a remarkable electrically conductive material that can be sustainably produced without harsh chemical processes from renewable feedstocks and that contain no toxic components in the final product. Thus, e-pili offer an unprecedented potential for developing novel materials, electronic devices, and sensors for diverse applications with a new "green" technology. Increasing e-pili conductivity will even further expand their potential applications. A proven strategy is to design synthetic e-pili that contain tryptophan, an aromatic amino acid not found in previously studied e-pili. The studies reported here demonstrate that a productive alternative approach is to search more broadly in the microbial world. Surprisingly, even though G.~metallireducens and G.~sulfurreducens are closely related, the conductivities of their e-pili differ by more than 3 orders of magnitude. The ability to produce e-pili with high conductivity without generating a genetically modified product enhances the attractiveness of this novel electronic material.

}, keywords = {Electric Conductivity, Electrodes, Electron Transport, Ferric Compounds, Fimbriae Proteins, Gene Expression, Geobacter, Oxidation-Reduction, Recombinant Proteins}, issn = {2150-7511}, doi = {10.1128/mBio.02203-16}, author = {Tan, Yang and Adhikari, Ramesh Y and Malvankar, Nikhil S and Ward, Joy E and Woodard, Trevor L and Nevin, Kelly P and Lovley, Derek R} } @article {3106, title = {Functional environmental proteomics: elucidating the role of a c-type cytochrome abundant during uranium bioremediation.}, journal = {ISME J}, volume = {10}, year = {2016}, month = {2016 Feb}, pages = {310-20}, abstract = {

Studies with pure cultures of dissimilatory metal-reducing microorganisms have demonstrated that outer-surface c-type cytochromes are important electron transfer agents for the reduction of metals, but previous environmental proteomic studies have typically not recovered cytochrome sequences from subsurface environments in which metal reduction is important. Gel-separation, heme-staining and mass spectrometry of proteins in groundwater from in situ uranium bioremediation experiments identified a putative c-type cytochrome, designated Geobacter subsurface c-type cytochrome A (GscA), encoded within the genome of strain M18, a Geobacter isolate previously recovered from the site. Homologs of GscA were identified in the genomes of other Geobacter isolates in the phylogenetic cluster known as subsurface clade 1, which predominates in a diversity of Fe(III)-reducing subsurface environments. Most of the gscA sequences recovered from groundwater genomic DNA clustered in a tight phylogenetic group closely related to strain M18. GscA was most abundant in groundwater samples in which Geobacter sp. predominated. Expression of gscA in a strain of Geobacter sulfurreducens that lacked the gene for the c-type cytochrome OmcS, thought to facilitate electron transfer from conductive pili to Fe(III) oxide, restored the capacity for Fe(III) oxide reduction. Atomic force microscopy provided evidence that GscA was associated with the pili. These results demonstrate that a c-type cytochrome with an apparent function similar to that of OmcS is abundant when Geobacter sp. are abundant in the subsurface, providing insight into the mechanisms for the growth of subsurface Geobacter sp. on Fe(III) oxide and suggesting an approach for functional analysis of other Geobacter proteins found in the subsurface.

}, keywords = {Amino Acid Sequence, Bacterial Proteins, Biodegradation, Environmental, Cytochrome c Group, Electron Transport, Ferric Compounds, Geobacter, Groundwater, Molecular Sequence Data, Oxidation-Reduction, Phylogeny, Proteomics, Sequence Alignment, Uranium}, issn = {1751-7370}, doi = {10.1038/ismej.2015.113}, author = {Yun, Jiae and Malvankar, Nikhil S and Ueki, Toshiyuki and Lovley, Derek R} } @article {3100, title = {The Low Conductivity of Geobacter uraniireducens Pili Suggests a Diversity of Extracellular Electron Transfer Mechanisms in the Genus Geobacter.}, journal = {Front Microbiol}, volume = {7}, year = {2016}, month = {2016}, pages = {980}, abstract = {

Studies on the mechanisms for extracellular electron transfer in Geobacter species have primarily focused on Geobacter sulfurreducens, but the poor conservation of genes for some electron transfer components within the Geobacter genus suggests that there may be a diversity of extracellular electron transport strategies among Geobacter species. Examination of the gene sequences for PilA, the type IV pilus monomer, in Geobacter species revealed that the PilA sequence of Geobacter uraniireducens was much longer than that of G. sulfurreducens. This is of interest because it has been proposed that the relatively short PilA sequence of G. sulfurreducens is an important feature conferring conductivity to G. sulfurreducens pili. In order to investigate the properties of the G. uraniireducens pili in more detail, a strain of G. sulfurreducens that expressed pili comprised the PilA of G. uraniireducens was constructed. This strain, designated strain GUP, produced abundant pili, but generated low current densities and reduced Fe(III) very poorly. At pH 7, the conductivity of the G. uraniireducens pili was 3 {\texttimes} 10(-4) S/cm, much lower than the previously reported 5 {\texttimes} 10(-2) S/cm conductivity of G. sulfurreducens pili at the same pH. Consideration of the likely voltage difference across pili during Fe(III) oxide reduction suggested that G. sulfurreducens pili can readily accommodate maximum reported rates of respiration, but that G. uraniireducens pili are not sufficiently conductive to be an effective mediator of long-range electron transfer. In contrast to G. sulfurreducens and G. metallireducens, which require direct contact with Fe(III) oxides in order to reduce them, G. uraniireducens reduced Fe(III) oxides occluded within microporous beads, demonstrating that G. uraniireducens produces a soluble electron shuttle to facilitate Fe(III) oxide reduction. The results demonstrate that Geobacter species may differ substantially in their mechanisms for long-range electron transport and that it is important to have information beyond a phylogenetic affiliation in order to make conclusions about the mechanisms by which Geobacter species are transferring electrons to extracellular electron acceptors.

}, issn = {1664-302X}, doi = {10.3389/fmicb.2016.00980}, author = {Tan, Yang and Adhikari, Ramesh Y and Malvankar, Nikhil S and Ward, Joy E and Nevin, Kelly P and Woodard, Trevor L and Smith, Jessica A and Snoeyenbos-West, Oona L and Franks, Ashley E and Tuominen, Mark T and Lovley, Derek R} } @article {3102, title = {Low Energy Atomic Models Suggesting a Pilus Structure that could Account for Electrical Conductivity of Geobacter sulfurreducens Pili.}, journal = {Sci Rep}, volume = {6}, year = {2016}, month = {2016 Mar 22}, pages = {23385}, abstract = {

The metallic-like electrical conductivity of Geobacter sulfurreducens pili has been documented with multiple lines of experimental evidence, but there is only a rudimentary understanding of the structural features which contribute to this novel mode of biological electron transport. In order to determine if it was feasible for the pilin monomers of G. sulfurreducens to assemble into a conductive filament, theoretical energy-minimized models of Geobacter pili were constructed with a previously described approach, in which pilin monomers are assembled using randomized structural parameters and distance constraints. The lowest energy models from a specific group of predicted structures lacked a central channel, in contrast to previously existing pili models. In half of the no-channel models the three N-terminal aromatic residues of the pilin monomer are arranged in a potentially electrically conductive geometry, sufficiently close to account for the experimentally observed metallic like conductivity of the pili that has been attributed to overlapping pi-pi orbitals of aromatic amino acids. These atomic resolution models capable of explaining the observed conductive properties of Geobacter pili are a valuable tool to guide further investigation of the metallic-like conductivity of the pili, their role in biogeochemical cycling, and applications in bioenergy and bioelectronics.

}, keywords = {Electric Conductivity, Fimbriae, Bacterial, Geobacter, Models, Theoretical}, issn = {2045-2322}, doi = {10.1038/srep23385}, author = {Xiao, Ke and Malvankar, Nikhil S and Shu, Chuanjun and Martz, Eric and Lovley, Derek R and Sun, Xiao} } @article {3096, title = {Reply to {\textquoteright}Measuring conductivity of living Geobacter sulfurreducens biofilms{\textquoteright}.}, journal = {Nat Nanotechnol}, volume = {11}, year = {2016}, month = {2016 Nov 08}, pages = {913-914}, keywords = {Biofilms, Electric Conductivity, Geobacter}, issn = {1748-3395}, doi = {10.1038/nnano.2016.191}, author = {Malvankar, Nikhil S and Rotello, Vincent M and Tuominen, Mark T and Lovley, Derek R} } @article {3101, title = {Synthetic Biological Protein Nanowires with High Conductivity.}, journal = {Small}, volume = {12}, year = {2016}, month = {2016 Sep}, pages = {4481-5}, abstract = {

Genetic modification to add tryptophan to PilA, the monomer for the electrically conductive pili of Geobacter sulfurreducens, yields conductive protein filaments 2000-fold more conductive than the wild-type pili while cutting the diameter in half to 1.5 nm.

}, keywords = {Amino Acid Sequence, Electric Conductivity, Fimbriae, Bacterial, Geobacter, Nanowires, Proteins, Tryptophan}, issn = {1613-6829}, doi = {10.1002/smll.201601112}, author = {Tan, Yang and Adhikari, Ramesh Y and Malvankar, Nikhil S and Pi, Shuang and Ward, Joy E and Woodard, Trevor L and Nevin, Kelly P and Xia, Qiangfei and Tuominen, Mark T and Lovley, Derek R} } @article {3121, title = {Centimeter-long electron transport in marine sediments via conductive minerals.}, journal = {ISME J}, volume = {9}, year = {2015}, month = {2015 Feb}, pages = {527-31}, abstract = {

Centimeter-long electron conduction through marine sediments, in which electrons derived from sulfide in anoxic sediments are transported to oxygen in surficial sediments, may have an important influence on sediment geochemistry. Filamentous bacteria have been proposed to mediate the electron transport, but the filament conductivity could not be verified and other mechanisms are possible. Surprisingly, previous investigations have never actually measured the sediment conductivity or its basic physical properties. Here we report direct measurements that demonstrate centimeter-long electron flow through marine sediments, with conductivities sufficient to account for previously estimated electron fluxes. Conductivity was lost for oxidized sediments, which contrasts with the previously described increase in the conductivity of microbial biofilms upon oxidation. Adding pyrite to the sediments significantly enhanced the conductivity. These results suggest that the role of conductive minerals, which are more commonly found in sediments than centimeter-long microbial filaments, need to be considered when modeling marine sediment biogeochemistry.

}, keywords = {Electron Transport, Geologic Sediments, Iron, Minerals, Oxidation-Reduction, Sulfides}, issn = {1751-7370}, doi = {10.1038/ismej.2014.131}, author = {Malvankar, Nikhil S and King, Gary M and Lovley, Derek R} } @article {3128, title = {Magnetite compensates for the lack of a pilin-associated c-type cytochrome in extracellular electron exchange.}, journal = {Environ Microbiol}, volume = {17}, year = {2015}, month = {2015 Mar}, pages = {648-55}, abstract = {

Nanoscale magnetite can facilitate microbial extracellular electron transfer that plays an important role in biogeochemical cycles, bioremediation and several bioenergy strategies, but the mechanisms for the stimulation of extracellular electron transfer are poorly understood. Further investigation revealed that magnetite attached to the electrically conductive pili of Geobacter species in a manner reminiscent of the association of the multi-heme c-type cytochrome OmcS with the pili of Geobacter sulfurreducens. Magnetite conferred extracellular electron capabilities on an OmcS-deficient strain unable to participate in interspecies electron transfer or Fe(III) oxide reduction. In the presence of magnetite wild-type cells repressed expression of the OmcS gene, suggesting that cells might need to produce less OmcS when magnetite was available. The finding that magnetite can compensate for the lack of the electron transfer functions of a multi-heme c-type cytochrome has implications not only for the function of modern microbes, but also for the early evolution of microbial electron transport mechanisms.

}, keywords = {Cytochrome c Group, Electron Transport, Electrons, Ferrosoferric Oxide, Fimbriae Proteins, Fimbriae, Bacterial, Gene Expression Regulation, Bacterial, Geobacter, Heme, Oxides}, issn = {1462-2920}, doi = {10.1111/1462-2920.12485}, author = {Liu, Fanghua and Rotaru, Amelia-Elena and Shrestha, Pravin M and Malvankar, Nikhil S and Nevin, Kelly P and Lovley, Derek R} } @article {3114, title = {Seeing is believing: novel imaging techniques help clarify microbial nanowire structure and function.}, journal = {Environ Microbiol}, volume = {17}, year = {2015}, month = {2015 Jul}, pages = {2209-15}, abstract = {

Novel imaging approaches have recently helped to clarify the properties of {\textquoteright}microbial nanowires{\textquoteright}. Geobacter sulfurreducens pili are actual wires. They possess metallic-like conductivity, which can be attributed to overlapping pi-pi orbitals of key aromatic amino acids. Electrostatic force microscopy recently confirmed charge propagation along the pili, in a manner similar to carbon nanotubes. The pili are essential for long-range electron transport to insoluble electron acceptors and interspecies electron transfer. Previous claims that Shewanella oneidensis also produce conductive pili have recently been recanted, based on novel live-imaging studies. The putative pili are, in fact, long extensions of the cytochrome-rich outer membrane and periplasm that, when dried, collapse to form filaments with dimensions similar to pili. It has yet to be demonstrated whether the cytochrome-to-cytochrome electron hopping documented in the dried membrane extensions takes place in intact hydrated membrane extensions or whether the membrane extensions enhance electron transport to insoluble electron acceptors such as Fe(III) oxides or electrodes. These findings demonstrate that G. sulfurreducens conductive pili and the outer membrane extensions of S. oneidensis are fundamentally different in composition, mechanism of electron transport and physiological role. New methods for evaluating filament conductivity will facilitate screening the microbial world for nanowires and elucidating their function.

}, keywords = {Cytochromes, Electric Conductivity, Electron Transport, Electrons, Ferric Compounds, Fimbriae, Bacterial, Geobacter, Microscopy, Atomic Force, Nanowires, Oxides, Periplasm, Shewanella}, issn = {1462-2920}, doi = {10.1111/1462-2920.12708}, author = {Lovley, Derek R and Malvankar, Nikhil S} } @article {3111, title = {Structural basis for metallic-like conductivity in microbial nanowires.}, journal = {mBio}, volume = {6}, year = {2015}, month = {2015 Mar 03}, pages = {e00084}, abstract = {

UNLABELLED: Direct measurement of multiple physical properties of Geobacter sulfurreducens pili have demonstrated that they possess metallic-like conductivity, but several studies have suggested that metallic-like conductivity is unlikely based on the structures of the G.~sulfurreducens pilus predicted from homology models. In order to further evaluate this discrepancy, pili were examined with synchrotron X-ray microdiffraction and rocking-curve X-ray diffraction. Both techniques revealed a periodic 3.2-{\r A} spacing in conductive, wild-type G.~sulfurreducens pili that was missing in the nonconductive pili of strain Aro5, which lack key aromatic acids required for conductivity. The intensity of the 3.2-{\r A} peak increased 100-fold when the pH was shifted from 10.5 to 2, corresponding with a previously reported 100-fold increase in pilus conductivity with this pH change. These results suggest a clear structure-function correlation for metallic-like conductivity that can be attributed to overlapping π-orbitals of aromatic amino acids. A homology model of the G.~sulfurreducens pilus was constructed with a Pseudomonas aeruginosa pilus model as a template as an alternative to previous models, which were based on a Neisseria gonorrhoeae pilus structure. This alternative model predicted that aromatic amino acids in G.~sulfurreducens pili are packed within 3 to 4~{\r A}, consistent with the experimental results. Thus, the predictions of homology modeling are highly sensitive to assumptions inherent in the model construction. The experimental results reported here further support the concept that the pili of G.~sulfurreducens represent a novel class of electronically functional proteins in which aromatic amino acids promote long-distance electron transport.

IMPORTANCE: The mechanism for long-range electron transport along the conductive pili of Geobacter sulfurreducens is of interest because these "microbial nanowires" are important in biogeochemical cycling as well as applications in bioenergy and bioelectronics. Although proteins are typically insulators, G.~sulfurreducens pilus proteins possess metallic-like conductivity. The studies reported here provide important structural insights into the mechanism of the metallic-like conductivity of G.~sulfurreducens pili. This information is expected to be useful in the design of novel bioelectronic materials.

}, keywords = {Amino Acids, Aromatic, Chemical Phenomena, Electrophysiological Phenomena, Fimbriae, Bacterial, Geobacter, Hydrogen-Ion Concentration, Models, Molecular, Nanowires, X-Ray Diffraction}, issn = {2150-7511}, doi = {10.1128/mBio.00084-15}, author = {Malvankar, Nikhil S and Vargas, Madeline and Nevin, Kelly and Tremblay, Pier-Luc and Evans-Lutterodt, Kenneth and Nykypanchuk, Dmytro and Martz, Eric and Tuominen, Mark T and Lovley, Derek R} } @article {3113, title = {Correlation between microbial community and granule conductivity in anaerobic bioreactors for brewery wastewater treatment.}, journal = {Bioresour Technol}, volume = {174}, year = {2014}, month = {2014 Dec}, pages = {306-10}, abstract = {

Prior investigation of an upflow anaerobic sludge blanket (UASB) reactor treating brewery wastes suggested that direct interspecies electron transfer (DIET) significantly contributed to interspecies electron transfer to methanogens. To investigate DIET in granules further, the electrical conductivity and bacterial community composition of granules in fourteen samples from four different UASB reactors treating brewery wastes were investigated. All of the UASB granules were electrically conductive whereas control granules from ANAMMOX (ANaerobic AMMonium OXidation) reactors and microbial granules from an aerobic bioreactor designed for phosphate removal were not. There was a moderate correlation (r=0.67) between the abundance of Geobacter species in the UASB granules and granule conductivity, suggesting that Geobacter contributed to granule conductivity. These results, coupled with previous studies, which have demonstrated that Geobacter species can donate electrons to methanogens that are typically predominant in anaerobic digesters, suggest that DIET may be a widespread phenomenon in UASB reactors treating brewery wastes.

}, keywords = {Alcoholic Beverages, Anaerobiosis, Bacteria, Bioreactors, Electric Conductivity, Ethanol, Sequence Analysis, DNA, Sewage, Waste Disposal, Fluid, Wastewater, Water Purification}, issn = {1873-2976}, doi = {10.1016/j.biortech.2014.10.004}, author = {Shrestha, Pravin Malla and Malvankar, Nikhil S and Werner, Jeffrey J and Franks, Ashley E and Elena-Rotaru, Amelia and Shrestha, Minita and Liu, Fanghua and Nevin, Kelly P and Angenent, Largus T and Lovley, Derek R} } @article {3131, title = {A Geobacter sulfurreducens strain expressing pseudomonas aeruginosa type IV pili localizes OmcS on pili but is deficient in Fe(III) oxide reduction and current production.}, journal = {Appl Environ Microbiol}, volume = {80}, year = {2014}, month = {2014 Feb}, pages = {1219-24}, abstract = {

The conductive pili of Geobacter species play an important role in electron transfer to Fe(III) oxides, in long-range electron transport through current-producing biofilms, and in direct interspecies electron transfer. Although multiple lines of evidence have indicated that the pili of Geobacter sulfurreducens have a metal-like conductivity, independent of the presence of c-type cytochromes, this claim is still controversial. In order to further investigate this phenomenon, a strain of G. sulfurreducens, designated strain PA, was constructed in which the gene for the native PilA, the structural pilin protein, was replaced with the PilA gene of Pseudomonas aeruginosa PAO1. Strain PA expressed and properly assembled P. aeruginosa PilA subunits into pili and exhibited a profile of outer surface c-type cytochromes similar to that of a control strain expressing the G. sulfurreducens PilA. Surprisingly, the strain PA pili were decorated with the c-type cytochrome OmcS in a manner similar to the control strain. However, the strain PA pili were 14-fold less conductive than the pili of the control strain, and strain PA was severely impaired in Fe(III) oxide reduction and current production. These results demonstrate that the presence of OmcS on pili is not sufficient to confer conductivity to pili and suggest that there are unique structural features of the G. sulfurreducens PilA that are necessary for conductivity.

}, keywords = {Amino Acid Sequence, Cytochromes c, Electricity, Ferric Compounds, Fimbriae Proteins, Fimbriae, Bacterial, Geobacter, Methanosarcinaceae, Molecular Sequence Data, Oxidation-Reduction, Pseudomonas aeruginosa, Sequence Alignment}, issn = {1098-5336}, doi = {10.1128/AEM.02938-13}, author = {Liu, Xing and Tremblay, Pier-Luc and Malvankar, Nikhil S and Nevin, Kelly P and Lovley, Derek R and Vargas, Madeline} } @article {3123, title = {Microbial nanowires for bioenergy applications.}, journal = {Curr Opin Biotechnol}, volume = {27}, year = {2014}, month = {2014 Jun}, pages = {88-95}, abstract = {

Microbial nanowires are electrically conductive filaments that facilitate long-range extracellular electron transfer. The model for electron transport along Shewanella oneidensis nanowires is electron hopping/tunneling between cytochromes adorning the filaments. Geobacter sulfurreducens nanowires are comprised of pili that have metal-like conductivity attributed to overlapping pi-pi orbitals of aromatic amino acids. The nanowires of Geobacter species have been implicated in direct interspecies electron transfer (DIET), which may be an important mode of syntrophy in the conversion of organic wastes to methane. Nanowire networks confer conductivity to Geobacter biofilms converting organic compounds to electricity in microbial fuel cells (MFCs) and increasing nanowire production is the only genetic manipulation shown to yield strains with improved current-producing capabilities. Introducing nanowires, or nanowire mimetics, might improve other bioenergy strategies that rely on extracellular electron exchange, such as microbial electrosynthesis. Similarities between microbial nanowires and synthetic conducting polymers suggest additional energy-related applications.

}, keywords = {Anaerobiosis, Bioelectric Energy Sources, Biofilms, Electric Conductivity, Electricity, Electron Transport, Electrons, Fimbriae, Bacterial, Geobacter, Methane, Nanowires, Shewanella, Synthetic Biology}, issn = {1879-0429}, doi = {10.1016/j.copbio.2013.12.003}, author = {Malvankar, Nikhil S and Lovley, Derek R} } @article {3124, title = {Promoting interspecies electron transfer with biochar.}, journal = {Sci Rep}, volume = {4}, year = {2014}, month = {2014 May 21}, pages = {5019}, abstract = {

Biochar, a charcoal-like product of the incomplete combustion of organic materials, is an increasingly popular soil amendment designed to improve soil fertility. We investigated the possibility that biochar could promote direct interspecies electron transfer (DIET) in a manner similar to that previously reported for granular activated carbon (GAC). Although the biochars investigated were 1000 times less conductive than GAC, they stimulated DIET in co-cultures of Geobacter metallireducens with Geobacter sulfurreducens or Methanosarcina barkeri in which ethanol was the electron donor. Cells were attached to the biochar, yet not in close contact, suggesting that electrons were likely conducted through the biochar, rather than biological electrical connections. The finding that biochar can stimulate DIET may be an important consideration when amending soils with biochar and can help explain why biochar may enhance methane production from organic wastes under anaerobic conditions.

}, keywords = {Charcoal, Coculture Techniques, Electron Transport, Electrons, Ethanol, Geobacter, Methanosarcina barkeri, Soil}, issn = {2045-2322}, doi = {10.1038/srep05019}, author = {Chen, Shanshan and Rotaru, Amelia-Elena and Shrestha, Pravin Malla and Malvankar, Nikhil S and Liu, Fanghua and Fan, Wei and Nevin, Kelly P and Lovley, Derek R} } @article {3116, title = {Visualization of charge propagation along individual pili proteins using ambient electrostatic force microscopy.}, journal = {Nat Nanotechnol}, volume = {9}, year = {2014}, month = {2014 Dec}, pages = {1012-7}, abstract = {

The nanoscale imaging of charge flow in proteins is crucial to understanding several life processes, including respiration, metabolism and photosynthesis. However, existing imaging methods are only effective under non-physiological conditions or are limited to photosynthetic proteins. Here, we show that electrostatic force microscopy can be used to directly visualize charge propagation along pili of Geobacter sulfurreducens with nanometre resolution and under ambient conditions. Charges injected at a single point into individual, untreated pili, which are still attached to cells, propagated over the entire filament. The mobile charge density in the pili, as well as the temperature and pH dependence of the charge density, were similar to those of carbon nanotubes and other organic conductors. These findings, coupled with a lack of charge propagation in mutated pili that were missing key aromatic amino acids, suggest that the pili of G. sulfurreducens function as molecular wires with transport via delocalized charges, rather than the hopping mechanism that is typical of biological electron transport.

}, keywords = {Fimbriae Proteins, Fimbriae, Bacterial, Geobacter, Microscopy, Electrochemical, Scanning}, issn = {1748-3395}, doi = {10.1038/nnano.2014.236}, author = {Malvankar, Nikhil S and Yalcin, Sibel Ebru and Tuominen, Mark T and Lovley, Derek R} } @article {3142, title = {Aromatic amino acids required for pili conductivity and long-range extracellular electron transport in Geobacter sulfurreducens.}, journal = {mBio}, volume = {4}, year = {2013}, month = {2013 Mar 12}, pages = {e00105-13}, abstract = {

UNLABELLED: It has been proposed that Geobacter sulfurreducens requires conductive pili for long-range electron transport to Fe(III) oxides and for high-density current production in microbial fuel cells. In order to investigate this further, we constructed a strain of G. sulfurreducens, designated Aro-5, which produced pili with diminished conductivity. This was accomplished by modifying the amino acid sequence of PilA, the structural pilin protein. An alanine was substituted for each of the five aromatic amino acids in the carboxyl terminus of PilA, the region in which G. sulfurreducens PilA differs most significantly from the PilAs of microorganisms incapable of long-range extracellular electron transport. Strain Aro-5 produced pili that were properly decorated with the multiheme c-type cytochrome OmcS, which is essential for Fe(III) oxide reduction. However, pili preparations of the Aro-5 strain had greatly diminished conductivity and Aro-5 cultures were severely limited in their capacity to reduce Fe(III) compared to the control strain. Current production of the Aro-5 strain, with a graphite anode serving as the electron acceptor, was less than 10\% of that of the control strain. The conductivity of the Aro-5 biofilms was 10-fold lower than the control strain{\textquoteright}s. These results demonstrate that the pili of G. sulfurreducens must be conductive in order for the cells to be effective in extracellular long-range electron transport.

IMPORTANCE: Extracellular electron transfer by Geobacter species plays an important role in the biogeochemistry of soils and sediments and has a number of bioenergy applications. For example, microbial reduction of Fe(III) oxide is one of the most geochemically significant processes in anaerobic soils, aquatic sediments, and aquifers, and Geobacter organisms are often abundant in such environments. Geobacter sulfurreducens produces the highest current densities of any known pure culture, and close relatives are often the most abundant organisms colonizing anodes in microbial fuel cells that harvest electricity from wastewater or aquatic sediments. The finding that a strain of G. sulfurreducens that produces pili with low conductivity is limited in these extracellular electron transport functions provides further insight into these environmentally significant processes.

}, keywords = {Amino Acids, Aromatic, Bioelectric Energy Sources, Biofilms, Electricity, Electrodes, Electron Transport, Ferric Compounds, Fimbriae Proteins, Fimbriae, Bacterial, Geobacter, Graphite}, issn = {2150-7511}, doi = {10.1128/mBio.00105-13}, author = {Vargas, Madeline and Malvankar, Nikhil S and Tremblay, Pier-Luc and Leang, Ching and Smith, Jessica A and Patel, Pranav and Snoeyenbos-West, Oona and Nevin, Kelly P and Lovley, Derek R} } @article {406, title = {Electrical conductivity in a mixed-species biofilm.}, journal = {Appl Environ Microbiol}, volume = {78}, year = {2012}, month = {2012 Aug}, pages = {5967-71}, abstract = {

Geobacter sulfurreducens can form electrically conductive biofilms, but the potential for conductivity through mixed-species biofilms has not been examined. A current-producing biofilm grown from a wastewater sludge inoculum was highly conductive with low charge transfer resistance even though microorganisms other than Geobacteraceae accounted for nearly half the microbial community.

}, keywords = {Biofilms, Electric Conductivity, Microbial Consortia, Sewage}, issn = {1098-5336}, doi = {10.1128/AEM.01803-12}, author = {Malvankar, Nikhil S and Lau, Joanne and Nevin, Kelly P and Franks, Ashley E and Tuominen, Mark T and Lovley, Derek R} } @article {407, title = {Microbial nanowires: a new paradigm for biological electron transfer and bioelectronics.}, journal = {ChemSusChem}, volume = {5}, year = {2012}, month = {2012 Jun}, pages = {1039-46}, abstract = {

The discovery that Geobacter sulfurreducens can produce protein filaments with metallic-like conductivity, known as microbial nanowires, that facilitate long-range electron transport is a paradigm shift in biological electron transfer and has important implications for biogeochemistry, microbial ecology, and the emerging field of bioelectronics. Although filaments in a wide diversity of microorganisms have been called microbial nanowires, the type IV pili of G. sulfurreducens and G. metallireducens are the only filaments that have been shown to be required for extracellular electron transport to extracellular electron acceptors or for conduction of electrons through biofilms. Studies of G. sulfurreducens pili preparations and intact biofilms under physiologically relevant conditions have provided multiple lines of evidence for metallic-like conduction along the length of pili and for the possibility of pili networks to confer high conductivity within biofilms. This mechanism of electron conduction contrasts with the previously known mechanism for biological electron transfer via electron tunneling or hopping between closely associated molecules, a strategy unlikely to be well adapted for long-range electron transport outside the cell. In addition to promoting electron exchange with abiotic electron acceptors, microbial nanowires have recently been shown to be involved in direct interspecies electron transfer between syntrophic partners. An improved understanding of the mechanisms for metallic-like conductivity in microbial nanowires, as well as engineering microorganisms with desirable catalytic abilities with nanowires, could lead to new applications in microbial electrosynthesis and bioelectronics.

}, keywords = {Bacterial Physiological Phenomena, Biofilms, Biotechnology, Electric Conductivity, Ferric Compounds, Fimbriae Proteins, Fimbriae, Bacterial, Nanostructures, Oxidation-Reduction}, issn = {1864-564X}, doi = {10.1002/cssc.201100733}, author = {Malvankar, Nikhil S and Lovley, Derek R} } @article {414, title = {Supercapacitors based on c-type cytochromes using conductive nanostructured networks of living bacteria.}, journal = {Chemphyschem}, volume = {13}, year = {2012}, month = {2012 Feb}, pages = {463-8}, abstract = {Supercapacitors have attracted interest in energy storage because they have the potential to complement or replace batteries. Here, we report that c-type cytochromes, naturally immersed in a living, electrically conductive microbial biofilm, greatly enhance the device capacitance by over two orders of magnitude. We employ genetic engineering, protein unfolding and Nernstian modeling for in vivo demonstration of charge storage capacity of c-type cytochromes and perform electrochemical impedance spectroscopy, cyclic voltammetry and charge-discharge cycling to confirm the pseudocapacitive, redox nature of biofilm capacitance. The biofilms also show low self-discharge and good charge/discharge reversibility. The superior electrochemical performance of the biofilm is related to its high abundance of cytochromes, providing large electron storage capacity, its nanostructured network with metallic-like conductivity, and its porous architecture with hydrous nature, offering prospects for future low cost and environmentally sustainable energy storage devices.}, keywords = {Bacteria, Biofilms, Cytochrome c Group, Dielectric Spectroscopy, Electric Capacitance, Electrodes, Geobacter, Nanostructures, Oxidation-Reduction}, issn = {1439-7641}, doi = {10.1002/cphc.201100865}, author = {Malvankar, Nikhil S and Mester, T{\"u}nde and Tuominen, Mark T and Lovley, Derek R} } @article {415, title = {Geobacter: the microbe electric{\textquoteright}s physiology, ecology, and practical applications.}, journal = {Adv Microb Physiol}, volume = {59}, year = {2011}, month = {2011}, pages = {1-100}, abstract = {Geobacter species specialize in making electrical contacts with extracellular electron acceptors and other organisms. This permits Geobacter species to fill important niches in a diversity of anaerobic environments. Geobacter species appear to be the primary agents for coupling the oxidation of organic compounds to the reduction of insoluble Fe(III) and Mn(IV) oxides in many soils and sediments, a process of global biogeochemical significance. Some Geobacter species can anaerobically oxidize aromatic hydrocarbons and play an important role in aromatic hydrocarbon removal from contaminated aquifers. The ability of Geobacter species to reductively precipitate uranium and related contaminants has led to the development of bioremediation strategies for contaminated environments. Geobacter species produce higher current densities than any other known organism in microbial fuel cells and are common colonizers of electrodes harvesting electricity from organic wastes and aquatic sediments. Direct interspecies electron exchange between Geobacter species and syntrophic partners appears to be an important process in anaerobic wastewater digesters. Functional and comparative genomic studies have begun to reveal important aspects of Geobacter physiology and regulation, but much remains unexplored. Quantifying key gene transcripts and proteins of subsurface Geobacter communities has proven to be a powerful approach to diagnose the in situ physiological status of Geobacter species during groundwater bioremediation. The growth and activity of Geobacter species in the subsurface and their biogeochemical impact under different environmental conditions can be predicted with a systems biology approach in which genome-scale metabolic models are coupled with appropriate physical/chemical models. The proficiency of Geobacter species in transferring electrons to insoluble minerals, electrodes, and possibly other microorganisms can be attributed to their unique "microbial nanowires," pili that conduct electrons along their length with metallic-like conductivity. Surprisingly, the abundant c-type cytochromes of Geobacter species do not contribute to this long-range electron transport, but cytochromes are important for making the terminal electrical connections with Fe(III) oxides and electrodes and also function as capacitors, storing charge to permit continued respiration when extracellular electron acceptors are temporarily unavailable. The high conductivity of Geobacter pili and biofilms and the ability of biofilms to function as supercapacitors are novel properties that might contribute to the field of bioelectronics. The study of Geobacter species has revealed a remarkable number of microbial physiological properties that had not previously been described in any microorganism. Further investigation of these environmentally relevant and physiologically unique organisms is warranted.}, keywords = {Biotechnology, Ecology, Environmental Remediation, Ferric Compounds, Geobacter}, issn = {0065-2911}, doi = {10.1016/B978-0-12-387661-4.00004-5}, author = {Lovley, Derek R and Ueki, Toshiyuki and Zhang, Tian and Malvankar, Nikhil S and Shrestha, Pravin M and Flanagan, Kelly A and Aklujkar, Muktak and Butler, Jessica E and Giloteaux, Ludovic and Rotaru, Amelia-Elena and Holmes, Dawn E and Franks, Ashley E and Orellana, Roberto and Risso, Carla and Nevin, Kelly P} } @article {418, title = {Tunable metallic-like conductivity in microbial nanowire networks.}, journal = {Nat Nanotechnol}, volume = {6}, year = {2011}, month = {2011 Sep}, pages = {573-9}, abstract = {Electronic nanostructures made from natural amino acids are attractive because of their relatively low cost, facile processing and absence of toxicity. However, most materials derived from natural amino acids are electronically insulating. Here, we report metallic-like conductivity in films of the bacterium Geobacter sulfurreducens and also in pilin nanofilaments (known as microbial nanowires) extracted from these bacteria. These materials have electronic conductivities of \~{}5~mS~cm(-1), which are comparable to those of synthetic metallic nanostructures. They can also conduct over distances on the centimetre scale, which is thousands of times the size of a bacterium. Moreover, the conductivity of the biofilm can be tuned by regulating gene expression, and also by varying the gate voltage in a transistor configuration. The conductivity of the nanofilaments has a temperature dependence similar to that of a disordered metal, and the conductivity could be increased by processing.}, keywords = {Electric Conductivity, Geobacter, Nanowires, Transistors, Electronic}, issn = {1748-3395}, doi = {10.1038/nnano.2011.119}, author = {Malvankar, Nikhil S and Vargas, Madeline and Nevin, Kelly P and Franks, Ashley E and Leang, Ching and Kim, Byoung-Chan and Inoue, Kengo and Mester, T{\"u}nde and Covalla, Sean F and Johnson, Jessica P and Rotello, Vincent M and Tuominen, Mark T and Lovley, Derek R} } @article {431, title = {Direct exchange of electrons within aggregates of an evolved syntrophic coculture of anaerobic bacteria.}, journal = {Science}, volume = {330}, year = {2010}, month = {2010 Dec 3}, pages = {1413-5}, abstract = {Microbial consortia that cooperatively exchange electrons play a key role in the anaerobic processing of organic matter. Interspecies hydrogen transfer is a well-documented strategy for electron exchange in dispersed laboratory cultures, but cooperative partners in natural environments often form multispecies aggregates. We found that laboratory evolution of a coculture of Geobacter metallireducens and Geobacter sulfurreducens metabolizing ethanol favored the formation of aggregates that were electrically conductive. Sequencing aggregate DNA revealed selection for a mutation that enhances the production of a c-type cytochrome involved in extracellular electron transfer and accelerates the formation of aggregates. Aggregate formation was also much faster in mutants that were deficient in interspecies hydrogen transfer, further suggesting direct interspecies electron transfer.}, keywords = {Anaerobiosis, Bacterial Proteins, Biological Evolution, Culture Media, Cytochrome c Group, Electron Transport, Electrons, Ethanol, Fimbriae Proteins, Geobacter, Hydrogen, Microbial Consortia, Microbial Interactions, Mutation, Oxidation-Reduction, Selection, Genetic}, issn = {1095-9203}, doi = {10.1126/science.1196526}, author = {Summers, Zarath M and Fogarty, Heather E and Leang, Ching and Franks, Ashley E and Malvankar, Nikhil S and Lovley, Derek R} }