@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 {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 {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 {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 {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 {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 {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} }