@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 {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 {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 {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 {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 {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 {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 {535, title = {Extracellular electron transfer via microbial nanowires.}, journal = {Nature}, volume = {435}, year = {2005}, month = {2005 Jun 23}, pages = {1098-101}, abstract = {Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(iii) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(iii) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(iii) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(iii) reducers in many environments, must directly contact Fe(iii) oxides to reduce them, and produce monolateral pili that were proposed, on the basis of the role of pili in other organisms, to aid in establishing contact with the Fe(iii) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(iii) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(iii) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell-cell interactions, and for bioengineering of novel conductive materials.}, keywords = {Biotechnology, Electric Conductivity, Electron Transport, Ferric Compounds, Fimbriae Proteins, Fimbriae, Bacterial, Genes, Bacterial, Geobacter, Microscopy, Atomic Force, Microscopy, Electron, Transmission, Mutation, Nanostructures, Phylogeny}, issn = {1476-4687}, doi = {10.1038/nature03661}, author = {Reguera, Gemma and McCarthy, Kevin D and Mehta, Teena and Nicoll, Julie S and Tuominen, Mark T and Lovley, Derek R} }