Bacterial Nanowires Research Articles

Electric stimulation: a versatile manipulation technique mediated microbial applications.

Electric stimulation (ES) is a versatile technique that uses an electric field to manipulate microorganisms individually. Over the past several decades, the capabilities of ES have expanded from bioremediation to the precise motion control of cells and microorganisms. However, there is limited information on the underlying mechanisms, latest advancement and broader microbial applications of ES in various fields, such as the production of extracellular polymers with upgraded properties. This review article summarizes recent advancements in ES and discusses it as a unique external manipulation technique for microorganisms with wide applications in bioremediation, industry, biofilm deactivation, disinfection, and controlled biosynthesis. One specific application of ES discussed in this review is the extracellular biosynthesis, regulation, and organization of extracellular polymers, such as bacterial cellulose nanofibrils, curdlan, and microbial nanowires. Overall, this review aims to provide a platform for microbial biotechnologists and synthetic biologists to leverage the manipulation of microorganisms using ES for bio-based applications, including the production of extracellular polymers with enhanced properties. Researchers can engineer, manipulate, and control microorganisms for various applications by harnessing the potential of electric fields.

Carbon Steel Corrosion Induced by Sulfate-Reducing Bacteria: A Review of Electrochemical Mechanisms and Pathways in Biofilms

Microbial metal corrosion has become an important topic in metal research, which is one of the main causes of equipment damage, energy loss, and economic loss. At present, the research on microbial metal corrosion focuses on the characteristics of corrosion products, the environmental conditions affecting corrosion, and the measures and means of corrosion prevention, etc. In contrast, the main microbial taxa involved in metal corrosion, their specific role in the corrosion process, and the electron transfer pathway research are relatively small. This paper summarizes the mechanism of microbial carbon steel corrosion caused by SRB, including the cathodic depolarization theory, acid metabolite corrosion theory, and the biocatalytic cathodic sulfate reduction mechanism. Based on the reversible nature of electron transfer in biofilms and the fact that electrons must pass through the extracellular polymers layer between the solid electrode and the cell, this paper focuses on three types of electrochemical mechanisms and electron transfer modes of extracellular electron transfer occurring in microbial fuel cells, including direct-contact electron transfer, electron transfer by conductive bacterial hair proteins or nanowires, and electron shuttling mediated by the use of soluble electron mediators. Finally, a more complete pathway of electron transfer in microbial carbon steel corrosion due to SRB is presented: an electron goes from the metal anode, through the extracellular polymer layer, the extracellular membrane, the periplasm, and the intracellular membrane, to reach the cytoplasm for sulfate allosteric reduction. This article also focuses on a variety of complex components in the extracellular polymer layer, such as extracellular DNA, quinoline humic acid, iron sulfide (FeSX), Fe3+, etc., which may act as an extracellular electron donor to provide electrons for the SRB intracellular electron transfer chain; the bioinduced mineralization that occurs in the SRB biofilm can inhibit metal corrosion, and it can be used for the development of green corrosion inhibitors. This provides theoretical guidance for the diagnosis, prediction, and prevention of microbial metal corrosion.

BIOELECTRICAL PRODUCTION BY MICROBIAL NANOWIRES

Bacterial nanowires, which are electrically conductive appendages or threads that facilitate the transfer of electrons outside the bacterial cell, produced by a number of bacteria, have shown the potential to be useful in many fields, especially bioenergy and bioremediation. Bacteria have developed a group of nanowires in different forms, including Pilli, which are fine structures protruding from the surface of bacteria and used for adhesion, movement, and the formation of biofilms in some species that participate in extracellular respiration and use their filaments to transfer electrons. The world is currently witnessing an energy crisis due to the continuous increase in energy requirements around the world, so the trend today is to use microbial fuel cell (MFC) technology, which is an encouraging technology for generating electricity and a source of clean and environmentally friendly energy that uses microorganisms to convert the chemical energy of organic compounds into electricity using bacterial nanowires. Most MFC studies are being done for electricity generation, which is the primary application of the technology at the moment, and it is a fascinating technology that can use a variety of substrates, MFC cell with bacteria to achieve bioenergy production despite the fact that the energy levels in all these systems were relatively low

Fabrication of Electronically Conductive Protein-Heme Nanowires for Power Harvesting.

Electronically conductive protein-based materials can enable the creation of bioelectronic components and devices from sustainable and nontoxic materials, while also being well-suited to interface with biological systems, such as living cells, for biosensor applications. However, as proteins are generally electrical insulators, the ability to render protein assemblies electroactive in a tailorable manner can usher in a plethora of useful materials. Here, an approach to fabricate electronically conductive protein nanowires is presented by aligning heme molecules in proximity along protein filaments, with these nanowires also possessing charge transfer abilities that enable energy harvesting from ambient humidity. The heme-incorporated protein nanowires demonstrate electron transfer over micrometer distances, with conductive atomic force microscopy showing individual nanowires having comparable conductance to other previously characterized heme-based bacterial nanowires. Exposure of multilayer nanowire films to humidity produces an electrical current, presumably through water molecules ionizing carboxyl groups in the filament and creating an unbalanced total charge distribution that is enhanced by the heme. Incorporation of heme and potentially other metal-center porphyrin molecules into protein nanostructures could pave the way for structurally- and electrically-defined protein-based bioelectronic devices.

Building realistic models and computational tools to model electron transfer processes in bacterial nanowires

Building realistic models and computational tools to model electron transfer processes in bacterial nanowires

Investigation of heavy metal removal from salty wastewater and voltage production using Shewanella oneidensisMR‐1 nanowires in a dual‐chamber microbial fuel cell

AbstractHeavy metal removal and simultaneous energy production were studied using a dual chambered Microbial Fuel Cell inoculated with Shewanella oneidensis MR‐1 in the anode. Synthetic wastewater was prepared with Cu (II), Mg (II), Mn (II), Zn (II), Na, and Phenol based on desalter effluent from refinery processes at different metal concentrations. In this study, a maximum open‐circuit voltage of 517.6 mV was reached at Conc. 5 with wastewater in the anode chamber, and 127.7 mV at Conc. 3 was produced with synthetic wastewater in the cathode chamber. Moreover, μ at Conc. 5 was 0.1133 h−1, demonstrating bacterial growth under metal and phenol concentrations. The highest metal removal in the anode for Cu (II), Mg (II), Mn (II), Zn (II), and Na was 93%, 85%, 93%, 88%, and 36%, respectively. In the cathode chamber the removal of Cu (II), Mg (II), Mn (II), Zn (II), and Na was 98%, 49%, 57%, 59%, and 36%, respectively. During the operation in the anode, SEM images showed that the bacterial nanowires are formed in response to toxic and anaerobic environments which contribute to the bacterial growth. These nanowires increased the metal removal and the voltage production as a consequence of a higher electron rate from the anode to the cathode due to the higher extracellular membrane surface area. S. oneidensis is a bacterium with metal‐reducing characteristics, and it is suitable for metal removal and electron transport from carbon sources, demonstrated in voltage production with microbial fuel cells.

Engineering nanowires in bacteria to elucidate electron transport structural–functional relationships

Bacterial pilin nanowires are protein complexes, suggested to possess electroactive capabilities forming part of the cells’ bioenergetic programming. Their role is thought to be linked to facilitating electron transfer between cells and the external environment to permit metabolism and cell-to-cell communication. There is a significant debate, with varying hypotheses as to the nature of the proteins currently lying between type-IV pilin-based nanowires and polymerised cytochrome-based filaments. Importantly, to date, there is a very limited structure–function analysis of these structures within whole bacteria. In this work, we engineered Cupriavidus necator H16, a model autotrophic organism to express differing aromatic modifications of type-IV pilus proteins to establish structure–function relationships on conductivity and the effects this has on pili structure. This was achieved via a combination of high-resolution PeakForce tunnelling atomic force microscopy (PeakForce TUNA™) technology, alongside conventional electrochemical approaches enabling the elucidation of conductive nanowires emanating from whole bacterial cells. This work is the first example of functional type-IV pili protein nanowires produced under aerobic conditions using a Cupriavidus necator chassis. This work has far-reaching consequences in understanding the basis of bio-electrical communication between cells and with their external environment.

Correlated particle transport enables biological free energy transduction

Studies of biological transport frequently neglect the explicit statistical correlations among particle site occupancies (i.e., they use a mean-field approximation). Neglecting correlations sometimes captures biological function, even for out-of-equilibrium and interacting systems. We show that neglecting correlations fails to describe free energy transduction, mistakenly predicting an abundance of slippage and energy dissipation, even for networks that are near reversible and lack interactions among particle sites. Interestingly, linear charge transport chains are well described without including correlations, even for networks that are driven and include site-site interactions typical of biological electron transfer chains. We examine three specific bioenergetic networks: a linear electron transfer chain (as found in bacterial nanowires), a near-reversible electron bifurcation network (as in complex III of respiration and other recently discovered structures), and a redox-coupled proton pump (as in complex IV of respiration).

Superflexible hybrid aerogel-based fabrics enable broadband electromagnetic wave management

As problematic interference from electromagnetic waves (EMWs) in modern human life continues to increase, broadband EMW management aerogels have received extensive attention in the safety and thermal management fields, but the mismatch in terms of their mechanical flexibility and functionality has limited their application. This work reports an ultra-lightweight (8.7 mg cm−3), superflexible, hyperelastic (≥95% strain), and superhydrophobic (contact angle: 157°) wearable hybrid aerogel-based fabric that offers both electromagnetic interference (EMI) shielding and infrared (IR) shielding functions. A nanotape-enabled multi-crosslinked hybridization strategy, in which freeze-drying-initiated hydrophobic -Si-O-Si- nanotape welds weak bacterial nanocellulose-silver nanowire interfaces perfectly, gives the fabric outstanding mechanical properties. Optimized synergy gain engineering between the metal and the semiconductor (antimony tin oxide nanoparticles) produces significant enhancements in the electrical conductivity (502.46 S m−1), the EMI shielding effectiveness (SE, 100 dB), and the IR shielding performance (ultralow thermal conductivity of 0.025 W m−1 K−1) of the fabric. The hybrid aerogel-based fabric is fabricated into broadband EMW management clothing, which has excellent prospects for safety and thermal management applications.

Recent Advances in Understanding the Electron Transport Through Metal-Azurin-Metal Junctions

Azurin proteins are the workhorse of protein electronics. This is a branch of biomolecular electronics, a recent research field which investigates electronics based on biomolecules such as proteins, peptides, amino acids, bacterial nanowires or DNA. In general, the possibility of including biosystems in solid-state junctions has opened the way to the development of novel electrical devices, and proteins have attracted enormous attention thanks to their many interesting properties. In the particular case of metal-azurin-metal junctions, experimental measurements have revealed extremely efficient electron transport over large distances, showing conductance values which are higher than certain conjugated molecules of similar lengths. Moreover, the electrical current has often been found to be temperature-independent, which has been used as an evidence of coherent transport or quantum tunneling. Interesting effects have been observed, moreover, upon insertion of single amino-acid mutations. In spite of a huge amount of work, the exact mechanism for the charge flow through these systems is still under debate. In this review, we will revise the recent advances made in the electron-transport measurements of azurin-based junctions as well as the corresponding theoretical modelling. We will discuss the interpretation of the currently-available experimental results as well as the open issues which still remain to be clarified.

Single molecule tracking of bacterial cell surface cytochromes reveals dynamics that impact long-distance electron transport

Using a series of multiheme cytochromes, the metal-reducing bacterium Shewanella oneidensis MR-1 can perform extracellular electron transfer (EET) to respire redox-active surfaces, including minerals and electrodes outside the cell. While the role of multiheme cytochromes in transporting electrons across the cell wall is well established, these cytochromes were also recently found to facilitate long-distance (micrometer-scale) redox conduction along outer membranes and across multiple cells bridging electrodes. Recent studies proposed that long-distance conduction arises from the interplay of electron hopping and cytochrome diffusion, which allows collisions and electron exchange between cytochromes along membranes. However, the diffusive dynamics of the multiheme cytochromes have never been observed or quantified in vivo, making it difficult to assess their hypothesized contribution to the collision-exchange mechanism. Here, we use quantum dot labeling, total internal reflection fluorescence microscopy, and single-particle tracking to quantify the lateral diffusive dynamics of the outer membrane-associated decaheme cytochromes MtrC and OmcA, two key components of EET in S. oneidensis. We observe confined diffusion behavior for both quantum dot-labeled MtrC and OmcA along cell surfaces (diffusion coefficients DMtrC = 0.0192 ± 0.0018 µm2/s, DOmcA = 0.0125 ± 0.0024 µm2/s) and the membrane extensions thought to function as bacterial nanowires. We find that these dynamics can trace a path for electron transport via overlap of cytochrome trajectories, consistent with the long-distance conduction mechanism. The measured dynamics inform kinetic Monte Carlo simulations that combine direct electron hopping and redox molecule diffusion, revealing significant electron transport rates along cells and membrane nanowires.

A Simple, Semiclassical Mechanism for Activationless, Long RangeCharge Transport in Molecular Junctions

Past reports on photocurrents in molecular junctions consisting of aromatic oligomers between electrical contacts reveal very low activation energies (<1 meV) and weak distance dependence for molecular layer thicknesses of 20–60 nm. Photocurrent transport mediated by sequential tunneling between adjacent subunit orbitals represents a “super highway” for charge transport with low activation barrier, field dependence and long range of at least 60 nm. In addition to photocurrents, such transport may be involved in dark currents for distances >10 nm, previously reported biological transport across μm in bacterial nanowires, and >1 cm in cable bacteria.

Microbial Fabrication of Nanomaterial and Its Role in Disintegration of Exopolymeric Matrices of Biofilm.

Bacterial biofilms are responsible for the development of various chronic wound-related and implant-mediated infections and confer protection to the pathogenic bacteria against antimicrobial drugs and host immune responses. Hence, biofilm-mediated chronic infections have created a tremendous burden upon healthcare systems worldwide. The development of biofilms upon the surface of medical implants has resulted in the failure of various implant-based surgeries and therapies. Although different conventional chemical and physical agents are used as antimicrobials, they fail to kill the sessile forms of bacterial pathogens due to the resistance exerted by the exopolysaccharide (EPS) matrices of the biofilm. One of the major techniques used in addressing such a problem is to directly check the biofilm formation by the use of novel antibiofilm materials, local drug delivery, and device-associated surface modifications, but the success of these techniques is still limited. The immense expansion in the field of nanoscience and nanotechnology has resulted in the development of novel nanomaterials as biocidal agents that can be either easily integrated within biomaterials to prevent the colonization of microbial cells or directly approach the pathogen overcoming the biofilm matrix. The antibiofilm efficacies of these nanomaterials are accomplished by the generation of oxidative stresses and through alterations of the genetic expressions. Microorganism-assisted synthesis of nanomaterials paved the path to success in such therapeutic approaches and is found to be more acceptable for its “greener” approach. Metallic nanoparticles functionalized with microbial enzymes, silver–platinum nanohybrids (AgPtNHs), bacterial nanowires, superparamagnetic iron oxide (Fe3O4), and nanoparticles synthesized by both magnetotactic and non-magnetotactic bacteria showed are some of the examples of such agents used to attack the EPS.

Coherence-assisted electron diffusion across the multi-heme protein-based bacterial nanowire

Biological electron transfer (ET) is one of the most studied biochemical processes due to its immense importance in biology. For many years, biological ET was explained using the classical incoherent transport mechanism, i.e. sequential hopping. One of the relatively recent major observations in this field is long-range extracellular ET (EET), where some bacteria were shown to mediate electrons for extremely long distances on the micrometer length scales across individual nanowires. This fascinating finding has resulted in several suggested mechanisms that might explain this intriguing EET. More recently, the structure of a conductive G. sulfurreducens nanowire has been solved, which showed a highly ordered quasi-1D wire of a hexaheme cytochrome protein, named OmcS. Based on this new structure, we suggest here several electron diffusion models for EET, involving either purely hopping or several degrees of mixed hopping and coherent transport, in which the coherent part is due to a local rigidification of the protein structure, associated with a decrease in the local reorganization energy. The effect is demonstrated for two closely packed heme sites as well as for longer chains containing up to several dozens porphyrins. We show that the pure hopping model probably cannot explain the reported conductivity values of the G. sulfurreducens nanowire using conventional values of reorganization energy and electronic coupling. On the other hand, we show that for a wide range of the latter energy values, the mixed hopping-coherent model results in superior electron diffusion compared to the pure hopping model, and especially for long-range coherent transport, involving multiple porphyrin sites.

Microbial Nanotechnology: Challenges and Prospects for Green Biocatalytic Synthesis of Nanoscale Materials for Sensoristic and Biomedical Applications.

Nanomaterials are increasingly being used in new products and devices with a great impact on different fields from sensoristics to biomedicine. Biosynthesis of nanomaterials by microorganisms is recently attracting interest as a new, exciting approach towards the development of ‘greener’ nanomanufacturing compared to traditional chemical and physical approaches. This review provides an insight about microbial biosynthesis of nanomaterials by bacteria, yeast, molds, and microalgae for the manufacturing of sensoristic devices and therapeutic/diagnostic applications. The last ten-year literature was selected, focusing on scientific works where aspects like biosynthesis features, characterization, and applications have been described. The knowledge, challenges, and potentiality of microbial-mediated biosynthesis was also described. Bacteria and microalgae are the main microorganism used for nanobiosynthesis, principally for biomedical applications. Some bacteria and microalgae have showed the ability to synthetize unique nanostructures: bacterial nanocellulose, exopolysaccharides, bacterial nanowires, and biomineralized nanoscale materials (magnetosomes, frustules, and coccoliths). Yeasts and molds are characterized by extracellular synthesis, advantageous for possible reuse of cell cultures and reduced purification processes of nanomaterials. The intrinsic variability of the microbiological systems requires a greater protocols standardization to obtain nanomaterials with increasingly uniform and reproducible chemical-physical characteristics. A deeper knowledge about biosynthetic pathways and the opportunities from genetic engineering are stimulating the research towards a breakthrough development of microbial-based nanosynthesis for the future scaling-up and possible industrial exploitation of these promising ‘nanofactories’.

Accelerating the startup of microbial fuel cells by facile microbial acclimation

Rapid startup of microbial fuel cells (MFCs) is critical for the practical application of MFCs, while it is still challenging to quickly initiate newly fabricated MFCs. Herein, we show that a simple yet handy strategy for acclimating the inoculum, by modulating brightness, anoxic environment and the presence of carbon powder, can accelerate the startup of MFCs. Using acclimated inoculums, the startup time of MFCs can be reduced to 43 h which is shortened by 63.2% than that of MFCs using unacclimated inoculums, while maximum power densities are well maintained (649–724 and 669 mW m−2 by using acclimated and unacclimated inoculums, respectively). Further studies show that the acclimation can induce the generation of conductive bacterial nanowires and increase the Rhizobiales population. Accordingly, we reason that conductive bacterial nanowires and the metabolically versatile Rhizobiales can synergistically facilitate the irreversible attachment of bacteria and subsequent biofilm formation, and therefore accelerate the startup.

Surface-Induced Formation and Redox-Dependent Staining of Outer Membrane Extensions in Shewanella oneidensis MR-1

The metal-reducing bacterium Shewanella oneidensis MR-1 produces extensions of its outer membrane (OM) and periplasm that contain cytochromes responsible for extracellular electron transfer (EET) to external redox-active surfaces, including minerals and electrodes. While the role of multi-heme cytochromes in transporting electrons across the cell wall is well established, their distribution along S. oneidensis OM extensions is also thought to allow lateral electron transport along these filaments. These proposed bacterial nanowires, which can be several times the cell length, would thereby extend EET to more distant electron acceptors. However, it is still unclear why these extensions form, and to what extent they contribute to respiration in living cells. Here, we investigate physical contributors to their formation using in vivo fluorescence microscopy. While previous studies focused on the display of S. oneidensis outer membrane extensions (OMEs) as a response to oxygen limitation, we find that cell-to-surface contact is sufficient to trigger the production of OMEs, including some that reach >100 µm in length, irrespective of medium composition, agitation, or aeration. To visualize the extent of heme redox centers along OMEs, and help distinguish these structures from other extracellular filaments, we also performed histochemical redox-dependent staining with transmission electron microscopy on wild type and cytochrome-deficient strains. We demonstrate that redox-active components are limited to OMEs and not present on other extracellular appendages, such as pili and flagella. We also observed that the loss of 8 functional periplasmic and outer membrane cytochromes significantly decreased both the frequency and intensity of redox-dependent staining found widespread on OMEs. These results will improve our understanding of the environmental conditions that influence the formation of S. oneidensis OMEs, as well as the distribution and functionality of EET components along extracellular appendages.

Cryo-EM reveals the structural basis of long-range electron transport in a cytochrome-based bacterial nanowire

Electrically conductive pili from Geobacter species, termed bacterial nanowires, are intensely studied for their biological significance and potential in the development of new materials. Using cryo-electron microscopy, we have characterized nanowires from conductive G. sulfurreducens pili preparations that are composed solely of head-to-tail stacked monomers of the six-heme C-type cytochrome OmcS. The unique fold of OmcS — closely wrapped around a continuous stack of hemes that can serve as an uninterrupted path for electron transport — generates a scaffold that supports the unbranched chain of hemes along the central axis of the filament. We present here, at 3.4 Å resolution, the structure of this cytochrome-based filament and discuss its possible role in long-range biological electron transport.

Assessing Possible Mechanisms of Micrometer-Scale Electron Transfer in Heme-Free Geobacter sulfurreducens Pili.

The electrically conductive pili of Geobacter sulfurreducens are of both fundamental and practical interest. They facilitate extracellular and interspecies electron transfer (ET) and also provide an electrical interface between living and nonliving systems. We examine the possible mechanisms of G. sulfurreducens electron transfer in regimes ranging from incoherent to coherent transport. For plausible ET parameters, electron transfer in G. sulfurreducens bacterial nanowires mediated only by the protein is predicted to be dominated by incoherent hopping between phenylalanine (Phe) and tyrosine (Tyr) residues that are 3 to 4 Å apart, where Phe residues in the hopping pathways may create delocalized "islands." This mechanism could be accessible in the presence of strong oxidants that are capable of oxidizing Phe and Tyr residues. We also examine the physical requirements needed to sustain biological respiration via nanowires. We find that the hopping regimes with ET rates on the order of 108 s-1 between Phe islands and Tyr residues, and conductivities on the order of mS/cm, can support ET fluxes that are compatible with cellular respiration rates, although sustaining this delocalization in the heterogeneous protein environment may be challenging. Computed values of fully coherent electron fluxes through the pili are orders of magnitude too low to support microbial respiration. We suggest experimental probes of the transport mechanism based on mutant studies to examine the roles of aromatic amino acids and yet to be identified redox cofactors.

Effect of hydraulic retention time on electricity generation using a solid plain-graphite plate microbial fuel cell anoxic/oxic process for treating pharmaceutical sewage

Treatment efficiency and electricity generation were evaluated using a solid plain-graphite plate microbial fuel cell (MFC) anoxic/oxic (A/O) process that treated pharmaceutical sewage using different hydraulic retention times (HRT). Short HRTs increased the volumetric organic loading rate, thereby reducing the MFC performance due to rapid depletion of the substrate (carbon/nitrogen source). The COD removal efficiency decreased from 96.28% at a HRT of 8 h to 90.67% at a HRT of 5 h. The removal efficiency of total nitrogen was reduced from 74.16% at a HRT of 8 h to 53.42% at a HRT of 5 h. A shorter HRT decreased the efficiency in treatment of the pharmaceutical products (PPs), which included acetaminophen, ibuprofen and sulfamethoxazole in an aerobic reactor because these antibiotic compounds inhibited the microbial activity of the aerobic activated sludge in the MFC A/O system. The average power density and coulombic efficiency values were 162.74 mW m−2 and 7.09% at a HRT of 8 h and 29.12 mW m−2 and 2.23% at a HRT of 5 h, respectively. The dominant bacterial species including Hydrogenophaga spp., Rubrivivax spp. and Leptothrix spp., which seem to be involved in PP biodegradation; these were identified in the MFC A/O system under all HRT conditions for the first time using next generation sequencing. Bacterial nanowires were involved in accelerating the transfer of electrons and served as mediators in the SPGRP biofilm. In conclusion, a SPGRP MFC A/O system at a HRT of 8 h gave better removal of COD, T-N and PPs, as well as generated more electricity.