Rate enhancement of bacterial extracellular electron transport involves bound flavin semiquinones

Dissimilatory Fe(III) Reduction Controls on Arsenic Mobilization: A Combined Biogeochemical and NanoSIMS Imaging Approach

Microbial metabolism plays a key role in controlling the fate of toxic groundwater contaminants, such as arsenic. Dissimilatory metal reduction catalyzed by subsurface bacteria can facilitate the mobilization of arsenic via the reductive dissolution of As(V)-bearing Fe(III) mineral assemblages. The mobility of liberated As(V) can then be amplified via reduction to the more soluble As(III) by As(V)-respiring bacteria. This investigation focused on the reductive dissolution of As(V) sorbed onto Fe(III)-(oxyhydr)oxide by model Fe(III)- and As(V)-reducing bacteria, to elucidate the mechanisms underpinning these processes at the single-cell scale. Axenic cultures of Shewanella sp. ANA-3 wild-type (WT) cells [able to respire both Fe(III) and As(V)] were grown using ¹³C-labeled lactate on an arsenical Fe(III)-(oxyhydr)oxide thin film, and after colonization, the distribution of Fe and As in the solid phase was assessed using nanoscale secondary ion mass spectrometry (NanoSIMS), complemented with aqueous geochemistry analyses. Parallel experiments were conducted using an arrA mutant, able to respire Fe(III) but not As(V). NanoSIMS imaging showed that most metabolically active cells were not in direct contact with the Fe(III) mineral. Flavins were released by both strains, suggesting that these cell-secreted electron shuttles mediated extracellular Fe(III)-(oxyhydr)oxide reduction, but did not facilitate extracellular As(V) reduction, demonstrated by the presence of flavins yet lack of As(III) in the supernatants of the arrA deletion mutant strain. 3D reconstructions of NanoSIMS depth-profiled single cells revealed that As and Fe were associated with the cell surface in the WT cells, whereas for the arrA mutant, only Fe was associated with the biomass. These data were consistent with Shewanella sp. ANA-3 respiring As(V) in a multistep process; first, the reductive dissolution of the Fe(III) mineral released As(V), and once in solution, As(V) was respired by the cells to As(III). As well as highlighting Fe(III) reduction as the primary release mechanism for arsenic, our data also identified unexpected cellular As(III) retention mechanisms that require further investigation.

Intra/extracellular electron transfer for aerobic denitrification mediated by in-situ biosynthesis palladium nanoparticles

The slow electron transfer rate is the bottleneck to the biological wastewater treatment process, and the nanoparticles (NPs) has been verified as a feasible strategy to improve the biological degradation efficiency by accelerating the electron transfer. Here, we employed the Gram-positive Bacillus megaterium Y-4, capable of synthetizing Pd(0), to investigate the intra/extracellular electron transfer (IET/EET) mechanisms mediated by NPs in aerobic denitrification for the first time. Kinetic and thermodynamic results showed that the bio-Pd(0) could significantly promote the removal of both nitrate and nitrite by improving affinity and decreasing activation energy. The enzymic activity and the respiration chain inhibition experiment indicated that the bio-Pd(0) could facilitate the nitrate biotic reduction by improving the Fe-S center activity and serving as parallel H carriers to replace coenzyme Q to selectively increase the electron flux toward nitrate in IET, while promoting the nitrite reduction by abiotic catalysis. Most importantly, the detection of DPV peak at -226~-287 mV proved that the one-electron EET via multiheme cytochrome-bound flavins also occurred in Gram-positive bacteria and enhanced in Pd-loaded cells. In addition, the remarkable increase of the formal charge in EPS indicated that the bio-Pd(0) could act as an electron shuttle to increase the redox site in EPS, eventually accelerating the electron hopping in long-distance electron transfer. Overall, this study expanded our understanding of the roles of bio-Pd(0) on the aerobic denitrification process and provided an insight into the IET/EET of Gram-positive strains.

Microbial extracellular electron transfer and strategies for engineering electroactive microorganisms

Electroactive microorganisms (EAMs) are ubiquitous in nature and have attracted considerable attention as they can be used for energy recovery and environmental remediation via their extracellular electron transfer (EET) capabilities. Although the EET mechanisms of Shewanella and Geobacter have been rigorously investigated and are well characterized, much less is known about the EET mechanisms of other microorganisms. For EAMs, efficient EET is crucial for the sustainable economic development of bioelectrochemical systems (BESs). Currently, the low efficiency of EET remains a key factor in limiting the development of BESs. In this review, we focus on the EET mechanisms of different microorganisms, (i.e., bacteria, fungi, and archaea). In addition, we describe in detail three engineering strategies for improving the EET ability of EAMs: (1) enhancing transmembrane electron transport via cytochrome protein channels; (2) accelerating electron transport via electron shuttle synthesis and transmission; and (3) promoting the microbe-electrode interface reaction via regulating biofilm formation. At the end of this review, we look to the future, with an emphasis on the cross-disciplinary integration of systems biology and synthetic biology to build high-performance EAM systems.

Electrolocation? The evidence for redox-mediated taxis in Shewanella oneidensis

Shewanella oneidensis is a dissimilatory metal reducing bacterium and model for extracellular electron transfer (EET), a respiratory mechanism in which electrons are transferred out of the cell. In the last 10 years, migration to insoluble electron acceptors for EET has been shown to be nonrandom and tactic, seemingly in the absence of molecular or energy gradients that typically allow for taxis. As the ability to sense, locate, and respire electrodes has applications in bioelectrochemical technology, a better understanding of taxis in S. oneidensis is needed. While the EET conduits of S. oneidensis have been studied extensively, its taxis pathways and their interplay with EET are not yet understood, making investigation into taxis phenomena nontrivial. Since S. oneidensis is a member of an EET-encoding clade, the genetic circuitry of taxis to insoluble acceptors may be conserved. We performed a bioinformatic analysis of Shewanella genomes to identify S. oneidensis chemotaxis orthologs conserved in the genus. In addition to the previously reported core chemotaxis gene cluster, we identify several other conserved proteins in the taxis signaling pathway. We present the current evidence for the two proposed models of EET taxis, "electrokinesis" and flavin-mediated taxis, and highlight key areas in need of further investigation.

Surface enhanced Raman scattering of bacteria using capped and uncapped silver nanoparticles

Surface enhanced Raman scattering (SERS) spectra of bacteria were obtained using citrate (capped) and borohydride (uncapped) generated silver nanoparticles (Ag NPs).The observed differences in SERS spectra are attributed to the manner in which these Ag NPs interact with bacteria. Capped Ag NPs are able to partition through the surface polysaccharides of the bacterial cell to bind to the inner and outer cell membranes, as well as the periplasmic space between them. The resultant spectra show contributions due to the components of the cell envelope and cellular secretions. Uncapped Ag NPs are unable to partition through the polysaccharide outer structures of the cells. Spectral features observed for these uncapped Ag NPs are secretions primarily due to the metabolites of purine degradation.

Internalized Carbon Dots for Enhanced Extracellular Electron Transfer in the Dark and Light

Cellular internalization of nanomaterials to endow cells with more functionalities is highly desirable. Herein, a straightforward strategy for internalizing red-emission carbon dots (CDs) into Shewanella xiamenensis is proposed. This suggests that the internalized CDs not only afford enhanced conductivity of bacteria but also trigger the cellular physiological response to secrete abundant electron shuttles to aid the boosting of extracellular electron transfer (EET) efficiency. Additionally, once illuminated, internalized CDs can also serve as light absorbers to allow for photogenerated electrons to be transferred into cellular metabolism to further facilitate light-enhanced EET processes. Specifically, the findings advance the fundamental understanding of the interaction between internalized carbon-based semiconductor and cells in the dark and light, and provide a facile and effective strategy for enhancing EET efficiency.

The Merging of Biological and Electronic Circuits

Biological circuits and systems within even a single cell need to be represented by large-scale feedback networks of nonlinear, stochastic, stiff, asynchronous, non-modular coupled differential equations governing complex molecular interactions. Thus, rational drug discovery and synthetic biological design is difficult. We suggest that a four-pronged interdisciplinary approach merging biology and electronics can help: (1) The mapping of biological circuits to electronic circuits via quantitatively exact schematics; (2) The use of existing electronic circuit software for hierarchical modeling, design, and analysis with such schematics; (3) The use of cytomorphic electronic hardware for rapid stochastic simulation of circuit schematics and associated parameter discovery to fit measured biological data; (4) The use of bio-electronic reporting circuits rather than bio-optical circuits for measurement. We suggest how these approaches can be combined to automate design, modeling, analysis, simulation, and quantitative fitting of measured data from a synthetic biological operational amplifier circuit in living microbial cells.

Extracellular electron transfer through visible light induced excited-state outer membrane C-type cytochromes of Geobacter sulfurreducens

Dissimilatory metal-reducing bacteria (DMRB) have a variety of c-type cytochromes (OM c-cyts) intercalated in their outer membrane, and this structure serves as the physiological basis for DMRB to carry out the extracellular electron transfer processes. Using Geobacter sulfurreducens as a model DMRB, we demonstrated that visible-light illumination could alter the electronic state of OM c-cyts from the ground state to the excited state in vivo. The existence of excited-state OM c-cyts in vivo was confirmed by spectroscopy. More importantly, excited-state OM c-cyts had a more negative potential compared to their ground-state counterparts, conferring DMRB with an extra pathway to transfer electrons to semi-conductive electron acceptors. To demonstrate this, using a TiO₂-coated electrode as an electron acceptor, we showed that G. sulfurreducens could directly utilise the conduction band of TiO₂ as an electron acceptor under visible-light illumination (λ > 420 nm) without causing TiO₂ charge separation. When G. sulfurreducens was subject to visible-light illumination, the rate of extracellular electron transfer (EET) to TiO₂ accelerated by over 8-fold compared to that observed under dark conditions. Results of additional electrochemical tests provided complementary evidence to support that G. sulfurreducens utilised excited-state OM c-cyts to enhance EET to TiO₂.

Microbial electro-fermentation for synthesis of chemicals and biofuels driven by bi-directional extracellular electron transfer

Electroactive bacteria could perform bi-directional extracellular electron transfer (EET) to exchange electrons and energy with extracellular environments, thus playing a central role in microbial electro-fermentation (EF) process. Unbalanced fermentation and microbial electrosynthesis are the main pathways to produce value-added chemicals and biofuels. However, the low efficiency of the bi-directional EET is a dominating bottleneck in these processes. In this review, we firstly demonstrate the main bi-directional EET mechanisms during EF, including the direct EET and the shuttle-mediated EET. Then, we review representative milestones and progresses in unbalanced fermentation via anode outward EET and microbial electrosynthesis via inward EET based on these two EET mechanisms in detail. Furthermore, we summarize the main synthetic biology strategies in improving the bi-directional EET and target products synthesis, thus to enhance the efficiencies in unbalanced fermentation and microbial electrosynthesis. Lastly, a perspective on the applications of microbial electro-fermentation is provided.

Extracellular Electron Shuttling Mediated by Soluble c-Type Cytochromes Produced by Shewanella oneidensis MR-1

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.

Spatiotemporal mapping of bacterial membrane potential responses to extracellular electron transfer

Extracellular electron transfer (EET) allows microorganisms to gain energy by linking intracellular reactions to external surfaces ranging from natural minerals to the electrodes of bioelectrochemical renewable energy technologies. In the past two decades, electrochemical techniques have been used to investigate EET in a wide range of microbes, with emphasis on dissimilatory metal-reducing bacteria, such as Shewanella oneidensis MR-1, as model organisms. However, due to the typically bulk nature of these techniques, they are unable to reveal the subpopulation variation in EET or link the observed electrochemical currents to energy gain by individual cells, thus overlooking the potentially complex spatial patterns of activity in bioelectrochemical systems. Here, to address these limitations, we use the cell membrane potential as a bioenergetic indicator of EET by S. oneidensis MR-1 cells. Using a fluorescent membrane potential indicator during in vivo single-cell-level fluorescence microscopy in a bioelectrochemical reactor, we demonstrate that membrane potential strongly correlates with EET. Increasing electrode potential and associated EET current leads to more negative membrane potential. This EET-induced membrane hyperpolarization is spatially limited to cells in contact with the electrode and within a near-electrode zone (<30 μm) where the hyperpolarization decays with increasing cell-electrode distance. The high spatial and temporal resolution of the reported technique can be used to study the single-cell-level dynamics of EET not only on electrode surfaces, but also during respiration of other solid-phase electron acceptors.

Single cell electron collectors for highly efficient wiring-up electronic abiotic/biotic interfaces

By electronically wiring-up living cells with abiotic conductive surfaces, bioelectrochemical systems (BES) harvest energy and synthesize electric-/solar-chemicals with unmatched thermodynamic efficiency. However, the establishment of an efficient electronic interface between living cells and abiotic surfaces is hindered due to the requirement of extremely close contact and high interfacial area, which is quite challenging for cell and material engineering. Herein, we propose a new concept of a single cell electron collector, which is in-situ built with an interconnected intact conductive layer on and cross the individual cell membrane. The single cell electron collector forms intimate contact with the cellular electron transfer machinery and maximizes the interfacial area, achieving record-high interfacial electron transfer efficiency and BES performance. Thus, this single cell electron collector provides a superior tool to wire living cells with abiotic surfaces at the single-cell level and adds new dimensions for abiotic/biotic interface engineering.

Active N dopant states of electrodes regulate extracellular electron transfer of Shewanella oneidensis MR-1 for bioelectricity generation: Experimental and theoretical investigations

Anodic N doping is an effective way to improve power generation of bioelectrochemical systems (BESs), but the role of various active N dopant states of the anode on BES performance is still unclear. Herein, the effect of anodic active N dopant states on bioelectricity generation of Shewanella oneidensis MR-1 inoculated BESs particularly including microbial extracellular electron transfer (EET) was explored using experiments and theoretical simulations. It was found a positive linear correlation between the peak current density of BESs and pyrrolic N content of the anode, which would mainly ascribe to the enhancement of both direct electron transfer (DET) and mediated electron transfer (MET) of S. oneidensis MR-1. Morever, the molecule dynamic simulation revealed that such EET improvements of S. oneidensis MR-1 could be due to more remarkable reduction in the thermodynamic and kinetic resistances of the DET and MET processes with anodic doping of pyrrolic N compared to pyridinic N and graphitic N. This work provides a valuable guideline to design of high-performance anodes for potential BES applications.

Metabolic Current Production by an Oral Biofilm Pathogen Corynebacterium matruchotii

The development of a simple and direct assay for quantifying microbial metabolic activity is important for identifying antibiotic drugs. Current production capabilities of environmental bacteria via the process called extracellular electron transport (EET) from the cell interior to the exterior is well investigated in mineral-reducing bacteria and have been used for various energy and environmental applications. Recently, the capability of human pathogens for producing current has been identified in different human niches, which was suggested to be applicable for drug assessment, because the current production of a few strains correlated with metabolic activity. Herein, we report another strain, a highly abundant pathogen in human oral polymicrobial biofilm, Corynebacterium matruchotii, to have the current production capability associated with its metabolic activity. It showed the current production of 50 nA/cm² at OD₆₀₀ of 0.1 with the working electrode poised at +0.4 V vs. a standard hydrogen electrode in a three-electrode system. The addition of antibiotics that suppress the microbial metabolic activity showed a significant current decrease (>90%), establishing that current production reflected the cellular activity in this pathogen. Further, the metabolic fixation of atomically labeled ¹³C (31.68% ± 2.26%) and ¹⁵N (19.69% ± 1.41%) confirmed by high-resolution mass spectrometry indicated that C. matruchotii cells were metabolically active on the electrode surface. The identified electrochemical activity of C. matruchotii shows that this can be a simple and effective test for evaluating the impact of antibacterial compounds, and such a method might be applicable to the polymicrobial oral biofilm on electrode surfaces, given four other oral pathogens have already been shown the current production capability.

Type IV Pili-Independent Photocurrent Production by the Cyanobacterium Synechocystis sp. PCC 6803

Biophotovoltaic devices utilize photosynthetic organisms such as the model cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis) to generate current for power or hydrogen production from light. These devices have been improved by both architecture engineering and genetic engineering of the phototrophic organism. However, genetic approaches are limited by lack of understanding of cellular mechanisms of electron transfer from internal metabolism to the cell exterior. Type IV pili have been implicated in extracellular electron transfer (EET) in some species of heterotrophic bacteria. Furthermore, conductive cell surface filaments have been reported for cyanobacteria, including Synechocystis. However, it remains unclear whether these filaments are type IV pili and whether they are involved in EET. Herein, a mediatorless electrochemical setup is used to compare the electrogenic output of wild-type Synechocystis to that of a ΔpilD mutant that cannot produce type IV pili. No differences in photocurrent, i.e., current in response to illumination, are detectable. Furthermore, measurements of individual pili using conductive atomic force microscopy indicate these structures are not conductive. These results suggest that pili are not required for EET by Synechocystis, supporting a role for shuttling of electrons via soluble redox mediators or direct interactions between the cell surface and extracellular substrates.

Enhanced Current Production by Exogenous Electron Mediators via Synergy of Promoting Biofilm Formation and the Electron Shuttling Process

Exogenous electron mediators (EMs) can facilitate extracellular electron transfer (EET) via electron shuttling processes, but it is still unclear whether and how biofilm formation is affected by the presence of EMs. Here, the impacts of EMs on EET and biofilm formation were investigated in bioelectrochemical systems (BESs) with Shewanella oneidensis MR-1, and the results showed that the presence of five different EMs led to high density current production. All the EMs substantially promoted biofilm formation with 15-36 times higher total biofilm DNA with EMs than without EMs, and they also increased the production of extracellular polymeric substances, which was favorable for biofilm formation. The current decreased substantially after removing EMs from the medium or by replacing electrodes without biofilm, suggesting that both biofilm and EMs are required for high density current production. EET-related gene expression was upregulated with EMs, resulting in the high flux of cell electron output. A synergistic mechanism was proposed: EMs in suspension were quickly reduced by the cells and reoxidized rapidly by the electrode, resulting in a microenvironment with sufficient oxidized EMs for biofilm formation, and thus, besides the well-known electron shuttling process, the EM-induced high biofilm formation and high Mtr gene expression could jointly contribute to the EET and subsequently produce a high density current. This study provides a new insight into EM-enhanced current production via regulating the biofilm formation and EET-related gene expression.

A hybrid cyt c maturation system enhances the bioelectrical performance of engineered Escherichia coli by improving the rate-limiting step

Bioelectronic devices can use electron flux to enable communication between biotic components and abiotic electrodes. We have modified Escherichia coli to electrically interact with electrodes by expressing the cytochrome c from Shewanella oneidensis MR-1. However, we observe inefficient electrical performance, which we hypothesize is due to the limited compatibility of the E. coli cytochrome c maturation (Ccm) systems with MR-1 cytochrome c. Here we test whether the bioelectronic performance of E. coli can be improved by constructing hybrid Ccm systems containing protein domains from both E. coli and S. oneidensis MR-1. The hybrid CcmH increased cytochrome c expression by increasing the abundance of CymA 60%, while only slightly changing the abundance of the other cytochromes c. Electrochemical measurements showed that the overall current from the hybrid ccm strain increased 121% relative to the wildtype ccm strain, with an electron flux per cell of 12.3 ± 0.3 fA·cell^-1. Additionally, the hybrid ccm strain doubled its electrical response with the addition of exogenous flavin, and quantitative analysis of this demonstrates CymA is the rate-limiting step in this electron conduit. These results demonstrate that this hybrid Ccm system can enhance the bioelectrical performance of the cyt c expressing E. coli, allowing the construction of more efficient bioelectronic devices.

Nitrogen doping to atomically match reaction sites in microbial fuel cells

Direct electron transfer at microbial anodes offers high energy conversion efficiency but relies on low concentrations of redox centers on bacterium membranes resulting in low power density. Here a heat-treatment is used to delicately tune nitrogen-doping for atomic matching with Flavin (a diffusive mediator) reaction sites resulting in strong adsorption and conversion of diffusive mediators to anchored redox centers. This impregnates highly concentrated fixed redox centers in the microbes-loaded biofilm electrode. This atomic matching enables short electron transfer pathways resulting in fast, direct electrochemistry as shown in Shewanella putrefaciens (S. putrefaciens) based microbial fuel cells (MFCs), showing a maximum power output higher than the conventional non-matched nitrogen-doped anode based MFCs by 21 times. This work sheds a light on diffusion mediation for fast direct electrochemistry, while holding promise for efficient and high power MFCs.

Genetic engineering for enhanced productivity in bioelectrochemical systems

A shift from petrochemical processes toward a bio-based economy is one of the most advocated developments for a sustainable future. To achieve this will require the biotechnological production of platform chemicals that can be further processed by chemical engineering. Bioelectrochemical systems (BESs) are a novel tool within the biotechnology field. In BESs, microbes serve as biocatalysts for the production of biofuels and value-added compounds, as well as for the production of electricity. Although the general feasibility of bioelectrochemical processes has been demonstrated in recent years, much research has been conducted to develop biocatalysts better suited to meet industrial demands. Initially, mainly natural exoelectrogenic organisms were investigated for their performance in BESs. Driven by possibilities of recent developments in genetic engineering and synthetic biology, the spectrum of microbial catalysts and their versatility (substrate and product range) have expanded significantly. Despite these developments, there is still a tremendous gap between currently achievable space-time yields and current densities on the one hand and the theoretical limits of BESs on the other. It will be necessary to move the performance of the biocatalysts closer to the theoretical possibilities in order to establish viable production routines. This review summarizes the status quo of engineering microbial biocatalysts for anode-applications with high space-time yields. Furthermore, we will address some of the theoretical limitations of these processes exemplarily and discuss which of the present strategies might be combined to achieve highly synergistic effects and, thus, meet industrial demands.

Geobacter cytochrome OmcZs binds riboflavin: implications for extracellular electron transfer

Geobacter sulfurreducens is an important model organism for understanding extracellular electron transfer (EET), i.e. transfer of electrons from the cell's interior (quinone pool) to an extracellular substrate. This exoelectrogenic functionality can be exploited in bioelectrochemical applications. Nonetheless, key questions remain regarding the mechanisms of this functionality. G. sulfurreducens has been hypothesized to employ both multi-heme cytochromes and soluble, small molecule redox shuttles, as the final, redox-active species in EET. However, interactions between flavin redox shuttles and outer membrane, redox proteins in Geobacter have not been demonstrated. Herein, the heterologous expression and purification from E. coli of a soluble form of the multi-heme cytochrome OmcZs from G. sulfurreducens is reported. UV-vis absorption assays show that riboflavin can be reduced by OmcZs with concomitant oxidation of the protein. Fluorescence assays show that oxidized OmcZs and riboflavin interact with a binding constant of 34 μM. Furthermore, expression of OmcZs in E. coli enables EET in the host, and the current produced by these E. coli in a bioelectrochemical cell increases when riboflavin is introduced. These results support the hypothesis that OmcZs functions in EET by transiently binding riboflavin, which shuttles electrons from the outer membrane to the extracellular substrate.

Carbon dots-fed Shewanella oneidensis MR-1 for bioelectricity enhancement

Bioelectricity generation, by Shewanella oneidensis (S. oneidensis) MR-1, has become particularly alluring, thanks to its extraordinary prospects for energy production, pollution treatment, and biosynthesis. Attempts to improve its technological output by modification of S. oneidensis MR-1 remains complicated, expensive and inefficient. Herein, we report on the augmentation of S. oneidensis MR-1 with carbon dots (CDs). The CDs-fed cells show accelerated extracellular electron transfer and metabolic rate, with increased intracellular charge, higher adenosine triphosphate level, quicker substrate consumption and more abundant extracellular secretion. Meanwhile, the CDs promote cellular adhesion, electronegativity, and biofilm formation. In bioelectrical systems the CDs-fed cells increase the maximum current value, 7.34 fold, and power output, 6.46 fold. The enhancement efficacy is found to be strongly dependent on the surface charge of the CDs. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of S. oneidensis MR-1, holding promise in all relevant technologies.

Bacterial fuel cells have generated attention with the prospect of green energy production; current research is focused on optimising the system to improve efficiency. Here, the authors report on the feeding of carbon dots to S. oneidensis MR-1 to enhance metabolic activity and bioelectric generation.

Rapid Electrochemical Monitoring of Bacterial Respiration for Gram-Positive and Gram-Negative Microbes: Potential Application in Antimicrobial Susceptibility Testing

Antimicrobial resistance is a grave threat to human life. Currently used time-consuming antibiotic susceptibility test (AST) methods limit physicians in selecting proper antibiotics. Hence, we developed a rapid AST using electroanalysis with a 15 min assay time, called EAST, which is live-monitored by time-lapse microscopy video. The present work reports systematical electrochemical analysis and standardization of protocol for EAST measurement. The proposed EAST is successfully applied for Gram-positive Bacillus subtilis and Gram-negative Escherichia coli as model organisms to monitor bacterial concentration, decay kinetics in the presence of various antibiotics (ciprofloxacin, cefixime, and amoxycillin), drug efficacy, and IC₅₀. Bacterial decay kinetics in the presence of antibiotics were validated by the colony counting method, field emission scanning electron microscopy, and atomic force microscopy image analysis. The EAST predicts the antibiotic susceptibility of bacteria within 15 min, which is a significant advantage over existing techniques that consume hours to days. The EAST was explored further by using bacteria-friendly l-lysine-functionalized cerium oxide nanoparticle coated indium tin oxide as a working electrode to observe the enhanced electron-transfer rate in the EAST. The results are very significant for future miniaturization and automation. The proposed EAST has huge potential in the development of a rapid AST device for applications in the clinical and pharmaceutical industries.

Microbial reduction of metal-organic frameworks enables synergistic chromium removal

Redox interactions between electroactive bacteria and inorganic materials underpin many emerging technologies, but commonly used materials (e.g., metal oxides) suffer from limited tunability and can be challenging to characterize. In contrast, metal-organic frameworks exhibit well-defined structures, large surface areas, and extensive chemical tunability, but their utility as microbial substrates has not been examined. Here, we report that metal-organic frameworks can support the growth of the metal-respiring bacterium Shewanella oneidensis, specifically through the reduction of Fe(III). In a practical application, we show that cultures containing S. oneidensis and reduced metal-organic frameworks can remediate lethal concentrations of Cr(VI) over multiple cycles, and that pollutant removal exceeds the performance of either component in isolation or bio-reduced iron oxides. Our results demonstrate that frameworks can serve as growth substrates and suggest that they may offer an alternative to metal oxides in applications seeking to combine the advantages of bacterial metabolism and synthetic materials.

Interactions between electroactive bacteria and metal oxides are used for bioremediation. Here, the authors report on the application of Fe(III)-containing metal organic frameworks as substrates for bacterial growth which allow for remediation of lethal levels of chromium with high efficacy over several cycles.

Evidence for the coexistence of direct and riboflavin-mediated interspecies electron transfer in Geobacter co-culture

Geobacter species can secrete free redox-active flavins, but the role of these flavins in the interspecies electron transfer (IET) of Geobacter direct interspecies electron transfer (DIET) co-culture is unknown. Here, we report the presence of a new riboflavin-mediated interspecies electron transfer (RMIET) process in a traditional Geobacter DIET co-culture; in this process, riboflavin contributes to IET by acting as a free-form electron shuttle between free Geobacter species and serving as a bound cofactor of some cytochromes in Geobacter co-culture aggregates. Multiple lines of evidence indicate that RMIET facilitates the primary initiation of syntrophic growth between Geobacter species before establishing the DIET co-culture and provides additional ways alongside the DIET to transfer electrons to achieve electric syntrophy between Geobacter species. Redox kinetic analysis of riboflavin on either Geobacter species demonstrated that the Gmet_2896 cytochrome acts as the key riboflavin reduction site, while riboflavin oxidation by Geobacter sulfurreducens is the rate-limiting step in RMIET, and the RMIET makes only a minor contribution to IET in Geobacter DIET co-culture. The discovery of a new RMIET process in Geobacter DIET co-culture suggests the complexity of IET in syntrophic bacterial communities and provides suggestions for the careful examination of the IET of other syntrophic co-cultures.

Response of syntrophic aggregates to the magnetite loss in continuous anaerobic bioreactor

Increasing studies indicate that magnetite addition could accelerate the methanogenesis via enhancing direct interspecies electron transfer (DIET)-based anaerobic syntrophy. However, magnetite is found to run off in continuous bioreactor, and the effect of magnetite loss on syntrophic aggregates is still underreported. In this study, two EGSB reactors (RM with magnetite-enhanced sludge, and RB as a control) were operated to investigate the magnetite behavior in continuous bioreactor and the corresponding response of syntrophic aggregates. Results showed that magnetite in RM was washed out gradually in form of iron ions, and a slightly acidic niche was supposed to be the major cause. Nevertheless, candidate DIET partners like Geobacter and Methanothrix along with syntrophic volatile fatty acids (VFAs)-degrading microbes were enriched in RM. In addition, the improved redox activity of extracellular polymeric substance (EPS), higher sludge conductivity and electron transport activity suggested that the DIET ability of sludge in RM was still enhanced, which favors the syntrophic metabolism of VFAs. Interestingly, syntrophic partners were loosely combined under the condition of high organic loading rate (OLR) in the presence of magnetite, but with gradual loss of magnetite, dense and active anaerobic granular sludge (AGS) was formed in RM. This study provided a comprehensive understanding of magnetite behavior in continuous bioreactor and the response of syntrophic aggregates. The robust DIET-based syntrophy after magnetite adding could favor the high-efficient anaerobic wastewater treatment and resource recovery in the future, and further investigations on magnetite resupply and the mechanism of magnetite enriching candidate DIET partners are recommended.

Differences in Applied Redox Potential on Cathodes Enrich for Diverse Electrochemically Active Microbial Isolates From a Marine Sediment

The diversity of microbially mediated redox processes that occur in marine sediments is likely underestimated, especially with respect to the metabolisms that involve solid substrate electron donors or acceptors. Though electrochemical studies that utilize poised potential electrodes as a surrogate for solid substrate or mineral interactions have shed some much needed light on these areas, these studies have traditionally been limited to one redox potential or metabolic condition. This work seeks to uncover the diversity of microbes capable of accepting cathodic electrons from a marine sediment utilizing a range of redox potentials, by coupling electrochemical enrichment approaches to microbial cultivation and isolation techniques. Five lab-scale three-electrode electrochemical systems were constructed, using electrodes that were initially incubated in marine sediment at cathodic or electron-donating voltages (five redox potentials between -400 and -750 mV versus Ag/AgCl) as energy sources for enrichment. Electron uptake was monitored in the laboratory bioreactors and linked to the reduction of supplied terminal electron acceptors (nitrate or sulfate). Enriched communities exhibited differences in community structure dependent on poised redox potential and terminal electron acceptor used. Further cultivation of microbes was conducted using media with reduced iron (Fe0, FeCl2) and sulfur (S0) compounds as electron donors, resulting in the isolation of six electrochemically active strains. The isolates belong to the genera Vallitalea of the Clostridia, Arcobacter of the Epsilonproteobacteria, Desulfovibrio of the Deltaproteobacteria, and Vibrio and Marinobacter of the Gammaproteobacteria. Electrochemical characterization of the isolates with cyclic voltammetry yielded a wide range of midpoint potentials (99.20 to -389.1 mV versus Ag/AgCl), indicating diverse metabolic pathways likely support the observed electron uptake. Our work demonstrates culturing under various electrochemical and geochemical regimes allows for enhanced cultivation of diverse cathode-oxidizing microbes from one environmental system. Understanding the mechanisms of solid substrate oxidation from environmental microbes will further elucidation of the ecological relevance of these electron transfer interactions with implications for microbe-electrode technologies.

Secreted Flavin Cofactors for Anaerobic Respiration of Fumarate and Urocanate by Shewanella oneidensis: Cost and Role

Shewanella oneidensis strain MR-1, a facultative anaerobe and model organism for dissimilatory metal reduction, uses a periplasmic flavocytochrome, FccA, both as a terminal fumarate reductase and as a periplasmic electron transfer hub for extracellular respiration of a variety of substrates. It is currently unclear how maturation of FccA and other periplasmic flavoproteins is achieved, specifically in the context of flavin cofactor loading, and the fitness cost of flavin secretion has not been quantified. We demonstrate that deletion of the inner membrane flavin adenine dinucleotide (FAD) exporter Bfe results in a 23% slower growth rate than that of the wild type during fumarate respiration and an 80 to 90% loss in fumarate reductase activity. Exogenous flavin supplementation does not restore FccA activity in a Δbfe mutant unless the gene encoding the periplasmic FAD hydrolase UshA is also deleted. We demonstrate that the small Bfe-independent pool of FccA is sufficient for anaerobic growth with fumarate. Strains lacking Bfe were unable to grow using urocanate as the sole electron acceptor, which relies on the periplasmic flavoprotein UrdA. We show that periplasmic flavoprotein maturation occurs in careful balance with periplasmic FAD hydrolysis, and that the current model for periplasmic flavin cofactor loading must account for a Bfe-independent mechanism for flavin transport. Finally, we determine that the metabolic burden of flavin secretion is not significant during growth with flavin-independent anaerobic electron acceptors. Our work helps frame the physiological motivations that drove evolution of flavin secretion by Shewanella IMPORTANCE Shewanella species are prevalent in marine and aquatic environments, throughout stratified water columns, in mineral-rich sediments, and in association with multicellular marine and aquatic organisms. The diversity of niches shewanellae can occupy are due largely to their respiratory versatility. Shewanella oneidensis is a model organism for dissimilatory metal reduction and can respire a diverse array of organic and inorganic compounds, including dissolved and solid metal oxides. The fumarate reductase FccA is a highly abundant multifunctional periplasmic protein that acts to bridge the periplasm and temporarily store electrons in a variety of respiratory nodes, including metal, nitrate, and dimethyl sulfoxide respiration. However, maturation of this central protein, particularly flavin cofactor acquisition, is poorly understood. Here, we quantify the fitness cost of flavin secretion and describe how free flavins are acquired by FccA and a homologous periplasmic flavoprotein, UrdA.

Effects of Flavin-Goethite Interaction on Goethite Reduction by Shewanella decolorationis S12

Flavin mononucleotide (FMN) and riboflavin are structurally similar flavins, except for the presence of a phosphate group on the FMN molecule. They are used by a variety of electroactive bacteria as extracellular electron shuttles in microbial Fe reduction and inevitably interact with Fe (hydr)oxides in the extracellular environment. It is currently unknown whether flavin/Fe (hydr)oxide interaction interferes with extracellular electron transfer (EET) to the mineral surface. In this study, we found that the goethite reduction rate was lower when mediated by FMN than by RF, suggesting that FMN was less effective in shuttling electrons between cells and minerals. Nevertheless, the phosphate group did not prevent the FMN molecule from accepting electrons from bacterial cells and transferring electrons to the mineral. Results of adsorption experiment, attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy, and bacterial attachment trend analyses showed that FMN exhibited strong adsorption on goethite surface by forming phosphate inner-sphere complex, which prevented bacterial cells from approaching goethite. Therefore, the interaction between FMN and goethite surface may increase the distance of electron transfer from bacterial cells to goethite and result in lower EET efficiency in comparison to those mediated by riboflavin. To our knowledge, these data reveal for the first time that the interaction between flavin and Fe (hydr)oxide affect flavin-mediated electron transfer to mineral surface and add a new dimension to our understanding of flavin-mediated microbial Fe reduction processes.

Bulk phase resource ratio alters carbon steel corrosion rates and endogenously produced extracellular electron transfer mediators in a sulfate-reducing biofilm

Desulfovibrio alaskensis G20 biofilms were cultivated on 316 steel, 1018 steel, or borosilicate glass under steady-state conditions in electron-acceptor limiting (EAL) and electron-donor limiting (EDL) conditions with lactate and sulfate in a defined medium. Increased corrosion was observed on 1018 steel under EDL conditions compared to 316 steel, and biofilms on 1018 carbon steel under the EDL condition had at least twofold higher corrosion rates compared to the EAL condition. Protecting the 1018 metal coupon from biofilm colonization significantly reduced corrosion, suggesting that the corrosion mechanism was enhanced through attachment between the material and the biofilm. Metabolomic mass spectrometry analyses demonstrated an increase in a flavin-like molecule under the 1018 EDL condition and sulfonates under the 1018 EAL condition. These data indicate the importance of S-cycling under the EAL condition, and that the EDL is associated with increased biocorrosion via indirect extracellular electron transfer mediated by endogenously produced flavin-like molecules.

Geometrical Changes in the Hemes of Bacterial Surface c-Type Cytochromes Reveal Flexibility in Their Binding Affinity with Minerals

Microbial extracellular electron transport occurs via the physical and electrical association of outer-membrane c-type cytochromes (OM c-Cyts) with extracellular solid surfaces. However, studies investigating the characteristics of cytochrome binding with solid materials have been limited to the use of purified units of OM c-Cyts dissolved in solution, rather than OM c-Cyts in intact cells, because of the lack of a methodology that specifically allows for the monitoring of OM c-Cyts in whole-cells. Here, we utilized circular dichroism (CD) spectroscopy to examine the molecular mechanisms and binding characteristics of the interaction between MtrC, a unit of OM c-Cyts, in whole Shewanella oneidensis MR-1 cells and hematite nanoparticles. The addition of hematite nanoparticles significantly decreased the intensity of the Soret CD peaks, indicating geometrical changes in the hemes in MtrC associated with their physical contact with hematite. The binding affinity of MtrC estimated using CD spectra changed predominantly depending upon the redox state of MtrC and the concentration of the hematite nanoparticles. In contrast, purified MtrC demonstrated a constant binding affinity following a Langmuir isotherm, with a standard Gibbs free energy of -43 kJ mol^-1, suggesting that the flexibility in the binding affinity of MtrC with hematite was specific in membrane-bound protein complex conditions. Overall, these findings suggest that the binding affinity as well as the heme geometry of OM c-Cyts are flexibly modulated in the membrane complex associated with microbe-mineral interactions.

Electrochemical properties of the interaction between cytochrome c and a hematite nanowire array electrode

We investigate the interaction of horse heart cytochrome c (cyt c) with hematite nanowire array electrodes by cyclic voltammetry to study the electron transfer between redox active proteins and mineral surfaces. Using this model system, we quantify electron transfer rates between cyt c and hematite under varying electric potential and pH conditions. The results are consistent with two cyt c conformations adsorbed at the hematite surface: the native and a partially unfolded form. The partially unfolded protein maintained redox activity, but at a lower redox potential than the native protein. Adsorption of cyt c allowed direct electron transfer between cyt c and hematite, with an interfacial electron transfer rate, k°_ET, of 0.4 s^-1 for the native form and 0.55 s^-1 for the partially unfolded protein at pH 7.07. At pH 4.66, protein adsorption decreased compared to neutral pH and the fraction of partially unfolded protein increased. Additionally, the diffusion controlled electron transfer rate between hematite and the electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) was determined to be k°_ET = 8.0·10^-3 cm·s^-1 at pH 7.07. Modulation of electron transfer rates as a result of conformational changes by redox active proteins has broad implications for describing chemical transformations at biological-mineral interfaces.

A Novel Bioelectronic Reporter System in Living Cells Tested with a Synthetic Biological Comparator

As the fields of biotechnology and synthetic biology expand, cheap and sensitive tools are needed to measure increasingly complicated genetic circuits. In order to bypass some drawbacks of optical fluorescent reporting systems, we have designed and created a co-culture microbial fuel cell (MFC) system for electronic reporting. This system leverages the syntrophic growth of Escheriachia. coli (E. coli) and an electrogenic bacterium Shewanella oneidensis MR-1 (S. oneidensis). The fermentative products of E. coli provide a carbon and electron source for S. oneidensis MR-1, which then reports on such activity electrically at the anode of the MFC. To further test the capability of electrical reporting of complicated synthetic circuits, a novel synthetic biological comparator was designed and tested with both fluorescent and electrical reporting systems. The results suggest that the electrical reporting system is a good alternative to commonly used optical fluorescent reporter systems since it is a non-toxic reporting system with a much wider dynamic range.

Improvement of the electron transfer rate in Shewanella oneidensis MR-1 using a tailored periplasmic protein composition

Periplasmic c-type cytochromes are essential for the electron transport between the cytoplasmic membrane bound menquinol oxidase CymA and the terminal ferric iron reductase MtrABC in the outer membrane of Shewanella oneidensis cells. Either STC or FccA are necessary for periplasmic electron transfer. We followed the hypothesis that the elimination of potential competing reactions in the periplasm and the simultaneous overexpression of STC (cctA) could lead to an accelerated electron transfer to the cell surface. The genes nrfA, ccpA, napB and napA were replaced by cctA. This led to a 1.7-fold increased ferric iron reduction rate and a 23% higher current generation in a bioelectrochemical system. Moreover, the quadruple mutant had a higher periplasmic flavin content. Further deletion of fccA and its replacement by cctA resulted in a strain with ferric iron reduction rates similar to the wild type and a lower concentration of periplasmic flavin compared to the quadruple mutant. A transcriptomic analysis revealed that the quadruple mutant had a 3.7-fold higher cctA expression which could not be further increased by the replacement of fccA. This work indicates that a synthetic adaptation of Shewanella towards extracellular respiration holds potential for increased respiratory rates and consequently higher current densities.

NETWORK ANALYSIS OF COMMON DIFFERENTIAL GENES IDENTIFIES KEY GENES AND IMPORTANT MODULES UNDERLYING EXTRACELLULAR ELECTRON TRANSFER PROCESSES

Electricigens can transfer electrons that produced in intracellular metabolic processes to cellular surface to restore extracellular insoluble electron acceptors (extracellular electron transfer, EET). To uncover the molecular mechanisms underlying EET processes, we integrated transcriptome changes accompanying such processes with molecular network. Firstly, time-series expression datasets for Shewanella oneidensis MR-1 under limited/changed [Formula: see text] conditions were obtained from the GEO database, and a total of 336 common differentially expressed genes (DEGs) were identified. Then, we constructed the protein–protein interaction (PPI) network that involved in EET processes from these DEGs. Furthermore, by using centralization analysis and community detection, network analysis of the PPI network was performed. Although the fundamental EET genes are similar to previous studies, important new genes have been discovered. Taking together, our study identified many literature-validated genes critical to EET processes, and also proposed some novel genes that were putatively involved in EET processes.