Mtr Pathway Research Articles

Mechanistic insights into sulfadimethoxine degradation via microbially driven Fenton reactions

Biodegradation, while cost-effective, is hindered by the requirement for specialized microorganisms and co-contaminants. Innovative biological technologies like the microbially driven Fenton reaction, hold promise for enhancing degradation efficiency. However, the intricate biochemical processes and essential steps for effective degradation in such systems have remained unclear. In this study, we harnessed the potential of the microbially driven Fenton reaction by employing Shewanella oneidensis MR-1 (MR-1). Our approach showcased remarkable efficacy in degrading a range of contaminants, including sulfadimethoxine (SDM), 4,4'-dibromodiphenyl ether (BDE-15) and atrazine (ATZ). Using SDM as a model contaminant of emergent contaminants (ECs), we unveiled that biodegradation relied on the generation of hydroxyl radicals (•OH) and involvement of oxidoreductases. Transcriptomic analysis shed light on the pivotal components of extracellular electron transfer (EET) during both anaerobic and aerobic periods. The presence of reactive oxidizing species induced cellular damage and impeded DNA repair, thereby affecting the Mtr pathway of EET. Moreover, the formation of vivianite hindered SDM degradation, underscoring the necessity of maintaining iron ions in the solution to ensure sustainable and efficient degradation. Overall, this study offers valuable insights into microbial technique for ECs degradation, providing a comprehensive understanding of degradation mechanisms during aerobic/anaerobic cycling.

Enhanced azo dye reduction at semiconductor-microbe interface: The key role of semiconductor band structure

Low-energy environmental remediation could be achieved by biocatalysis with assistance of light-excited semiconductor, in which the energy band structure of semiconductor has a significant influence on the metabolic process and electron transfer of microbes. In this study, direct Z-scheme and type II heterojunction semiconductor with different energy band structure were successfully synthesized for constructing semiconductor-microbe interface with Shewanella oneidensis MR-1 to achieve acid orange7 (AO7) biodegradation. UV–vis diffuse reflection spectroscopy, photoluminescence spectra and photoelectrochemical analysis revealed that the direct Z-scheme heterojunction semiconductor had stronger reduction power and faster separation of photoelectron-hole, which was beneficial for the AO7 biodegradation at semiconductor-microbe interface. Riboflavin was also involved in electron transfer between the semiconductor and microbes during AO7 reduction. Transcriptome results illustrated that functional gene expression of Shewanella oneidensis MR-1 was upregulated significantly with photo-stimulation of direct Z-scheme semiconductor, and Mtr pathway and conductive pili played the important roles in the photoelectron utilization by Shewanella oneidensis MR-1. This work is expected to provide alternative ideas for designing semiconductor-microbial interface with efficient electron transfer and broadening their applications in bioremediation.

Microbial electrosynthetic nitrate reduction to ammonia by reversing the typical electron transfer pathway in Shewanella oneidensis

Ammonia production is a critical industrial process, and mild routes to recycle nitrates in wastewater could be a promising route to ammonia synthesis. In this study, ammonia production is demonstrated in a microbial electrosynthesis system with nitrate and an electrode as electron acceptors and donors, respectively. Based on the bidirectional extracellular electron transfer capability of Shewanella oneidensis MR-1, our microbial electrosynthetic system achieves a maximum ammonia production rate of 24.3 μg h−1·mg protein−1 with 82.5% selectivity and 33.1% cathodic efficiency and functions for several cycles over 30 days. Electrochemical analysis suggests that cytochromes c, flavins, and the flavin/c-cytochrome combination play a pivotal role. Charge transfer resistance weakens over the course of weeks, resulting in easier electron transfer. Parallel reaction monitoring proteomics suggest that reversing a typical “Mtr pathway” plays a role, and a dissimilatory nitrate to ammonia pathway is used. This work proposes a progressive route to carry out ammonia synthesis under mild conditions.

A New Electron Shuttling Pathway Mediated by Lipophilic Phenoxazine via the Interaction with Periplasmic and Inner Membrane Proteins of Shewanella oneidensis MR-1.

Although it has been established that electron mediators substantially promote extracellular electron transfer (EET), electron shuttling pathways are not fully understood. Here, a new electron shuttling pathway was found in the EET process by Shewanella oneidensis MR-1 with resazurin, a lipophilic electron mediator. With resazurin, the genes encoding outer-membrane cytochromes (mtrCBA and omcA) were downregulated. Although cytochrome deletion substantially reduced biocurrent generation to 1-12% of that of wild-type (WT) cells, the presence of resazurin restored biocurrent generation to 168 μA·cm-2 (ΔmtrA/omcA/mtrC), nearly equivalent to that of WT cells (194 μA·cm-2), indicating that resazurin-mediated electron transfer was not dependent on the Mtr pathway. Biocurrent generation by resazurin was much lower in ΔcymA and ΔmtrA/omcA/mtrC/fccA/cctA mutants (4 and 6 μA·cm-2) than in WT cells, indicating a key role of FccA, CctA, and CymA in this process. The effectiveness of resazurin in EET of Mtr cytochrome mutants is also supported by cyclic voltammetry, resazurin reduction kinetics, and in situ c-type cytochrome spectroscopy results. The findings demonstrated that low molecular weight, lipophilic electron acceptors, such as phenoxazine and phenazine, may facilitate electron transfer directly from periplasmic and inner membrane proteins, thus providing new insight into the roles of exogenous electron mediators in electron shuttling in natural and engineered biogeochemical systems.

NADH dehydrogenases drive inward electron transfer in Shewanella oneidensis MR-1.

Shewanella oneidensis MR-1 is a promising chassis organism for microbial electrosynthesis because it has a well-defined biochemical pathway (the Mtr pathway) that can connect extracellular electrodes to respiratory electron carriers inside the cell. We previously found that the Mtr pathway can be used to transfer electrons from a cathode to intracellular electron carriers and drive reduction reactions. In this work, we hypothesized that native NADH dehydrogenases form an essential link between the Mtr pathway and NADH in the cytoplasm. To test this hypothesis, we compared the ability of various mutant strains to accept electrons from a cathode and transfer them to an NADH-dependent reaction in the cytoplasm, reduction of acetoin to 2,3-butanediol. We found that deletion of genes encoding NADH dehydrogenases from the genome blocked electron transfer from a cathode to NADH in the cytoplasm, preventing the conversion of acetoin to 2,3-butanediol. However, electron transfer to fumarate was not blocked by the gene deletions, indicating that NADH dehydrogenase deletion specifically impacted NADH generation and did not cause a general defect in extracellular electron transfer. Proton motive force (PMF) is linked to the function of the NADH dehydrogenases. We added a protonophore to collapse PMF and observed that it blocked inward electron transfer to acetoin but not fumarate. Together these results indicate a link between the Mtr pathway and intracellular NADH. Future work to optimize microbial electrosynthesis in S. oneidensis MR-1 should focus on optimizing flux through NADH dehydrogenases.

Multichannel Bioelectronic Sensing Using Engineered Escherichia coli

Whole cell bioelectronic sensors enable the conversion of chemical signals into electrical signals using extracellular electron transfer pathways. However, these sensors use a single extracellular electron pathway as a single electrochemical channel, severely limiting the capacity of the information exchange. To expand the information content, we report a multichannel bioelectronic sensor in which different chemical modules modulate distinct extracellular electron transfer pathways in Escherichia coli. We have introduced two reporting channels into a single Escherichia coli: 1) Mtr pathway from Shewanella oneidensis; 2) riboflavin synthesis pathway from Bacillus subtilis. By upregulating the exogenous riboflavin pathway, we doubled the extracellular flavin concentration. The presence of flavins yielded a distinct electrochemical signal at more negative potentials than the Mtr pathway. To demonstrate multichannel bioelectronic sensing of heavy metals, expression of the Mtr and RF pathways were placed under control of As and Cd responsive promoters. Using a redox-potential-dependent algorithm, multichannel bioelectronic sensors could distinguish As, Cd, and a combination of As and Cd in situ in environmental samples. These accomplishments provide a new platform for multichannel bioelectronic sensors that simultaneously detect different chemicals and expand the information content from biosensors to help safeguard human and environmental health.

Investigating the interaction between Shewanella oneidensis and phenazine 1-carboxylic acid in the microbial electrochemical processes

Many exoelectrogens utilize small redox mediators for extracellular electron transfer (EET). Notable examples include Shewanella species, which synthesize flavins, and Pseudomonas species, which produce phenazines. In natural and engineered environments, redox-active metabolites from different organisms coexist. The interaction between Shewanella oneidensis and phenazine 1-carboxylic acid (PCA, a representative phenazine compound) was investigated to demonstrate exoelectrogens utilizing metabolites secreted by other organisms as redox mediators. After 24 h in a reactor with and without added PCA (1 μM), the anodic current generated by Shewanella was 235 ± 11 and 51.7 ± 2.8 μA, respectively. Shewanella produced oxidative current approximately three times as high with medium containing PCA as with medium containing the same concentration of riboflavin. PCA also stimulated inward EET in Shewanella. The strong effect of PCA on EET was attributed to its enrichment at the biofilm/electrode interface. The PCA voltammetric peak heights with a Shewanella bioanode were 25–30 times higher than under abiotic conditions. The electrochemical properties of PCA were also altered by the transition from two-electron to single-electron electrochemistry, which suggests PCA was bound between the electrode and cell surface redox proteins. This behavior would benefit electroactive bacteria, which usually dwell in open systems where mediators are present in low concentrations. Like flavins, PCA can be immobilized under both bioanode and biocathode conditions but not under metabolically inactive conditions. Shewanella rapidly transfers electrons to PCA via its Mtr pathway. Compared with wild-type Shewanella, the PCA reduction ability was decreased in gene knockout mutants lacking Mtr pathway cytochromes, especially in the mutants with severely undermined electrode-reduction capacities. These strains also lost the ability to immobilize PCA, even under current-generating conditions.

Proteomics Reveal the Effect of Exogenous Electrons on Electroactive Escherichia coli.

Microbial cells utilizing electricity to produce high-value fuels and chemicals are the foundation of the biocathodic bioelectrochemical system. However, molecular mechanisms of electron transfer and utilization have not been elucidated. In this work, Escherichia coli engineered by introducing the Mtr pathway from Shewanella oneidensis exhibited stronger electrochemical activity than control and could utilize exogenous electrons to stimulate metabolite profiles and boost succinate production in the bioelectrochemical system. Proteomic analysis and real-time PCR were performed to investigate the effect of exogenous electrons on electroactive E. coli. Bioinformatics analysis suggested that the proteins of molecular function associated with oxidoreductase activity, 4 iron, 4 sulfur([4Fe-4S]) cluster binding, iron-sulfur cluster binding, and metal cluster binding were positively affected by exogenous electrons. Moreover, mapping to the Kyoto Encyclopedia of Genes and Genomes pathway database showed that the up-regulated proteins were mainly involved in metabolic pathways of tricarboxylic acid cycle, pyruvate metabolism, and nitrogen metabolism pathway, providing support for the metabolic balance of microbial cells shifting toward reduced end-products due to electron utilization. Using a biochemical method, the ompF-overexpressed strain was employed to investigate the function of the channel protein. These findings provided a theoretical basis for further improving electron transfer and utilization efficiency, and contributed to the potential applications of the bioelectrochemical system.

Biogenic iron sulfide functioning as electron-mediating interface to accelerate dissimilatory ferrihydrite reduction by Shewanella oneidensis MR-1

Microbially driven iron and sulfur geochemical cycles co-exist ubiquitously in subsurface environments and are of environmental relevance. Shewanella species (dissimilatory metal-reducing bacteria) are capable of reducing Fe(III)-(oxyhydr)oxide minerals and diverse sulfur sources using corresponding metabolic pathways and producing FeS secondary minerals. In spite of the ability in promoting bacterial extracellular electron transfer (EET), the specific role of FeS in mediating EET between microbe/mineral interface is still unclear. In this work, the electron-mediating function of biogenic FeS on promoting the reduction of ferrihydrite by S. oneidensis MR-1 using thiosulfate as sulfur source was investigated in terms of Fe(III) reduction percentage, X-ray diffraction and scanning electron microscopy. The results showed that the microbial ferrihydrite reduction was pH-dependent and positively correlated with the addition of thiosulfate. In the presence of thiosulfate, biogenic FeS in nano-scale were formed and deposited on the surfaces of S. oneidensis MR-1 and ferrihydrite to build an interfacial electron transfer bridge between them. The addition of either thiosulfate and in-vitro FeS could rescue the entirely inactivated ability of the mutant (△omcA/mtrC) in ferrihydrite reduction to some extent, but which was obviously inferior to the wild-type strain. Meanwhile, the effect of the biogenic FeS in-situ coating on the surfaces of S. oneidensis MR-1 cells on promoting microbial ferrihydrite reduction was significantly superior to the in-vitro ones. Thus, the in-situ formed biogenic FeS secondary minerals were demonstrated to mediate and accelerate interfacial electron transfer from S. oneidensis MR-1 cells to ferrihydrite through interfacing with the bacterial EET routes, especially Mtr pathway. This work provides an insight into the secondary minerals-mediating interfacial electron transfer between microbes and minerals in the presence of biological S (-II), which has important biogeochemical and environmental implications.

Pyruvate accelerates palladium reduction by regulating catabolism and electron transfer pathway in Shewanella oneidensis.

Shewanella oneidensis is a model strain of the electrochemical active bacteria (EAB) because of its strong capability of extracellular electron transfer (EET) and genetic tractability. In this study, we investigated the effect of carbon sources on EET in S. oneidensis by using reduction of palladium ions (Pd(II)) as a model and found that pyruvate greatly accelerated the Pd(II) reduction compared with lactate by resting cells. Both Mtr pathway and hydrogenases played a role in Pd(II) reduction when pyruvate was used as a carbon source. Furthermore, in comparison with lactate-feeding S. oneidensis, the transcriptional levels of formate dehydrogenases involving in pyruvate catabolism, Mtr pathway, and hydrogenases in pyruvate-feeding S. oneidensis were up-regulated. Mechanistically, the enhancement of electron generation from pyruvate catabolism and electron transfer to Pd(II) explains the pyruvate effect on Pd(II) reduction. Interestingly, a 2-h time window is required for pyruvate to regulate transcription of these genes and profoundly improve Pd(II) reduction capability, suggesting a hierarchical regulation for pyruvate sensing and response in S. oneidensis IMPORTANCE The unique respiration of EET is crucial for the biogeochemical cycling of metal elements and diverse applications of EAB. Although a carbon source is a determinant factor of bacterial metabolism, the research into the regulation of carbon source on EET is rare. In this work, we reported the pyruvate-specific regulation and improvement of EET in S. oneidensis and revealed the underlying mechanism, which suggests potential targets to engineer and improve the EET efficiency of this bacterium. This study sheds light on the regulatory role of carbon sources in anaerobic respiration in EAB, providing a way to regulate EET for diverse applications from a novel perspective.

Elevated intracellular cyclic-di-GMP level in Shewanella oneidensis increases expression of c-type cytochromes.

Electrochemically active biofilms are capable of exchanging electrons with solid electron acceptors and have many energy and environmental applications such as bioelectricity generation and environmental remediation. The performance of electrochemically active biofilms is usually dependent on c-type cytochromes, while biofilm development is controlled by a signal cascade mediated by the intracellular secondary messenger bis-(3'-5') cyclic dimeric guanosine monophosphate (c-di-GMP). However, it is unclear whether there are any links between the c-di-GMP regulatory system and the expression of c-type cytochromes. In this study, we constructed a S. oneidensis MR-1 strain with a higher cytoplasmic c-di-GMP level by constitutively expressing a c-di-GMP synthase and it exhibited expected c-di-GMP-influenced traits, such as lowered motility and increased biofilm formation. Compared to MR-1 wild-type strain, the high c-di-GMP strain had a higher Fe(III) reduction rate (21.58 vs 11.88pM of Fe(III)/h cell) and greater expression of genes that code for the proteins involved in the Mtr pathway, including CymA, MtrA, MtrB, MtrC and OmcA. Furthermore, single-cell Raman microspectroscopy (SCRM) revealed a great increase of c-type cytochromes in the high c-di-GMP strain as compared to MR-1 wild-type strain. Our results reveal for the first time that the c-di-GMP regulation system indirectly or directly positively regulates the expression of cytochromes involved in the extracellular electron transport (EET) in S.oneidensis, which would help to understand the regulatory mechanism of c-di-GMP on electricity production in bacteria.

Direct electron uptake from a cathode using the inward Mtr pathway in Escherichia coli

Research on the biocathode-based bioelectrochemical system (BES) has attracted extensive attention because of its ability to increase the electricity-driven production of high-value fuels or chemicals by relying on microbial cells as catalysts. An extracellular electron transfer (EET) that makes electrical connections to microorganisms plays a key role in the BES. Compared with the better understanding of the EET-to-anode connection, the understanding of the mechanism and elements involved in inward EET from cathodes to microbes remains limited. Additionally, the low capability of the EET limits its applications in BESs for producing chemicals. Here, we introduced the Mtr pathway into Escherichia coli cells by expressing ccmABCDEFGH from E. coli and mtrABC from Shewanella oneidensis. Through selection by electrochemical pressure, the evolved E. coli could use electricity to increase the production of succinate in direct BES and enhance the electroactivity. In addition, in studying the mechanism of inward EET, menaquinone was found to be one of the key components of inward EET, and it is essential for the fumarate reduction reaction. Lastly, the intracellular NADH and ATP levels showed that there were differences in the energy conservation coupling between the electron transfer routes of the inward Mtr pathway and the electron mediator.

Quantum dot interactions with and toxicity to Shewanella oneidensis MR-1

Combining abiotic photosensitisers such as quantum dots (QDs) with non-photosynthetic bacteria presents an intriguing concept into the design of artificial photosynthetic organisms and solar-driven fuel production. Shewanella oneidensis MR-1 (MR-1) is a versatile bacterium concerning respiration, metabolism and biocatalysis, and is a promising organism for artificial photosynthesis as the bacterium’s synthetic and catalytic ability provides a potential system for bacterial biohydrogen production. MR-1’s hydrogenases are present in the periplasmatic space. It follows that for photoenergised electrons to reach these enzymes, QDs will need to be able to enter the periplasm, or electrons need to enter the periplasm via the Mtr pathway that is responsible for MR-1’s extracellular electron transfer ability. As a step towards this goal, various QDs were tested for their photo-reducing potential, nanotoxicology and further for their interaction with MR-1. CdTe/CdS/TGA, CdTe/CdS/Cysteamine, a commercial, negatively charged CdTe and CuInS2/ZnS/PMAL QDs were examined. The photoreduction potential of the QDs was confirmed by measuring their ability to photoreduce methyl viologen with different sacrificial electron donors. The commercial CdTe and CuInS2/ZnS/PMAL QDs showed no toxicity towards MR-1 as evaluated by a colony-forming units method and a fluorescence viability assay. Only the commercial negatively charged CdTe QDs showed good interaction with MR-1. With transmission electron microscopy, QDs were observed both in the cytoplasm and periplasm. These results inform on the possibilities and bottlenecks when developing bionanotechnological systems for the photosynthetic production of biohydrogen by MR-1.

NADH dehydrogenases Nuo and Nqr1 contribute to extracellular electron transfer by Shewanella oneidensis MR-1 in bioelectrochemical systems

Shewanella oneidensis MR-1 is quickly becoming a synthetic biology workhorse for bioelectrochemical technologies due to a high level of understanding of its interaction with electrodes. Transmembrane electron transfer via the Mtr pathway has been well characterized, however, the role of NADH dehydrogenases in feeding electrons to Mtr has been only minimally studied in S. oneidensis MR-1. Four NADH dehydrogenases are encoded in the genome, suggesting significant metabolic flexibility in oxidizing NADH under a variety of conditions. A strain lacking the two dehydrogenases essential for aerobic growth exhibited a severe growth defect with an anode (+0.4 VSHE) or Fe(III)-NTA as the terminal electron acceptor. Our study reveals that the same NADH dehydrogenase complexes are utilized under oxic conditions or with a high potential anode. Our study also supports the previously indicated importance of pyruvate dehydrogenase activity in producing NADH during anerobic lactate metabolism. Understanding the role of NADH in extracellular electron transfer may help improve biosensors and give insight into other applications for bioelectrochemical systems.

Mechanism study of photo-induced gold nanoparticles formation by Shewanella oneidensis MR-1

Shewanella oneidensis MR-1, a bioelectricity generating bacterium, is broadly used in bioremediation, microbial fuel cell and dissimilatory reduction and recovery of precious metals. Herein, we report for the first time that photo induction as a trigger to stimulate gold nanoparticles (Au@NPs) formation by MR-1, with wavelength and light intensity as two key variables. Results indicated that sigmoidal model is the best fit for Au@NPs formation at various wavelengths (with R2 > 0.97). Light intensity in terms of photosynthetic photon flux density (PPFD) critically influences the rate constant in the low-light intensity region (PPFD < 20), while wavelength controls the maximum rate constant in the high-light region (PPFD > 20). By deletion of Mtr pathway genes in MR-1, we proposed the mechanism for light induced Au@NP formation is the excitation effect of light on certain active groups and extracellular polymeric substances (EPS) on the cell surface. Also, the release of electrons from proteins and co-enzyme complexes enhance electron generation. To the best of our knowledge, this is the first-attempt to explore the effect of photo-induction on Au@NPs production by MR-1, which provides an alternative cost-effective and eco-friendly process in green chemical industry.

Kinetics of trifurcated electron flow in the decaheme bacterial proteins MtrC and MtrF

The bacterium Shewanella oneidensis has evolved a sophisticated electron transfer (ET) machinery to export electrons from the cytosol to extracellular space during extracellular respiration. At the heart of this process are decaheme proteins of the Mtr pathway, MtrC and MtrF, located at the external face of the outer bacterial membrane. Crystal structures have revealed that these proteins bind 10 c-type hemes arranged in the peculiar shape of a staggered cross that trifurcates the electron flow, presumably to reduce extracellular substrates while directing electrons to neighboring multiheme cytochromes at either side along the membrane. Especially intriguing is the design of the heme junctions trifurcating the electron flow: they are made of coplanar and T-shaped heme pair motifs with relatively large and seemingly unfavorable tunneling distances. Here, we use electronic structure calculations and molecular simulations to show that the side chains of the heme rings, in particular the cysteine linkages inserting in the space between coplanar and T-shaped heme pairs, strongly enhance electronic coupling in these two motifs. This results in an [Formula: see text]-fold speedup of ET steps at heme junctions that would otherwise be rate limiting. The predicted maximum electron flux through the solvated proteins is remarkably similar for all possible flow directions, suggesting that MtrC and MtrF shuttle electrons with similar efficiency and reversibly in directions parallel and orthogonal to the outer membrane. No major differences in the ET properties of MtrC and MtrF are found, implying that the different expression levels of the two proteins during extracellular respiration are not related to redox function.

Multiheme Cytochrome Mediated Redox Conduction through Shewanella oneidensis MR-1 Cells.

Multiheme cytochromes function as extracellular electron transfer (EET) conduits that extend the metabolic reach of microorganisms to external solid surfaces. These conduits are also proposed to facilitate long-distance electron transport along cellular membranes and across multiple cells. Here we report electrochemical gating measurements of Shewanella oneidensis MR-1 cells linking interdigitated electrodes. The dependence of the source-drain current on gate potential demonstrates a redox conduction mechanism, which we link to the presence of multiheme cytochromes of the Mtr pathway. We also find that the measured thermal activation energy of 0.29 ± 0.03 eV is consistent with these obtained from electron hopping calculations through the S. oneidensis Mtr outer-membrane decaheme cytochromes. Our measurements and calculations have implications for understanding and controlling micrometer-scale electron transport in microbial systems.

Turn on the Mtr pathway genes under pLacI promoter in Shewanella oneidensis MR-1

BackgroundShewanella genus is famous for applications like electron transfer in microbe fuel cells and bioremediation of heavy metals through the Mtr pathway. A potential way to enhance the electron genesis ability of Shewanella is to express exogenous mtr genes via recombinant DNA technology. Thus, to design and develop expression vectors capable of replicating in Shewanella and enhance the genetic toolbox of the same is important.ResultIn this study, a plasmid construct with a replication origin, repB, and pLacI promoter is reported for the first time to drive the expression of green fluorescent protein in S. oneidensis MR-1. Based on the same vector, the Mtr pathway genes mtrA, mtrC, and mtrCAB were also successfully expressed in MR-1. The recombinant strains had higher ferric reductase activity compared to the wild type. The highest enzymatic activity of 508.33 U/L in genetic Shewanella with mtrC gene is obtained, which is 1.53-fold higher than that of wild strain. The plasmids were stable up to 90 generations.ConclusionWe have demonstrated an expression system based on pLacI promoter and repB ori in Shewanella. Consequently, the combination of repB and pLacI will have great potential in Shewanella to turn on expression of different genes constitutively.

Influence of cytochrome charge and potential on the cathodic current of electroactive artificial biofilms

An electroactive artificial biofilm has been optimized for the cathodic reduction of fumarate by Shewanella oneidensis. The system is based on the self-assembly of multi-walled carbon nanotubes with bacterial cells in the presence of a c-type cytochrome. The aggregates are then deposited on an electrode to form the electroactive artificial biofilm. Six c-type cytochromes have been studied, from bovine heart or Desulfuromonas and Desulfuvibrio strains. The isoelectric point of the cytochrome controls the self-assembly process that occurs only with positively-charged cytochromes. The redox potential of the cytochrome is critical for electron transfer reactions with membrane cytochromes of the Mtr pathway. Optimal results have been obtained with c3 from Desulfovibrio vulgaris Hildenborough having an isoelectric point of 10.2 and redox potentials of the four hemes ranging between −290 and −375 mV vs SHE. A current density of 170 μA cm−2 could be achieved in the presence of 50 mM fumarate. The stability of the electrochemical response was evaluated, showing a regular decrease of the current within 13 h, possibly due to the inactivation or leaching of loosely-bound cytochromes from the biofilm.

Biosynthesis and Characterization of Copper Nanoparticles Using Shewanella oneidensis: Application for Click Chemistry.

Copper nanoparticles (Cu-NPs) have a wide range of applications as heterogeneous catalysts. In this study, a novel green biosynthesis route for producing Cu-NPs using the metal-reducing bacterium, Shewanella oneidensis is demonstrated. Thin section transmission electron microscopy shows that the Cu-NPs are predominantly intracellular and present in a typical size range of 20-40 nm. Serial block-face scanning electron microscopy demonstrates the Cu-NPs are well-dispersed across the 3D structure of the cells. X-ray absorption near-edge spectroscopy and extended X-ray absorption fine-structure spectroscopy analysis show the nanoparticles are Cu(0), however, atomic resolution images and electron energy loss spectroscopy suggest partial oxidation of the surface layer to Cu2 O upon exposure to air. The catalytic activity of the Cu-NPs is demonstrated in an archetypal "click chemistry" reaction, generating good yields during azide-alkyne cycloadditions, most likely catalyzed by the Cu(I) surface layer of the nanoparticles. Furthermore, cytochrome deletion mutants suggest a novel metal reduction system is involved in enzymatic Cu(II) reduction and Cu-NP synthesis, which is not dependent on the Mtr pathway commonly used to reduce other high oxidation state metals in this bacterium. This work demonstrates a novel, simple, green biosynthesis method for producing efficient copper nanoparticle catalysts.