Dissimilatory Metal-reducing Bacteria Research Articles

Degradation of 2, 2′, 4, 4′-Tetrabrominated diphenyl ether (BDE-47) via the Fenton reaction driven by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1

A microbially facilitated approach was developed to degrade 2, 2′, 4, 4′-tetrabrominated diphenyl ether (BDE-47). This approach consisted of biological production of Fe(II) and H2O2 by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 during the repetitive anoxic/oxic cycles and abiotic production of hydroxyl radical (HO●) with the biologically produced Fe(II) and H2O2 via Fenton reaction. Under the condition tested, BDE-47 did not inhibit the growth of S. oneidensis MR-1. Water soluble Fe(III)-citrate and the solid minerals ferrihydrite [Fe(III)2O3•0.5H2O] and goethite [Fe(III)OOH] were tested in this study. Under anoxic condition, the amounts of Fe(II) produced by S. oneidensis MR-1 varied among the Fe(III)s tested, which decreased in the order of Fe(III)-citrate > ferrihydrite > goethite. Under subsequent oxic condition, H2O2 was produced via O2 reduction by S. oneidensis MR-1. The amounts of H2O2 detected also varied, which decreased in the order of the reactions with Fe(III)-citrate > goethite > ferrihydrite. S. oneidensis MR-1 maintained its ability to produce Fe(II) and H2O2 for up to seven anoxic/oxic cycles. At each end of anoxic/oxic cycle, HO● was detected. The amount of HO● produced decreased in the order of the reactions with ferrihydrite > goethite > Fe(III)-citrate, which was opposite to that of H2O2 detected. Compared to the controls without HO●, the amounts of BDE-47 in the reactions with HO● decreased. The more HO● in the reaction, the less amount of BDE-47 detected. Furthermore, no BDE-47 degradation was observed when HO● was scavenged or ferrihydrite was either omitted or replaced by nitrate. Finally, identification of degradation products, such as hydroxylated BDE-47 and trisBDE, dibromophenol and monobromophenol, suggested that OH-addition and Br-substitution by HO● were the main mechanisms for degrading BDE-47. Collectively, all these results demonstrated for the first time that the Fenton reaction driven by S. oneidensis MR-1 degraded BDE-47 effectively.

Synthesis and characterization of molybdenum disulfide nanoparticles in Shewanella oneidensis MR-1 biofilms.

Shewanella oneidensis MR-1 is a dissimilatory metal-reducing bacterium capable of reducing various metal and sulfur compounds and precipitating them in nanoparticulate form. Here, we report the synthesis of molybdenum disulfide nanomaterials at the site of S. oneidensis biofilms grown in the presence of molybdenum trioxide and sodium thiosulfate. Samples from the growth medium were imaged using scanning electron microscopy and characterized using transmission electron microscopy, energy-dispersive x-ray spectroscopy, absorbance spectroscopy, and x-ray diffraction. These methods revealed the presence of molybdenum disulfide nanoparticle aggregates 50-300 nm in diameter with both hexagonal and rhombohedral polytypes. As a biosynthesis method for molybdenum sulfide, the use of S. oneidensis offers the advantage of significantly reduced heat and chemical solvent input compared to conventional methods of synthesizing molybdenum disulfide nanoparticles.

Interaction between Biocompatible Graphene Oxide and Live Shewanella in the Self-Assembled Hydrogel: The Role of Physicochemical Properties.

Understanding the interaction of graphene materials with bacterial cells is crucial for exploiting their environmental applications. Meanwhile, knowledge on the mechanism of graphene oxide (GO) action to bacteria is still incomplete. This study focused on the inter-relationship of biocompatible GO and the well-known dissimilatory metal-reducing bacteria Shewanella, in view of the biographene hydrogel (BGH), a self-assembly of GO and live bacteria. The results showed that, among various inter-related physicochemical properties of GO, the sheet area determined the bacterial survival and the gelation potential with the same Shewanella strain. For the biocompatible GO sheet above 0.30 μm2, the larger the GO, the higher the speed of BGH assembling. Only 22 h was needed to obtain BGH using GO with an average area of 1.83 μm2 (maximum in this study). The GO oxidation degree was found to be another critical factor affecting whether BGH formed or not, with a referential threshold of C/O > 1.75. Finally, surface force of GO was detected and correlated with the bacterial adhesion behavior for the first time, confirming that the large GO in the low oxidation state has high resultant force to attract bacteria. All these findings pave a promising way to develop a GO-bacteria complex like BGH to treat industrial wastewater in the future.

Manganese Carbonate Precipitation Induced by Calcite-Forming Bacteria

Several dissimilatory metal-reducing bacteria and a halophilic bacterium are able to induce manganese carbonate (rhodochrosite) precipitation. In this study, it was revealed that Ensifer adhaerens JCM 21105T, Microbacterium testaceum JCM 1353T, Pseudomonas protegens DSM 19095T, and Rheinheimera texasensis DSM 17496T, which are calcite-forming bacteria, were able to aerobically induce the precipitation of manganese carbonate crystals on an agar medium. In the case of all four strains, the principal morphology of the precipitated manganese carbonate crystals was that of micro-sized spheres, when they were aerobically cultivated over the entire surface of the agar medium at 28 °C for 7 days.

Sensing and Approaching Toxic Arsenate by Shewanella putrefaciens CN-32.

Although arsenic at a high concentration imposes strong selective pressure on microbes, various microbes have been found to grow in As-rich environments. So far, little is known about how microbes can sense and move toward arsenate in the environment, and the underlying molecular mechanisms have not been revealed. Here, we report the chemotaxis response toward arsenate (As(V)) by Shewanella putrefaciens CN-32, a model dissimilatory metal-reducing bacterium (DMRB), and elucidate the mechanisms. We find that S. putrefaciens CN-32 exhibits a chemotactic behavior toward As(V) and diverse electron acceptors. To sense As(V), S. putrefaciens CN-32 requires functional arsenate respiratory reductase but does not depend on its metal-reducing-like respiratory pathway. We observe that such a sense is governed by an energy taxis mechanism and mediated by several methyl-accepting chemotaxis proteins (MCPs), rather than a specific MCP. Moreover, we reveal that the chemotactic signal transduction pathway is conserved in Shewanella, and histidine kinase and flagella-mediated motility are essential for taxis toward As(V). This work reverses the conventional view about arsenic as a chemotactic inhibitor to microbes by revealing the positive chemotaxis of Shewanella to As(V).

Efficient and Precise Genome Editing in Shewanella with Recombineering and CRISPR/Cas9-Mediated Counter-Selection.

Dissimilatory metal-reducing bacteria, particularly those from the genus Shewanella, are of importance for bioremediation of metal contaminated sites and sustainable energy production. However, studies on this species have suffered from a lack of effective genetic tools for precise and high throughput genome manipulation. Here we report the development of a highly efficient system based on single-stranded DNA oligonucleotide recombineering coupled with CRISPR/Cas9-mediated counter-selection. Our system uses two plasmids: a sgRNA targeting vector and an editing vector, the latter harboring both Cas9 and the phage recombinase W3 Beta. Following the experimental analysis of Cas9 activity, we demonstrate the ability of this system to efficiently and precisely engineer different Shewanella strains with an average efficiency of >90% among total transformed cells, compared to ≃5% by recombineering alone, and regardless of the gene modified. We also show that different genetic changes can be introduced: mismatches, deletions, and small insertions. Surprisingly, we found that use of CRISPR/Cas9 alone allows selection of recombinase-independent S.oneidensis mutations, albeit at lower efficiency and frequency. With synthesized single-stranded DNA as substrates for homologous recombination and Cas9 as a counter-selectable marker, this new system provides a rapid, scalable, versatile, and scarless tool that will accelerate progress in Shewanella genomic engineering.

Biomimetic Peptide Nanowires Designed for Conductivity

The filamentous peptide-based nanowires produced by some dissimilatory metal-reducing bacteria, such as Geobacter sulfurreducens, display excellent natural conductivity. Their mechanism of conduction is assumed to be a combination of delocalized electrons through closely aligned aromatic amino acids and hopping/charge transfer. The proteins that form these microbial nanowires are structured from a coiled-coil, for which the design rules have been reported in the literature. Furthermore, at least one biomimetic system using related synthetic peptides has shown that the incorporation of aromatic residues can be used to enhance conductivity of peptide fibers. Herein, the de novo design of peptide sequences is used to enhance the conductivity of peptide gels, as inspired by microbial nanowires. A critical factor hampering investigations in both microbiology and materials development is inconsistent reporting of biomaterial conductivity measurements, with consistent methodologies needed for such investigations. We have reported a method herein to analyze non-Ohmic behavior using existing parameters, which is a statistically insightful approach for detecting small changes in biologically based samples. Aromatic residues were found to contribute to peptide gel conductivity, with the importance of the peptide confirmation and fibril assembly demonstrated both experimentally and computationally. This is a small step (in combination with parallel research under way by other researchers) toward developing effective peptide-based conducting nanowires, opening the door to the use of electronics in water and physiological environments for bioelectronic and bioenergy applications.

Quantifying Redox Dynamics of c-Type Cytochromes in a Living Cell Suspension of Dissimilatory Metal-reducing Bacteria.

To quickly and accurately quantify the redox dynamics of c-type cytochromes (c-Cyts) in a living cell suspension, diffuse-transmission UV/visible (DT-UV/Vis) and normal UV/Vis spectroscopy were used to record spectra of c-Cyts in living Shewanella oneidensis MR-1 bacteria. DT-UV/Vis showed a higher absorbance of c-Cyts and lower background compared with normal UV/Vis, because interference from cell surface scattering was removed. The extinction coefficients of oxidized c-Cyts (410 nm) and reduced c-Cyts (419 and 552 nm) were observed. Using this method and the obtained c-Cyts extinction coefficients, the redox transformation kinetics of c-Cyts under anoxic conditions were successfully examined in the presence of various electron acceptors, including 9,10-anthraquinone-2,6-disulfonic acid, Cr(VI), Fe(III) citrate and oxygen. Therefore, the in situ spectral analysis of outer-membrane proteins of intact cells using DT-UV/Vis spectroscopy appears a promising method for investigating microbial metal reduction processes in living cell systems.

Pan-genome analyses of 24 Shewanella strains re-emphasize the diversification of their functions yet evolutionary dynamics of metal-reducing pathway

BackgroundShewanella strains are important dissimilatory metal-reducing bacteria which are widely distributed in diverse habitats. Despite efforts to genomically characterize Shewanella, knowledge of the molecular components, functional information and evolutionary patterns remain lacking, especially for their compatibility in the metal-reducing pathway. The increasing number of genome sequences of Shewanella strains offers a basis for pan-genome studies.ResultsA comparative pan-genome analysis was conducted to study genomic diversity and evolutionary relationships among 24 Shewanella strains. Results revealed an open pan-genome of 13,406 non-redundant genes and a core-genome of 1878 non-redundant genes. Selective pressure acted on the invariant members of core genome, in which purifying selection drove evolution in the housekeeping mechanisms. Shewanella strains exhibited extensive genome variability, with high levels of gene gain and loss during the evolution, which affected variable gene sets and facilitated the rapid evolution. Additionally, genes related to metal reduction were diversely distributed in Shewanella strains and evolved under purifying selection, which highlighted the basic conserved functionality and specificity of respiratory systems.ConclusionsThe diversity of genes present in the accessory and specific genomes of Shewanella strains indicates that each strain uses different strategies to adapt to diverse environments. Horizontal gene transfer is an important evolutionary force in shaping Shewanella genomes. Purifying selection plays an important role in the stability of the core-genome and also drives evolution in mtr–omc cluster of different Shewanella strains.

Dissociation between Iron and Heme Biosyntheses Is Largely Accountable for Respiration Defects of Shewanella oneidensis fur Mutants.

Iron, a major protein cofactor, is essential for most organisms but can simultaneously be toxic. Iron homeostasis thus has to be effectively maintained under a range of iron regimes. This may be particularly true with Shewanella oneidensis, a representative of dissimilatory metal-reducing bacteria (DMRB), which are capable of respiring a variety of chemicals as electron acceptors (EAs), including iron ores. Although iron respiration and its regulation have been extensively studied in this bacterium, how iron homeostasis is maintained remains largely unknown. Here, we report that the loss of the iron homeostasis master regulator Fur negatively affects the respiration of all EAs tested. This defect appears mainly to be a result of reduced cytochrome c (cyt c) production, despite a decrease in the expression of reductases that are under the direct control of Fur. We also show that S. oneidensis Fur interacts with canonical Fur box motifs in F-F-x-R configuration rather than the palindromic motif proposed before. The fur mutant has lowered total iron and increased free iron contents. Under iron-rich conditions, overproduction of the major iron storage protein Bfr elevates the total iron levels of the fur mutant over those of the wild-type but does not affect free iron levels. Intriguingly, such an operation only marginally improves cyt c production by affecting heme b biosynthesis. It is established that iron dictates heme b/cyt c biosynthesis in S. oneidensisfur+ strains, but the fur mutation annuls the dependence of heme b/cyt c biosynthesis on iron. Overall, our results suggest that Fur has a profound impact on the iron homeostasis of S. oneidensis, through which many physiological processes, especially respiration, are transformed.IMPORTANCE Iron reduction is a signature of S. oneidensis, and this process relies on a large number of type c cytochromes, which per se are iron-containing proteins. Thus, iron plays an essential and special role in iron respiration, but to date, the nature of iron metabolism and regulation of the bacterium remains largely unknown. In this study, we investigated impacts of Fur, the master regulator of iron homeostasis, on respiration. The loss of Fur causes a general defect in respiration, a result of impaired cyt c production rather than specific regulation. Additionally, the fur mutant is unresponsive to iron, resulting in imbalanced iron homeostasis and dissociation between iron and cyt c production. These findings provide important insights into the iron biology of DMRB.

Survival of Anaerobic Fe2+ Stress Requires the ClpXP Protease.

Shewanella oneidensis strain MR-1 is a versatile bacterium capable of respiring extracellular, insoluble ferric oxide minerals under anaerobic conditions. The respiration of iron minerals results in the production of soluble ferrous ions, which at high concentrations are toxic to living organisms. It is not fully understood how Fe2+ is toxic to cells anaerobically, nor is it fully understood how S. oneidensis is able to resist high levels of Fe2+ Here we describe the results of a transposon mutant screen and subsequent deletion of the genes clpX and clpP in S. oneidensis, which demonstrate that the protease ClpXP is required for anaerobic Fe2+ resistance. Many cellular processes are known to be regulated by ClpXP, including entry into stationary phase, envelope stress response, and turnover of stalled ribosomes. However, none of these processes appears to be responsible for mediating anaerobic Fe2+ resistance in S. oneidensis Protein trapping studies were performed to identify ClpXP targets in S. oneidensis under Fe2+ stress, implicating a wide variety of protein targets. Escherichia coli strains lacking clpX or clpP also display increased sensitivity to Fe2+ anaerobically, indicating Fe2+ resistance may be a conserved role for the ClpXP protease system. Hypotheses regarding the potential role(s) of ClpXP during periods of high Fe2+ are discussed. We speculate that metal-containing proteins are misfolded under conditions of high Fe2+ and that the ClpXP protease system is necessary for their turnover.IMPORTANCE Prior to the evolution of cyanobacteria and oxygenic photosynthesis, life arose and flourished in iron-rich oceans. Today, aqueous iron-rich environments are less common, constrained to low-pH conditions and anaerobic systems such as stratified lakes and seas, digestive tracts, subsurface environments, and sediments. The latter two ecosystems often favor dissimilatory metal reduction, a process that produces soluble Fe2+ from iron oxide minerals. Dissimilatory metal-reducing bacteria must therefore have mechanisms to tolerate anaerobic Fe2+ stress, and studying resistance in these organisms may help elucidate the basis of toxicity. Shewanella oneidensis is a model dissimilatory metal-reducing bacterium isolated from metal-rich sediments. Here we demonstrate a role for ClpXP, a protease system widely conserved in bacteria, in anaerobic Fe2+ resistance in both S. oneidensis and Escherichia coli.

MgtE Homolog FicI Acts as a Secondary Ferrous Iron Importer in Shewanella oneidensis Strain MR-1.

The transport of metals into and out of cells is necessary for the maintenance of appropriate intracellular concentrations. Metals are needed for incorporation into metalloproteins but become toxic at higher concentrations. Many metal transport proteins have been discovered in bacteria, including the Mg2+ transporter E (MgtE) family of passive Mg2+/Co2+ cation-selective channels. Low sequence identity exists between members of the MgtE family, indicating that substrate specificity may differ among MgtE transporters. Under anoxic conditions, dissimilatory metal-reducing bacteria, such as Shewanella and Geobacter species, are exposed to high levels of soluble metals, including Fe2+ and Mn2+ Here we characterize SO_3966, which encodes an MgtE homolog in Shewanella oneidensis that we name FicI (ferrous iron and cobalt importer) based on its role in maintaining metal homeostasis. A SO_3966 deletion mutant exhibits enhanced growth over that of the wild type under conditions with high Fe2+ or Co2+ concentrations but exhibits wild-type Mg2+ transport and retention phenotypes. Conversely, deletion of feoB, which encodes an energy-dependent Fe2+ importer, causes a growth defect under conditions of low Fe2+ concentrations but not high Fe2+ concentrations. We propose that FicI represents a secondary, less energy-dependent mechanism for iron uptake by S. oneidensis under high Fe2+ concentrations.IMPORTANCEShewanella oneidensis MR-1 is a target of microbial engineering for potential uses in biotechnology and the bioremediation of heavy-metal-contaminated environments. A full understanding of the ways in which S. oneidensis interacts with metals, including the means by which it transports metal ions, is important for optimal genetic engineering of this and other organisms for biotechnology purposes such as biosorption. The MgtE family of metal importers has been described previously as Mg2+ and Co2+ transporters. This work broadens that designation with the discovery of an MgtE homolog in S. oneidensis that imports Fe2+ but not Mg2+ The research presented here also expands our knowledge of the means by which microorganisms have adapted to take up essential nutrients such as iron under various conditions.

Dual application of Shewanella oneidensis MR-1 in green biosynthesis of Pd nanoparticles supported on TiO2 nanotubes and assisted photocatalytic degradation of methylene blue.

Biosynthesised nanocomposites have attracted growing interests attributed to their 'green' synthesis nature in recent years. Shewanella oneidensis MR-1, a dissimilatory metal-reducing bacterium, was used to reduce palladium (II) nitrate to palladium (0) nanoparticles (Pd NPs) under anaerobic conditions, resulting in the in situ formation of Pd NPs immobilised on TiO2 nanotubes (TNTs) (Pd/TNTs nanocomposites). The Pd/TNTs nanocomposites were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray, and electron spin resonance, respectively. The results indicated that Pd NPs are successfully grown on the TNTs without aggregation. Photocatalytic degradation of methylene blue (MB) by Pd/TNTs nanocomposites under simulated sunlight was also investigated. Pd/TNTs nanocomposites had photocatalytic efficiency superior to that of single TiO2 nanomaterials. The photocatalytic activity of Pd/TNTs nanocomposites can be enhanced by S. oneidensis MR-1. The results showed that after only 10 min, the degradation ratio of MB reached 98.7% by Pd/TNTs nanocomposites when simultaneously assisted with S. oneidensis MR-1.

The enhancement and inhibition of mercury reduction by natural organic matter in the presence of Shewanella oneidensis MR-1

Reduction-oxidation (Redox) processes of mercury (Hg) are of significant importance in influencing Hg speciation, bioavailability, and fate in anoxic environments where natural organic matter (NOM) and dissimilatory metal reducing bacteria (DMRB) are widely observed. However, the redox reaction between Hg and NOM, has not yet been studied in the presence of S. oneidensis MR-1 in anoxic environments. We have found that the reduction rate of mercuric mercury [Hg(II)] in the presence of Elliott soil humic acid (ESHA) was 0.02 h−1. It was faster than the rate (0.01 h−1) in the direct microbial Hg(II) reduction, suggesting that ESHA acts as an electron transfer mediator between cells and Hg, which enhances Hg(II) reduction under anoxic conditions. The overall rate of Hg(II) reduction in the presence of ESHA is determined by the rate of electron transfer from S. oneidensis MR-1 to ESHA (rate-limiting step) since the rate of electron transfer from reduced ESHA to Hg(II) was so rapid. In the reaction between S. oneidensis MR-1 and a variety of NOM analogs, the production rate of elemental mercury [Hg (0)] was linearly correlated with the free radical concentrations and aromaticities in reduced NOM analogs. However, at the high ESHA concentrations or cell contents, Hg(II) reduction might be inhibited by thiol functional groups in reduced ESHA and on cells. We suggest that the presence of NOM, cell concentration and NOM source can significantly affect the redox processes of Hg and therefore, have important implications for elucidating Hg redox processes under environmentally relevant complex conditions.

NMR studies of the interaction between inner membrane-associated and periplasmic cytochromes from Geobacter sulfurreducens.

Geobactersulfurreducens is a dissimilatory metal-reducing bacterium with notable properties and significance in biotechnological applications. Biochemical studies suggest that the inner membrane-associated diheme cytochrome MacA and the periplasmic triheme cytochrome PpcA from G.sulfurreducens can exchange electrons. In this work, NMR chemical shift perturbation measurements were used to map the interface region and to measure the binding affinity between PpcA and MacA. The results show that MacA binds to PpcA in a cleft defined by hemes I and IV, favoring the contact between PpcA heme IV and the MacA high-potential heme. The dissociation constant values indicate the formation of a low-affinity complex between the proteins, which is consistent with the transient interaction observed in electron transfer complexes.

Enhancing Extracellular Electron Transfer of Shewanella oneidensis MR-1 through Coupling Improved Flavin Synthesis and Metal-Reducing Conduit for Pollutant Degradation.

Dissimilatory metal reducing bacteria (DMRB) are capable of extracellular electron transfer (EET) to insoluble metal oxides, which are used as external electron acceptors by DMRB for their anaerobic respiration. The EET process has important contribution to environmental remediation mineral cycling, and bioelectrochemical systems. However, the low EET efficiency remains to be one of the major bottlenecks for its practical applications for pollutant degradation. In this work, Shewanella oneidensis MR-1, a model DMRB, was used to examine the feasibility of enhancing the EET and its biodegradation capacity through genetic engineering. A flavin biosynthesis gene cluster ribD-ribC-ribBA-ribE and metal-reducing conduit biosynthesis gene cluster mtrC-mtrA-mtrB were coexpressed in S. oneidensis MR-1. Compared to the control strain, the engineered strain was found to exhibit an improved EET capacity in microbial fuel cells and potentiostat-controlled electrochemical cells, with an increase in maximum current density by approximate 110% and 87%, respectively. The electrochemical impedance spectroscopy (EIS) analysis showed that the current increase correlated with the lower interfacial charge-transfer resistance of the engineered strain. Meanwhile, a three times more rapid removal rate of methyl orange by the engineered strain confirmed the improvement of its EET and biodegradation ability. Our results demonstrate that coupling of improved synthesis of mediators and metal-reducing conduits could be an efficient strategy to enhance EET in S. oneidensis MR-1, which is essential to the applications of DMRB for environmental remediation, wastewater treatment, and bioenergy recovery from wastes.

Modulation of the reactivity of multiheme cytochromes by site-directed mutagenesis: moving towards the optimization of microbial electrochemical technologies.

Dissimilatory metal-reducing bacteria perform extracellular electron transfer, a metabolic trait that is at the core of a wide range of biotechnological applications. To better understand how these microorganisms transfer electrons from their metabolism to an extracellular electron acceptor, it is necessary to characterize in detail the key players in this process, the multiheme c-type cytochromes. Shewanella oneidensis MR-1 is a model organism for studying extracellular electron transfer, where the heme protein referred to as small tetraheme cytochrome is one of the most abundant multiheme cytochromes found in the periplasmic space of this bacterium. The small tetraheme cytochrome is responsible for the delivery of electrons to the porin-cytochrome supercomplexes that permeate the outer-membrane and reduce metallic minerals or electrodes. In this work, well-established thermodynamic and kinetic models that discriminate the electron transfer activity of the four individual hemes were employed to characterize a set of single amino-acid mutants of the small tetraheme cytochrome and their interaction with small inorganic electron donors and acceptors. The results show that electrostatics play an important role in the reactivity of the small tetraheme cytochrome with small inorganic electron partners, in particularly in the kinetics of the electron transfer processes. This thorough exploration using site-directed mutants provides key mechanistic insights to guide the rational manipulation of the proteins that are key players in extracellular electron transfer processes, towards the improvement of microbial electrochemical applications using dissimilatory metal-reducing bacteria.

Comparative proteomics implicates a role for multiple secretion systems in electrode-respiring Geobacter sulfurreducens biofilms.

Geobacter sulfurreducens is a dissimilatory metal-reducing bacterium capable of forming thick electron-conducting biofilms on solid electrodes. Here, we employ for the first time comparative proteomics to identify key physiological changes involved in G. sulfurreducens adaptation from fumarate-respiring planktonic cells to electron-conducting biofilms. Increased levels of proteins involved in outer membrane biogenesis, cell motility and secretion are expressed in biofilms. Of particular importance to the electron-conducting biofilms are proteins associated with secretion systems of Type I, II, V and Type IV pili. Furthermore, enzymes involved in lipopolysaccharide and peptidoglycan biosynthesis show increased levels of expression in electron-conducting biofilms compared to planktonic cells. These observations point to similarities in long-range electron transfer mechanisms between G. sulfurreducens and Shewanella oneidensis, while highlighting the wider significance of secretion systems beyond that of Type IV pili identified to date in the adaptation of G. sulfurreducens to electrode respiration.

Biogenic FeS accelerates reductive dechlorination of carbon tetrachloride by Shewanella putrefaciens CN32.

Dissimilatory metal reducing bacteria (DMRB) widely exist in the subsurface environment and are involved in various contaminant degradation and element geochemical cycling processes. Recent studies suggest that DMRB can biosynthesize metal nanoparticles during metal reduction, but it is unclear yet how such biogenic nanomaterials would affect their decontamination behaviors. In this study, we found that the dechlorination rates of carbon tetrachloride (CT) by Shewanella putrefaciens CN32 was significantly increased by 8 times with the formation of biogenic ferrous sulfide (FeS) nanoparticles. The pasteurized biogenic FeS enabled 5 times faster dechlorination than abiotic FeS that had larger sizes and irregular structure, confirming a significant contribution of the biogenic FeS to CT bioreduction resulting from its good dispersion and relatively high dechlorination activity. This study highlights a potentially important role of biosynthesized nanoparticles in environmental bioremediation.

Imaging the Hydrated Microbe-Metal Interface Using Nanoscale Spectrum Imaging

PdAu nanocrystals are synthesised by Geobacter sulfurreducens, a dissimilatory metal-reducing bacterium, and the resulting bimetallic nanocrystal-decorated microbes are imaged using a range of advanced electron microscopy techniques. Specifically, the first example of elemental mapping of fully hydrated biological nanostructures using scanning transmission electron microscope (STEM) energy dispersive X-ray (EDX) spectrum imaging within an environmental liquid-cell is reported. These results are combined with cryo-TEM and ex situ STEM imaging and EDX analysis with the aim of better understanding microbial synthesis of bimetallic nanoparticles. It is demonstrated that although Au and Pd are colocalized across the cells, the population of nanoparticles produced is bimodal, containing ultrasmall alloyed nanocrystals with diameters 200 nm in diameter) which show higher Pd contents and exhibit a Pd enriched shell only a few nanometers thick. The application of high-resolution imaging techniques described here offers the potential to visualize the microbe-metal interface during the bioproduction of a range of functional materials by microbial “green” synthesis routes, and also key interfaces underpinning globally relevant environmental processes (e.g., metal cycling).