Outer Membrane Cytochromes OmcA Research Articles

The Outer Membrane Cytochrome OmcA Is Essential for Infection of Shewanella oneidensis by a Zebrafish-Associated Bacteriophage.

The microbiota-the mixture of microorganisms in the intestinal tract of animals-plays an important role in host biology. Bacteriophages are a prominent, though often overlooked, component of the microbiota. The mechanisms that phage use to infect susceptible cells associated with animal hosts, and the broader role they could play in determining the substituents of the microbiota, are poorly understood. In this study, we isolated a zebrafish-associated bacteriophage, which we named Shewanella phage FishSpeaker. This phage infects Shewanella oneidensis strain MR-1, which cannot colonize zebrafish, but it is unable to infect Shewanella xiamenensis strain FH-1, a strain isolated from the zebrafish gut. Our data suggest that FishSpeaker uses the outer membrane decaheme cytochrome OmcA, which is an accessory component of the extracellular electron transfer (EET) pathway in S. oneidensis, as well as the flagellum to recognize and infect susceptible cells. In a zebrafish colony that lacks detectable FishSpeaker, we found that most Shewanella spp. are sensitive to infection and that some strains are resistant to infection. Our results suggest that phage could act as a selectivity filter for zebrafish-associated Shewanella and show that the EET machinery can be targeted by phage in the environment. IMPORTANCE Phage exert selective pressure on bacteria that influences and shapes the composition of microbial populations. However, there is a lack of native, experimentally tractable systems for studying how phage influence microbial population dynamics in complex communities. Here, we show that a zebrafish-associated phage requires both the outer membrane-associated extracellular electron transfer protein OmcA and the flagellum to infect Shewanella oneidensis strain MR-1. Our results suggest that the newly discovered phage-FishSpeaker-could exert selective pressure that restricts which Shewanella spp. colonize zebrafish. Moreover, the requirement of OmcA for infection by FishSpeaker suggests that the phage preferentially infects cells that are oxygen limited, a condition required for OmcA expression and an ecological feature of the zebrafish gut.

Improvement of microbial extracellular electron transfer via outer membrane cytochromes expression of engineered bacteria

The effective contact between proteins on the electroactive membranes or nanowires determines the efficiency of the electroactive in bioelectrochemical systems (BES) biofilm. However, most electroactive biofilms are multi-layer bacterial aggregates, and the microorganisms in the outer layer cannot directly contact the electrode surface electron transfer. Herein, we construct a genetically engineered bacterium (GEB) that can express outer membrane cytochrome OmcA to improve the microbial extracellular electron transfer in the biofilm. The engineered bacterium improves the maximum power density of the mixed bacteria microbial fuel cell (MFC) by 2.36 times to 3.76 A cm−2, and increases the current density to 1.98 W m−2. This improved behavior was stable for at least three feeds. Adding the engineered bacteria generates a more compact biofilm structure and significantly improves the electrochemical performance of the anode biofilm. The engineered bacteria with the electron transfer function also increase the content of the OmcA gene in the bacterial community, thereby promoting the microbial extracellular electron transfer and increasing the proportion of electrogenic bacteria in the biofilm. With its expression of the conductive protein, the GEB can effectively improve the extracellular electron transfer efficiency of the outer layer bacteria in the biofilm of the MFC. Our study provides a new choice for building an efficient electrochemical system.

Interfacial Electron Transfer from the Outer Membrane Cytochrome OmcA to Graphene Oxide in a Microbial Fuel Cell: Spectral and Electrochemical Insights

A microbial fuel cell (MFC) is able to convert waste energy into electricity, and its performance is mainly constrained by extracellular electron transfer (EET) at biological/inorganic interfaces. In this work, the interaction process between the microbe’s outer membrane cytochrome (OmcA) and graphene oxide (GO) is investigated in vitro to elucidate the EET mechanism in the MFC. The feasibility of the electron transfer from the purified OmcA to GO is examined by protein film voltammetry tests. Stopped flow analysis demonstrates that the EET kinetic process obeys the Michaelis–Menten equation and exhibits the characteristics of enzyme catalysis. The structural changes of OmcA/GO monitored by a circular dichroism spectroscopy combined with infrared spectroscopy techniques indicate that the formation of a hydrogen bond between the −NH2 group of the residues and the −COOH/–OH group of GO shortens the electron-transfer distance to mitigate the energy barrier. These results help to understand the EET mechanism ...

Particle size effect and the mechanism of hematite reduction by the outer membrane cytochrome OmcA of Shewanella oneidensis MR-1

The cycling of iron at the Earth’s near surface is profoundly influenced by dissimilatory metal reducing microorganisms, and many studies have focused on unraveling electron transfer mechanisms between these bacteria and Fe(III)-(oxyhydr)oxides. However, these efforts have been complicated by the fact that these minerals often occur in the micro- to nanosize regime, and in relevant natural environments as well as in the laboratory are subject to aggregation. The nature of the physical interface between the cellular envelope, the outer-membrane cytochromes responsible for facilitating the interfacial electron transfer step, and these complex mineral particulates is thus difficult to probe. Previous studies using whole cells have reported reduction rates that do not correlate with particle size. In the present study we isolate the interaction between the decaheme outer-membrane cytochrome OmcA of Shewanella oneidensis and nanoparticulate hematite, examining the reduction rate as a function of particle size and reaction products through detailed characterization of the electron balance and the structure and valence of iron at particle surfaces. By comparison with abiotic reduction via the smaller molecule ascorbic acid, we show that the reduction rate is systematically controlled by the sterically accessible interfacial contact area between OmcA and hematite in particle aggregates; rates increase once pore throat sizes in aggregates become as large as OmcA. Simultaneous measure of OmcA oxidation against Fe(II) release shows a ratio of 1:10, consistent with a cascade OmcA oxidation mechanism heme by heme. X-ray absorption spectroscopies reveal incipient magnetite on the reacted surfaces of the hematite nanoparticles after reaction. The collective findings establish the importance of accessibility of physical contact between the terminal reductases and iron oxide surfaces, and through apparent consistency of observations help reconcile behavior reported at the larger more complex scale of whole cell studies.

Transcriptional mechanisms for differential expression of outer membrane cytochrome genes omcA and mtrC in Shewanella oneidensis MR-1.

BackgroundShewanella oneidensis MR-1 is capable of reducing extracellular electron acceptors, such as metals and electrodes, through the Mtr respiratory pathway, which consists of the outer membrane cytochromes OmcA and MtrC and associated proteins MtrA and MtrB. These proteins are encoded in the mtr gene cluster (omcA-mtrCAB) in the MR-1 chromosome.ResultsHere, we investigated the transcriptional mechanisms for the mtr genes and demonstrated that omcA and mtrC are transcribed from two upstream promoters, PomcA and PmtrC, respectively. In vivo transcription and in vitro electrophoretic mobility shift assays revealed that a cAMP receptor protein (CRP) positively regulates the expression of the mtr genes by binding to the upstream regions of PomcA and PmtrC. However, the expression of omcA and mtrC was differentially regulated in response to culture conditions; specifically, the expression from PmtrC was higher under aerobic conditions than that under anaerobic conditions with fumarate as an electron acceptor, whereas expression from PomcA exhibited the opposite trend. Deletion of the region upstream of the CRP-binding site of PomcA resulted in a significant increase in promoter activity under aerobic conditions, demonstrating that the deleted region is involved in the negative regulation of PomcA.ConclusionsTaken together, the present results indicate that transcription of the mtr genes is regulated by multiple promoters and regulatory systems, including the CRP/cAMP-dependent regulatory system and yet-unidentified negative regulators.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0406-8) contains supplementary material, which is available to authorized users.

Role of outer membrane c‐type cytochromes MtrC and OmcA in Shewanella oneidensis MR‐1 cell production, accumulation, and detachment during respiration on hematite

The iron-reducing bacterium Shewanella oneidensis MR-1 has the capacity to contribute to iron cycling over the long term by respiring on crystalline iron oxides such as hematite when poorly crystalline phases are depleted. The ability of outer membrane cytochromes OmcA and MtrC of MR-1 to bind to and transfer electrons to hematite has led to the suggestion that they function as terminal reductases when this mineral is used as a respiratory substrate. Differences in their redox behavior and hematite-binding properties, however, indicate that they play different roles in the electron transfer reaction. Here, we investigated how these differences in cytochrome behavior with respect to hematite affected biofilm development when the mineral served as terminal electron acceptor (TEA). Upon attachment to hematite, cells of the wild-type (WT) strain as well as those of a ΔomcA mutant but not those of a ΔmtrC mutant replicated and accumulated on the mineral surface. The results indicate that MtrC but not OmcA is required for growth when this mineral serves as TEA. While an OmcA deficiency did not impede cell replication and accumulation on hematite prior to achievement of a maximum surface cell density comparable to that established by WT cells, OmcA was required for efficient electron transfer and cell attachment to hematite once maximum surface cell density was achieved. OmcA may therefore play a role in overcoming barriers to electron transfer and cell attachment to hematite imposed by reductive dissolution of the mineral surface from cell respiration associated with achievement of high surface cell densities.

Crystallization and preliminary X-ray crystallographic studies of the outer membrane cytochrome OmcA from Shewanella oneidensis MR-1.

The outer membrane cytochrome OmcA functions as a terminal metal reductase in the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. The ten-heme centers shuttle electrons from the transmembrane donor complex to extracellular electron acceptors. Here, the crystallization and preliminary crystallographic analysis of OmcA are reported. Crystals of OmcA were grown by the sitting-drop vapor-diffusion method using PEG 20,000 as a precipitant. The OmcA crystals belonged to space group P2(1), with unit-cell parameters a = 93.0, b = 246.0, c = 136.6 Å, α = 90, β = 97.8, γ = 90°. X-ray diffraction data were collected to a maximum resolution of 3.25 Å.

Electrochemical interaction of Shewanella oneidensis MR-1 and its outer membrane cytochromes OmcA and MtrC with hematite electrodes

Bacterial metal reduction is an important biogeochemical process in anaerobic environments. An understanding of electron transfer pathways from dissimilatory metal-reducing bacteria (DMRB) to solid phase metal (hydr)oxides is important for understanding metal redox cycling in soils and sediments, for utilizing DMRB in bioremedation, and for developing technologies such as microbial fuel cells. Here we hypothesize that the outer membrane cytochromes OmcA and MtrC from Shewanella oneidensis MR-1 are the only terminal reductases capable of direct electron transfer to a hematite working electrode. Cyclic voltammetry (CV) was used to study electron transfer between hematite electrodes and protein films, S. oneidensis MR-1 wild-type cell suspensions, and cytochrome deletion mutants. After controlling for hematite electrode dissolution at negative potential, the midpoint potentials of adsorbed OmcA and MtrC were measured (−201 mV and −163 mV vs. Ag/AgCl, respectively). Cell suspensions of wild-type MR-1, deletion mutants deficient in OmcA (Δ omcA), MtrC (Δ mtrC), and both OmcA and MtrC (Δ mtrC–Δ omcA) were also studied; voltammograms for Δ mtrC–Δ omcA were indistinguishable from the control. When the control was subtracted from the single deletion mutant voltammograms, redox peaks were consistent with the present cytochrome (i.e., Δ omcA consistent with MtrC and Δ mtrC consistent with OmcA). The results indicate that OmcA and MtrC are capable of direct electron exchange with hematite electrodes, consistent with a role as terminal reductases in the S. oneidensis MR-1 anaerobic respiratory pathway involving ferric minerals. There was no evidence for other terminal reductases operating under the conditions investigated. A Marcus-based approach to electron transfer kinetics indicated that the rate constant for electron transfer k et varies from 0.025 s −1 in the absence of a barrier to 63.5 s −1 with a 0.2 eV barrier.

The c -Type Cytochrome OmcA Localizes to the Outer Membrane upon Heterologous Expression in Escherichia coli

We have functionally produced the outer membrane cytochrome OmcA from Shewanella oneidensis in Escherichia coli. Substrate accessibility experiments indicate that OmcA is surface exposed in an E. coli B strain but not in a K-12 strain. We show that a functional type II secretion system is required for surface localization.

Cell surface exposure of the outer membrane cytochromes of Shewanella oneidensis MR-1.

To determine if the outer membrane (OM) cytochromes of the metal-reducing bacterium Shewanella oneidensis MR-1 are exposed on the cell surface. MR-1 cells were incubated with proteinase K or buffer and the resulting degradation of the OM cytochromes was examined by Western blotting. The periplasmic fumarate reductase (control) was not degraded. The OM cytochromes OmcA and OmcB were significantly degraded by proteinase K (71 and 31%, respectively). Immunofluorescence confirmed a prominent cell surface exposure of OmcA and a partial exposure of OmcB and the noncytochrome OM protein MtrB. The cytochromes OmcA and OmcB are exposed on the outer face of the OM. The cell surface exposure of these cytochromes could allow them to directly contact extracellular insoluble electron acceptors (e.g. manganese oxides) and is consistent with their in vivo role.

Role for Outer Membrane Cytochromes OmcA and OmcB of Shewanella putrefaciens MR-1 in Reduction of Manganese Dioxide

Shewanella putrefaciens MR-1 can use a wide variety of terminal electron acceptors for anaerobic respiration, including certain insoluble manganese and iron oxides. To examine whether the outer membrane (OM) cytochromes of MR-1 play a role in Mn(IV) and Fe(III) reduction, mutants lacking the OM cytochrome OmcA or OmcB were isolated by gene replacement. Southern blotting and PCR confirmed replacement of the omcA and omcB genes, respectively, and reverse transcription-PCR analysis demonstrated loss of the respective mRNAs, whereas mRNAs for upstream and downstream genes were retained. The omcA mutant (OMCA1) resembled MR-1 in its growth on trimethylamine N-oxide (TMAO), dimethyl sulfoxide, nitrate, fumarate, thiosulfate, and tetrathionate and its reduction of nitrate, nitrite, ferric citrate, FeOOH, and anthraquinone-2,6-disulfonic acid. Similarly, the omcB mutant (OMCB1) grew on fumarate, nitrate, TMAO, and thiosulfate and reduced ferric citrate and FeOOH. However, OMCA1 and OMCB1 were 45 and 75% slower than MR-1, respectively, at reducing MnO(2). OMCA1 lacked only OmcA. While OMCB1 lacked OmcB, other OM cytochromes were also missing or markedly depressed. The total cytochrome content of the OM of OMCB1 was less than 15% of that of MR-1. Western blots demonstrated that OMCB1 still synthesized OmcA, but most of it was localized in the cytoplasmic membrane and soluble fractions rather than in the OM. OMCB1 had therefore lost the ability to properly localize multiple OM cytochromes to the OM. Together, the results suggest that the OM cytochromes of MR-1 participate in the reduction of Mn(IV) but are not required for the reduction of Fe(III) or other electron acceptors.