MgtE Homolog FicI Acts as a Secondary Ferrous Iron Importer inShewanella oneidensisStrain MR-1

ABSTRACTThe 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 asShewanellaandGeobacterspecies, are exposed to high levels of soluble metals, including Fe2+and Mn2+. Here we characterize SO_3966, which encodes an MgtE homolog inShewanella oneidensisthat we name FicI (ferrousiron andcobaltimporter) 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 offeoB, 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 byS. oneidensisunder high Fe2+concentrations.IMPORTANCEShewanella oneidensisMR-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 whichS. oneidensisinteracts 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 inS. oneidensisthat 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.

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Complex Iron Uptake by the Putrebactin-Mediated and Feo Systems in Shewanella oneidensis

Applied and Environmental Microbiology ◽

10.1128/aem.01752-18 ◽

2018 ◽

Vol 84 (20) ◽

Cited By ~ 3

Author(s):

Lulu Liu ◽

Shisheng Li ◽

Sijing Wang ◽

Ziyang Dong ◽

Haichun Gao

Keyword(s):

Iron Uptake ◽

Iron Homeostasis ◽

Shewanella Oneidensis ◽

Special Role ◽

Uptake System ◽

Biological Processes ◽

Content Type ◽

Iron Uptake System ◽

Reducing Bacteria ◽

Metal Reducing Bacteria

ABSTRACT Shewanella oneidensis is an extensively studied bacterium capable of respiring minerals, including a variety of iron ores, as terminal electron acceptors (EAs). Although iron plays an essential and special role in iron respiration of S. oneidensis, little has been done to date to investigate the characteristics of iron transport in this bacterium. In this study, we found that all proteins encoded by the pub-putA-putB cluster for putrebactin (S. oneidensis native siderophore) synthesis (PubABC), recognition-transport of Fe3+-putrebactin across the outer membrane (PutA), and reduction of ferric putrebactin (PutB) are essential to putrebactin-mediated iron uptake. Although homologs of PutA are many, none can function as its replacement, but some are able to work with other bacterial siderophores. We then showed that Fe2+-specific Feo is the other primary iron uptake system, based on the synthetical lethal phenotype resulting from the loss of both iron uptake routes. The role of the Feo system in iron uptake appears to be more critical, as growth is significantly impaired by the absence of the system but not of putrebactin. Furthermore, we demonstrate that hydroxyl acids, especially α-types such as lactate, promote iron uptake in a Feo-dependent manner. Overall, our findings underscore the importance of the ferrous iron uptake system in metal-reducing bacteria, providing an insight into iron homeostasis by linking these two biological processes. IMPORTANCE S. oneidensis is among the first- and the best-studied metal-reducing bacteria, with great potential in bioremediation and biotechnology. However, many questions regarding mechanisms closely associated with those applications, such as iron homeostasis, including iron uptake, export, and regulation, remain to be addressed. Here we show that Feo is a primary player in iron uptake in addition to the siderophore-dependent route. The investigation also resolved a few puzzles regarding the unexpected phenotypes of the putA mutant and lactate-dependent iron uptake. By elucidating the physiological roles of these two important iron uptake systems, this work revealed the breadth of the impacts of iron uptake systems on the biological processes.

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SREBP-Dependent Triazole Susceptibility in Aspergillus fumigatus Is Mediated through Direct Transcriptional Regulation oferg11A(cyp51A)

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.05027-11 ◽

2011 ◽

Vol 56 (1) ◽

pp. 248-257 ◽

Cited By ~ 41

Author(s):

Sara J. Blosser ◽

Robert A. Cramer

Keyword(s):

Aspergillus Fumigatus ◽

Regulatory Element ◽

Critical Role ◽

Clinical Importance ◽

Growth Defect ◽

Ergosterol Biosynthesis ◽

Wild Type ◽

Transcript Levels ◽

Content Type ◽

Conditional Expression

ABSTRACTAs triazole antifungal drug resistance during invasiveAspergillus fumigatusinfection has become more prevalent, the need to understand mechanisms of resistance inA. fumigatushas increased. The presence of twoerg11(cyp51) genes inAspergillusspp. is hypothesized to account for the inherent resistance of this mold to the triazole fluconazole (FLC). Recently, anA. fumigatusnull mutant of a transcriptional regulator in the sterol regulatory element binding protein (SREBP) family, the ΔsrbAstrain, was found to have increased susceptibility to FLC and voriconazole (VCZ). In this study, we examined the mechanism engendering the observed increase inA. fumigatustriazole susceptibility in the absence of SrbA. We observed a significant reduction in theerg11Atranscript in the ΔsrbAstrain in response to FLC and VCZ. Transcript levels oferg11Bwere also reduced but not to the extent oferg11A. Interestingly,erg11Atranscript levels increased upon extended VCZ, but not FLC, exposure. Construction of anerg11Aconditional expression strain in the ΔsrbAstrain was able to restoreerg11Atranscript levels and, consequently, wild-type MICs to the triazole FLC. The VCZ MIC was also partially restored upon increasederg11Atranscript levels; however, total ergosterol levels remained significantly reduced compared to those of the wild type. Induction of theerg11Aconditional strain did not restore the hypoxia growth defect of the ΔsrbAstrain. Taken together, our results demonstrate a critical role for SrbA-mediated regulation of ergosterol biosynthesis and triazole drug interactions inA. fumigatusthat may have clinical importance.

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The Streptococcus iniae Transcriptional Regulator CpsY Is Required for Protection from Neutrophil-Mediated Killing and Proper GrowthInVitro

Infection and Immunity ◽

10.1128/iai.05567-11 ◽

2011 ◽

Vol 79 (11) ◽

pp. 4638-4648 ◽

Cited By ~ 16

Author(s):

Jonathan P. Allen ◽

Melody N. Neely

Keyword(s):

Whole Blood ◽

Local Environment ◽

Growth Defect ◽

Infection Model ◽

Streptococcus Iniae ◽

Virulence Determinants ◽

Wild Type ◽

Content Type ◽

Proteose Peptone ◽

Lysr Family

ABSTRACTThe ability of a pathogen to metabolically adapt to the local environment for optimal expression of virulence determinants is a continued area of research. Orthologs of theStreptococcus iniaeLysR family regulator CpsY have been shown to regulate methionine biosynthesis and uptake pathways but appear to influence expression of several virulence genes as well. AnS. iniaemutant with an in-frame deletion ofcpsY(ΔcpsYmutant) is highly attenuated in a zebrafish infection model. The ΔcpsYmutant displays a methionine-independent growth defect in serum, which differs from the methionine-dependent defect observed for orthologous mutants ofStreptococcus mutansandStreptococcus agalactiae. On the contrary, the ΔcpsYmutant can grow in excess of the wild type (WT) when supplemented with proteose peptone, suggesting an inability to properly regulate growth. CpsY is critical for protection ofS. iniaefrom clearance by neutrophils in whole blood but is dispensable for intracellular survival in macrophages. Susceptibility of the ΔcpsYmutant to killing in whole blood is not due to a growth defect, because inhibition of neutrophil phagocytosis rescues the mutant to WT levels. Thus, CpsY appears to have a pleiotropic regulatory role forS. iniae, integrating metabolism and virulence. Furthermore,S. iniaeprovides a unique model to investigate the paradigm of CpsY-dependent regulation during systemic streptococcal infection.

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Uncovering an Important Role for YopJ in the Inhibition of Caspase-1 in Activated Macrophages and Promoting Yersinia pseudotuberculosis Virulence

Infection and Immunity ◽

10.1128/iai.00843-15 ◽

2016 ◽

Vol 84 (4) ◽

pp. 1062-1072 ◽

Cited By ~ 10

Author(s):

Taylor J. Schoberle ◽

Lawton K. Chung ◽

Joseph B. McPhee ◽

Ben Bogin ◽

James B. Bliska

Keyword(s):

Yersinia Pseudotuberculosis ◽

Host Cells ◽

Enzyme Release ◽

Activated Macrophages ◽

Innate Immune Responses ◽

Wild Type ◽

Content Type ◽

Caspase 1 ◽

Epistasis Analysis ◽

Dependent Mechanism

PathogenicYersiniaspecies utilize a type III secretion system to translocate Yop effectors into infected host cells. Yop effectors inhibit innate immune responses in infected macrophages to promoteYersiniapathogenesis. In turn,Yersinia-infected macrophages respond to translocation of Yops by activating caspase-1, but different mechanisms of caspase-1 activation occur, depending on the bacterial genotype and the state of phagocyte activation. In macrophages activated with lipopolysaccharide (LPS) prior toYersinia pseudotuberculosisinfection, caspase-1 is activated by a rapid inflammasome-dependent mechanism that is inhibited by translocated YopM. The possibility that other effectors cooperate with YopM to inhibit caspase-1 activation in LPS-activated macrophages has not been investigated. Toward this aim, epistasis analysis was carried out in which the phenotype of aY. pseudotuberculosisyopMmutant was compared to that of ayopJ yopM,yopE yopM,yopH yopM,yopT yopM, orypkA yopMmutant. Activation of caspase-1 was measured by cleavage of the enzyme, release of interleukin-1β (IL-1β), and pyroptosis in LPS-activated macrophages infected with wild-type or mutantY. pseudotuberculosisstrains. Results show enhanced activation of caspase-1 after infection with theyopJ yopMmutant relative to infection by any other single or double mutant. Similar results were obtained with theyopJ,yopM, andyopJ yopMmutants ofYersinia pestis. Following intravenous infection of mice, theY. pseudotuberculosisyopJmutant was as virulent as the wild type, while theyopJ yopMmutant was significantly more attenuated than theyopMmutant. In summary, through epistasis analysis this work uncovered an important role for YopJ in inhibiting caspase-1 in activated macrophages and in promotingYersiniavirulence.

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Dependency of Coxiella burnetii Type 4B Secretion on the Chaperone IcmS

Journal of Bacteriology ◽

10.1128/jb.00431-19 ◽

2019 ◽

Vol 201 (23) ◽

Cited By ~ 2

Author(s):

Charles L. Larson ◽

Paul A. Beare ◽

Robert A. Heinzen

Keyword(s):

Host Cell ◽

Coxiella Burnetii ◽

Legionella Pneumophila ◽

Q Fever ◽

Secretion System ◽

Host Cells ◽

Growth Defect ◽

Wild Type ◽

Signal Sequences ◽

Content Type

ABSTRACT Macrophage parasitism by Coxiella burnetii, the cause of human Q fever, requires the translocation of proteins with effector functions directly into the host cell cytosol via a Dot/Icm type 4B secretion system (T4BSS). Secretion by the analogous Legionella pneumophila T4BSS involves signal sequences within the C-terminal and internal domains of effector proteins. The cytoplasmic chaperone pair IcmSW promotes secretion and binds internal sites distinct from signal sequences. In the present study, we investigated requirements of C. burnetii IcmS for host cell parasitism and effector translocation. A C. burnetii icmS deletion mutant (ΔicmS) exhibited impaired replication in Vero epithelial cells, deficient formation of the Coxiella-containing vacuole, and aberrant T4BSS secretion. Three secretion phenotypes were identified from a screen of 50 Dot/Icm substrates: IcmS dependent (secreted by only wild-type bacteria), IcmS independent (secreted by both wild-type and ΔicmS bacteria), or IcmS inhibited (secreted by only ΔicmS bacteria). Secretion was assessed for N-terminal or C-terminal truncated forms of CBU0794 and CBU1525. IcmS-inhibited secretion of CBU1525 required a C-terminal secretion signal whereas IcmS-dependent secretion of CBU0794 was directed by C-terminal and internal signals. Interchange of the C-terminal 50 amino acids of CBU0794 and CBU1525 revealed that sites within the C terminus regulate IcmS dependency. Glutathione S-transferase-tagged IcmSW bound internal sequences of IcmS-dependent and -inhibited substrates. Thus, the growth defect of the C. burnetii ΔicmS strain is associated with a loss of T4BSS chaperone activity that both positively and negatively regulates effector translocation. IMPORTANCE The intracellular pathogen Coxiella burnetii employs a type 4B secretion system (T4BSS) that promotes growth by translocating effectors of eukaryotic pathways into host cells. T4BSS regulation modeled in Legionella pneumophila indicates IcmS facilitates effector translocation. Here, we characterized type 4B secretion by a Coxiella ΔicmS mutant that exhibits intracellular growth defects. T4BSS substrates demonstrated increased, equivalent, or decreased secretion by the ΔicmS mutant relative to wild-type Coxiella. Similar to the Legionella T4BSS, IcmS dependency in Coxiella was determined by C-terminal and/or internal secretion signals. However, IcmS inhibited secretion of some effectors by Coxiella that were previously shown to be translocated by Legionella. Thus, Coxiella has a unique IcmS regulatory mechanism that both positively and negatively regulates T4BSS export.

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Loss of RNA Chaperone Hfq Unveils a Toxic Pathway in Pseudomonas aeruginosa

Journal of Bacteriology ◽

10.1128/jb.00232-19 ◽

2019 ◽

Vol 201 (20) ◽

Cited By ~ 2

Author(s):

Ian T. Hill ◽

Thomas Tallo ◽

Matthew J. Dorman ◽

Simon L. Dove

Keyword(s):

Pseudomonas Aeruginosa ◽

Opportunistic Pathogen ◽

Toxic Effects ◽

Growth Defect ◽

Wild Type ◽

Rna Chaperone ◽

Small Protein ◽

Content Type ◽

Transcription Regulator ◽

Mutant Cells

ABSTRACT Hfq is an RNA chaperone that serves as a master regulator of bacterial physiology. Here we show that in the opportunistic pathogen Pseudomonas aeruginosa, the loss of Hfq can result in a dramatic reduction in growth in a manner that is dependent upon MexT, a transcription regulator that governs antibiotic resistance in this organism. Using a combination of chromatin immunoprecipitation with high-throughput sequencing and transposon insertion sequencing, we identify the MexT-activated genes responsible for mediating the growth defect of hfq mutant cells. These include a newly identified MexT-controlled gene that we call hilR. We demonstrate that hilR encodes a small protein that is acutely toxic to wild-type cells when produced ectopically. Furthermore, we show that hilR expression is negatively regulated by Hfq, offering a possible explanation for the growth defect of hfq mutant cells. Finally, we present evidence that the expression of MexT-activated genes is dependent upon GshA, an enzyme involved in the synthesis of glutathione. Our findings suggest that Hfq can influence the growth of P. aeruginosa by limiting the toxic effects of specific MexT-regulated genes. Moreover, our results identify glutathione to be a factor important for the in vivo activity of MexT. IMPORTANCE Here we show that the conserved RNA chaperone Hfq is important for the growth of the opportunistic pathogen Pseudomonas aeruginosa. We found that the growth defect of hfq mutant cells is dependent upon the expression of genes that are under the control of the transcription regulator MexT. These include a gene that we refer to as hilR, which we show is negatively regulated by Hfq and encodes a small protein that can be toxic when ectopically produced in wild-type cells. Thus, Hfq can influence the growth of P. aeruginosa by limiting the toxic effects of MexT-regulated genes, including one encoding a previously unrecognized small protein. We also show that MexT activity depends on an enzyme that synthesizes glutathione.

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Availability of Zinc Impacts Interactions between Streptococcus sanguinis and Pseudomonas aeruginosa in Coculture

Journal of Bacteriology ◽

10.1128/jb.00618-19 ◽

2019 ◽

Vol 202 (2) ◽

Author(s):

Kewei Li ◽

Alex H. Gifford ◽

Thomas H. Hampton ◽

George A. O’Toole

Keyword(s):

Pseudomonas Aeruginosa ◽

Zinc Uptake ◽

Microbial Interactions ◽

Growth Defect ◽

Zinc Toxicity ◽

Wild Type ◽

Content Type ◽

Airway Infections ◽

Zinc Levels

ABSTRACT Airway infections associated with cystic fibrosis (CF) are polymicrobial. We reported previously that clinical isolates of Pseudomonas aeruginosa promote the growth of a variety of streptococcal species. To explore the mechanistic basis of this interaction, we performed a genetic screen to identify mutants of Streptococcus sanginuis SK36 whose growth was no longer enhanced by P. aeruginosa PAO1. Mutations in the zinc uptake systems of S. sanguinis SK36 reduced growth of these strains by 1 to 3 logs compared to that of wild-type S. sanguinis SK36 when grown in coculture with P. aeruginosa PAO1, and exogenous zinc (0.1 to 10 μM) rescued the coculture defect of zinc uptake mutants of S. sanguinis SK36. Zinc uptake mutants of S. sanguinis SK36 had no obvious growth defect in monoculture. Consistent with competition for zinc driving coculture dynamics, S. sanguinis SK36 grown in coculture with P. aeruginosa showed increased expression of zinc uptake genes compared to that of S. sanguinis grown alone. Strains of P. aeruginosa PAO1 defective in zinc transport also supported ∼2-fold more growth by S. sanguinis compared to that in coculture with wild-type P. aeruginosa PAO1. An analysis of 118 CF sputum samples revealed that total zinc levels varied from ∼5 to 145 μM. At relatively low zinc levels, Pseudomonas and Streptococcus spp. were found in approximately equal abundance; at higher zinc levels, we observed a decline in relative abundance of Streptococcus spp., perhaps as a result of increasing zinc toxicity. Together, our data indicate that the relative abundances of these microbes in the CF airway may be impacted by zinc levels. IMPORTANCE Polymicrobial infections in CF cases likely impact patient health, but the mechanism(s) underlying such interactions is poorly understood. Here, we show using an in vitro model system that interactions between Pseudomonas and Streptococcus are modulated by zinc availability, and clinical data are consistent with this model. Together with previous studies, our work supports a role for metal homeostasis as a key factor driving microbial interactions.

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Inactivation of NMB0419, Encoding a Sel1-Like Repeat (SLR) Protein, in Neisseria meningitidis Is Associated with Differential Expression of Genes Belonging to the Fur Regulon and Reduced Intraepithelial Replication

Infection and Immunity ◽

10.1128/iai.00574-16 ◽

2017 ◽

Vol 85 (5) ◽

Cited By ~ 1

Author(s):

Ming-Shi Li ◽

Paul R. Langford ◽

J. Simon Kroll

Keyword(s):

Epithelial Cells ◽

Neisseria Meningitidis ◽

Iron Acquisition ◽

Regulatory Function ◽

Growth Defect ◽

Wild Type ◽

Content Type ◽

Expression Of Genes ◽

New Perspective

ABSTRACT Neisseria meningitidis is a commensal microbe that colonizes the human nasopharynx but occasionally invades the bloodstream to cause life-threatening infection. N. meningitidis MC58 NMB0419 encodes a Sel1-like repeat (SLR)-containing protein, previously implicated in invasion of epithelial cells. A gene-regulatory function was revealed in Escherichia coli expressing plasmid-borne NMB0419 and showing significantly increased epithelial adherence compared to the wild type, due to increased expression of mannose-sensitive type 1 pili. While a meningococcal NMB0419 mutant did not have altered epithelial adherence, in a transcriptome-wide comparison of the wild type and an NMB0419 mutant, a large proportion of genes differentially regulated in the mutant were involved in iron acquisition and metabolism. Fifty-one percent and 38% of genes, respectively, up- and downregulated in the NMB0419 mutant had previously been identified as being induced and repressed by meningococcal Fur. An in vitro growth defect of the NMB0419 mutant under iron restriction was consistent with the downregulation of tbpAB and hmbR, while an intraepithelial replication defect was consistent with the downregulation of tonB, exbB, and exbD, based on a known phenotype of a meningococcal tonB mutant. Disruption of the N-terminal NMB0419 signal peptide, predicted to export the protein beyond the cytoplasmic membrane, resulted in loss of functional traits in N. meningitidis and E. coli. Our study indicates that the expression of NMB0419 is associated with transcriptional changes counterbalancing the regulatory function of Fur, offering a new perspective on regulatory mechanisms involved in meningococcal interaction with epithelial cells, and suggests new insights into the roles of SLR-containing genes in other bacteria.

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The LysR-Type Transcriptional Regulator CrgA Negatively Regulates the Flagellar Master Regulator flhDC in Ralstonia solanacearum GMI1000

Journal of Bacteriology ◽

10.1128/jb.00419-20 ◽

2020 ◽

Vol 203 (1) ◽

Author(s):

Xiaojing Fan ◽

Zhiwen Zhao ◽

Tingyan Sun ◽

Wei Rou ◽

Caiying Gui ◽

...

Keyword(s):

Cell Motility ◽

Ralstonia Solanacearum ◽

Host Plants ◽

Transcriptional Regulator ◽

Growth Defect ◽

Wild Type ◽

Mobility Shift ◽

Content Type ◽

Gus Reporter ◽

Electrophoretic Mobility Shift Assays

ABSTRACT The invasion and colonization of host plants by the destructive pathogen Ralstonia solanacearum rely on its cell motility, which is controlled by multiple factors. Here, we report that the LysR-type transcriptional regulator CrgA (RS_RS16695) represses cell motility in R. solanacearum GMI1000. CrgA possesses common features of a LysR-type transcriptional regulator and contains an N-terminal helix-turn-helix motif as well as a C-terminal LysR substrate-binding domain. Deletion of crgA results in an enhanced swim ring and increased transcription of flhDC. In addition, the ΔcrgA mutant possesses more polar flagella than wild-type GMI1000 and exhibits higher expression of the flagellin gene fliC. Despite these alterations, the ΔcrgA mutant did not have a detectable growth defect in culture. Yeast one-hybrid and electrophoretic mobility shift assays revealed that CrgA interacts directly with the flhDC promoter. Expressing the β-glucuronidase (GUS) reporter under the control of the crgA promoter showed that crgA transcription is dependent on cell density. Soil-soaking inoculation with the crgA mutant caused wilt symptoms on tomato (Solanum lycopersicum L. cv. Hong yangli) plants earlier than inoculation with the wild-type GMI1000 but resulted in lower disease severity. We conclude that the R. solanacearum regulator CrgA represses flhDC expression and consequently affects the expression of fliC to modulate cell motility, thereby conditioning disease development in host plants. IMPORTANCE Ralstonia solanacearum is a widely distributed soilborne plant pathogen that causes bacterial wilt disease on diverse plant species. Motility is a critical virulence attribute of R. solanacearum because it allows this pathogen to efficiently invade and colonize host plants. In R. solanacearum, motility-defective strains are markedly affected in pathogenicity, which is coregulated with multiple virulence factors. In this study, we identified a new LysR-type transcriptional regulator (LTTR), CrgA, that negatively regulates motility. The mutation of the corresponding gene leads to the precocious appearance of wilt symptoms on tomato plants when the pathogen is introduced using soil-soaking inoculation. This study indicates that the regulation of R. solanacearum motility is more complex than previously thought and enhances our understanding of flagellum regulation in R. solanacearum.

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Production of Manganese Oxide Nanoparticles by Shewanella Species

Applied and Environmental Microbiology ◽

10.1128/aem.00663-16 ◽

2016 ◽

Vol 82 (17) ◽

pp. 5402-5409 ◽

Cited By ~ 23

Author(s):

Mitchell H. Wright ◽

Saad M. Farooqui ◽

Alan R. White ◽

Anthony C. Greene

Keyword(s):

Manganese Oxide ◽

Manganese Oxides ◽

Shewanella Oneidensis ◽

Culture Conditions ◽

Natural Environments ◽

Aerobic Conditions ◽

Content Type ◽

Shewanella Species ◽

Oxide Particles ◽

Reducing Bacteria

ABSTRACTSeveral species of the bacterial genusShewanellaare well-known dissimilatory reducers of manganese under anaerobic conditions. In fact,Shewanella oneidensisis one of the most well studied of all metal-reducing bacteria. In the current study, a number ofShewanellastrains were tested for manganese-oxidizing capacity under aerobic conditions. All were able to oxidize Mn(II) and to produce solid dark brown manganese oxides.Shewanellaloihicastrain PV-4 was the strongest oxidizer, producing oxides at a rate of 20.3 mg/liter/day and oxidizing Mn(II) concentrations of up to 9 mM. In contrast,S. oneidensisMR-1 was the weakest oxidizer tested, producing oxides at 4.4 mg/liter/day and oxidizing up to 4 mM Mn(II). Analysis of products from the strongest oxidizers, i.e.,S.loihicaPV-4 andShewanella putrefaciensCN-32, revealed finely grained, nanosize, poorly crystalline oxide particles with identical Mn oxidation states of 3.86. The biogenic manganese oxide products could be subsequently reduced within 2 days by all of theShewanellastrains when culture conditions were made anoxic and an appropriate nutrient (lactate) was added. WhileShewanellaspecies were detected previously as part of manganese-oxidizing consortia in natural environments, the current study has clearly shown manganese-reducingShewanellaspecies bacteria that are able to oxidize manganese in aerobic cultures.IMPORTANCEMembers of the genusShewanellaare well known as dissimilatory manganese-reducing bacteria. This study shows that a number of species fromShewanellaare also capable of manganese oxidation under aerobic conditions. Characterization of the products of the two most efficient oxidizers,S. loihicaandS. putrefaciens, revealed finely grained, nanosize oxide particles. With a change in culture conditions, the manganese oxide products could be subsequently reduced by the same bacteria. The ability ofShewanellaspecies both to oxidize and to reduce manganese indicates that the genus plays a significant role in the geochemical cycling of manganese. Due to the high affinity of manganese oxides for binding other metals, these bacteria may also contribute to the immobilization and release of other metals in the environment.

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