A novel cytochrome c peroxidase from Neisseria gonorrhoeae: a lipoprotein from a Gram-negative bacterium

A systematic review on chlorine tolerance among bacteria and standardization of their assessment protocol in wastewater

Though chlorine is a cost-effective disinfectant for water and wastewaters, the bacteria surviving after chlorination pose serious public health and environmental problems. This review critically assesses the mechanism of chlorine disinfection as described by various researchers; factors affecting chlorination efficacy; and the re-growth potential of microbial contaminations in treated wastewater post chlorination to arrive at meaningful doses for ensuring health safety. Literature analysis shows procedural inconsistencies in the assessment of chlorine tolerant bacteria, making it extremely difficult to compare the tolerance characteristics of different reported tolerant bacteria. A comparison of logarithmic reduction after chlorination and the concentration-time values for prominent pathogens led to the generation of a standard protocol for the assessment of chlorine tolerance. The factors that need to be critically monitored include applied chlorine doses, contact time, determination of chlorine demands of the medium, and the consideration of bacterial counts immediately after chlorination and in post chlorinated samples (regrowth). The protocol devised here appropriately assesses the chlorine-tolerant bacteria and urges the scientific community to report the regrowth characteristics as well. This would increase the confidence in data interpretation that can provide a better understanding of chlorine tolerance in bacteria and aid in formulating strategies for effective chlorination.

Transcriptional regulation of a gonococcal gene encoding a virulence factor (L-lactate permease)

GdhR is a GntR-type regulator of Neisseria gonorrhoeae encoded by a gene (gdhR) belonging to the MtrR regulon, which comprises multiple genes required for antibiotic resistance such as the mtrCDE efflux pump genes. In previous work we showed that loss of gdhR results in enhanced gonococcal fitness in a female mouse model of lower genital tract infection. Here, we used RNA-Seq to perform a transcriptional profiling study to determine the GdhR regulon. GdhR was found to regulate the expression of 2.3% of all the genes in gonococcal strain FA19, of which 39 were activated and 11 were repressed. Within the GdhR regulon we found that lctP, which encodes a unique L-lactate transporter and has been associated with gonococcal pathogenesis, was the highest of GdhR-repressed genes. By using in vitro transcription and DNase I footpriting assays we mapped the lctP transcriptional start site (TSS) and determined that GdhR directly inhibits transcription by binding to an inverted repeat sequence located 9 bases downstream of the lctP TSS. Epistasis analysis revealed that, while loss of lctP increased susceptibility of gonococci to hydrogen peroxide (H2O2) the loss of gdhR enhanced resistance; however, this GdhR-endowed property was reversed in a double gdhR lctP null mutant. We assessed the effect of different carbon sources on lctP expression and found that D-glucose, but not L-lactate or pyruvate, repressed lctP expression within a physiological concentration range but in a GdhR-independent manner. Moreover, we found that adding glucose to the medium enhanced susceptibility of gonococci to hydrogen peroxide. We propose a model for the role of lctP regulation via GdhR and glucose in the pathogenesis of N. gonorrhoeae.

Reduction of hydrogen peroxide in gram-negative bacteria - bacterial peroxidases

Bacteria display an array of enzymes to detoxify reactive oxygen species that cause damage to DNA and to other biomolecules leading to cell death. Hydrogen peroxide is one of these species, with endogenous and exogenous sources, such as lactic acid bacteria, oxidative burst of the immune system or chemical reactions at oxic-anoxic interfaces. The enzymes that detoxify hydrogen peroxide will be the focus of this review, with special emphasis on bacterial peroxidases that reduce hydrogen peroxide to water. Bacterial peroxidases are periplasmic cytochromes with either two or three c-type haems, which have been classified as classical and non-classical bacterial peroxidases, respectively. Most of the studies have been focus on the classical bacterial peroxidases, showing the presence of a reductive activation in the presence of calcium ions. Mutagenesis studies have clarified the catalytic mechanism of this enzyme and were used to propose an intramolecular electron transfer pathway, with far less being known about the intermolecular electron transfer that occurs between reduced electron donors and the enzyme. The physiological function of these enzymes was not very clear until it was shown, for the non-classical bacterial peroxidase, that this enzyme is required for the bacteria to use hydrogen peroxide as terminal electron acceptor under anoxic conditions. These non-classical bacterial peroxidases are quinol peroxidases that do not require reductive activation but need calcium ions to attain maximum activity and share similar catalytic intermediates with the classical bacterial peroxidases.

Neisseria gonorrhoeae Exposed to Sublethal Levels of Hydrogen Peroxide Mounts a Complex Transcriptional Response

Neisseria gonorrhoeae mounts a substantial transcriptional program in response to hydrogen peroxide (HP), a prominent reactive oxygen species (ROS) encountered during infection. We tested which strain FA1090 genes show differential transcript abundance in response to sublethal amounts of HP to differentiate HP-responsive signaling from widespread cellular death and dysregulation. RNA sequencing (RNA-Seq) revealed that 150 genes were significantly upregulated and 143 genes downregulated following HP exposure. We annotated HP-responsive operons and all transcriptional start sites (TSSs) and identified which TSSs responded to HP treatment. We compared the HP responses and other previously reported genes and found only partial overlapping of other regulatory networks, indicating that the response to HP involves multiple biological functions. Using a representative subset of responsive genes, we validated the RNA-Seq results and found that the HP transcriptome was similar to that of sublethal organic peroxide. None of the genes in the representative subset, however, responded to sublethal levels of HOCl or O2 -. These results support the idea that N. gonorrhoeae may use variations in HP levels as a signal for different stages of infection. IMPORTANCE The strict human pathogen Neisseria gonorrhoeae is the only causative agent of the sexually transmitted disease gonorrhea. This bacterium encounters hydrogen peroxide produced from host cells during infection, but the organism survives in the presence of this antimicrobial agent. This work shows that the bacterium responds to hydrogen peroxide by regulating the expression of many genes involved in multiple processes.

Interaction between Neisseria gonorrhoeae bacterial peroxidase and its electron donor, the lipid-modified azurin

The Neisseria gonorrhoeae bacterial cytochrome c peroxidase plays a key role in detoxifying the cells from H₂ O₂ by reducing it to water using the lipid-modified azurin, LAz, a small type 1 copper protein, as electron donor. Here, the interaction between these two proteins was characterized by steady-state kinetics, two-dimensional NMR and molecular docking simulations. LAz is an efficient electron donor capable of activating this enzyme. This electron transfer complex is weak with a hydrophobic character, with LAz binding close to the electron transferring heme of the enzyme. The high catalytic rate (39 ± 0.03 s^-1 ) is explained by the LAz pre-orientation, due to a positive dipole moment, and by the fast-dynamic ensemble of orientations, suggested by the small chemical shifts.

YhjA - An Escherichia coli trihemic enzyme with quinol peroxidase activity

The trihemic bacterial cytochrome c peroxidase from Escherichia coli, YhjA, is a membrane-anchored protein with a C-terminal domain homologous to the classical bacterial peroxidases and an additional N-terminal (NT) heme binding domain. Recombinant YhjA is a 50 kDa monomer in solution with three c-type hemes covalently bound. Here is reported the first biochemical and spectroscopic characterization of YhjA and of the NT domain demonstrating that NT heme is His63/Met125 coordinated. The reduction potentials of P (active site), NT and E hemes were established to be -170 mV, +133 mV and +210 mV, respectively, at pH 7.5. YhjA has quinol peroxidase activity in vitro with optimum activity at pH 7.0 and millimolar range K_M values using hydroquinone and menadiol (a menaquinol analogue) as electron donors (K_M = 0.6 ± 0.2 and 1.8 ± 0.5 mM H₂O₂, respectively), with similar turnover numbers (k_cat = 19 ± 2 and 13 ± 2 s^-1, respectively). YhjA does not require reductive activation for maximum activity, in opposition to classical bacterial peroxidases, as P heme is always high-spin 6-coordinated with a water-derived molecule as distal axial ligand but shares the need for the presence of calcium ions in the kinetic assays. Formation of a ferryl Fe(IV) = O species was observed upon incubation of fully oxidized YhjA with H₂O₂. The data reported improve our understanding of the biochemical properties and catalytic mechanism of YhjA, a three-heme peroxidase that uses the quinol pool to defend the cells against hydrogen peroxide during transient exposure to oxygenated environments.

Peptide Inhibitors Targeting the Neisseria gonorrhoeae Pivotal Anaerobic Respiration Factor AniA

Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, which is highly prevalent worldwide and has a major impact on reproductive and neonatal health. The superbug status of N. gonorrhoeae necessitates the development of drugs with different mechanisms of action. Here, we focused on targeting the nitrite reductase AniA, which is a pivotal component of N. gonorrhoeae anaerobic respiration and biofilm formation. Our studies showed that gonococci expressing AniA containing the altered catalytic residues D137A and H280A failed to grow under anaerobic conditions, demonstrating that the nitrite reductase function is essential. To facilitate the pharmacological targeting of AniA, new crystal structures of AniA were refined to 1.90-Å and 2.35-Å resolutions, and a phage display approach with libraries expressing randomized linear dodecameric peptides or heptameric peptides flanked by a pair of cysteine residues was utilized. Biopanning experiments led to the identification of 29 unique peptides, with 1 of them, C7-3, being identified multiple times. Evaluation of their ability to interact with AniA using enzyme-linked immunosorbent assay and computational docking studies revealed that C7-3 was the most promising inhibitor, binding near the type 2 copper site of the enzyme, which is responsible for interaction with nitrite. Subsequent enzymatic assays and biolayer interferometry with a synthetic C7-3 and its derivatives, C7-3m1 and C7-3m2, demonstrated potent inhibition of AniA. Finally, the MIC₅₀ value of C7-3 and C7-3m2 against anaerobically grown N. gonorrhoeae was 0.6 mM. We present the first peptide inhibitors of AniA, an enzyme that should be further exploited for antigonococcal drug development.

Escherichia coli cytochrome c peroxidase is a respiratory oxidase that enables the use of hydrogen peroxide as a terminal electron acceptor

Microbial cytochrome c peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Escherichia coli Intriguingly, ccp transcription requires both the presence of H₂O₂ and the absence of O₂ Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous H₂O₂ However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors. Indeed, Ccp enabled E. coli to grow on a nonfermentable carbon source when H₂O₂ was supplied. Salmonella behaved similarly. This role rationalizes ccp repression in oxic environments. We speculate that micromolar H₂O₂ is created both biologically and abiotically at natural oxic/anoxic interfaces. The OxyR response appears to exploit this H₂O₂ as a terminal oxidant while simultaneously defending the cell against its toxicity.

Biochemical characterization of the bacterial peroxidase from the human pathogen Neisseria gonorrhoeae

Neisseria gonorrhoeae is an obligate human pathogen that expresses an array of molecular systems to detoxify reactive oxygen species as defense mechanisms during colonization and infection. One of these is the bacterial peroxidase that reduces H₂O₂ to water in its periplasm. The soluble form of this enzyme was heterologously expressed in E. coli in the holo-form binding two c-types hemes, a high-potential E heme and a low-potential P heme, with redox potentials of (+310mV) and (-190mV/-300mV), respectively in the presence of calcium ions, at pH7.5. Visible and EPR spectroscopic analysis together with activity assays indicate the presence of a calcium dependent reductive activation mechanism in thgonorrhoeaeNeisseria gonorrhoeae bacterial peroxidase, in which P heme is bis-His coordinated low-spin in the fully oxidized state of the enzyme, and becomes penta-coordinated high-spin upon reduction of E heme in the presence of calcium ions. The activated enzyme has a high affinity for H₂O₂ (K_M of 4±1μM), with maximum activity being attained at pH7.0 and 37°C, with the rate-limiting step in the catalytic cycle being the electron transfer between the two hemes. In this enzyme, dimer formation is not promoted at high ionic strength, thus differing from the classical bacterial peroxidases. These results contribute to the understanding of the involvement of Neisseria gonorrhoeae bacterial peroxidase has a first line defense mechanism against exogenously produced hydrogen peroxide in the host environment.

Cytochrome c-based domain modularity governs genus-level diversification of electron transfer to dissimilatory nitrite reduction

The genus Neisseria contains two pathogenic species (N. meningitidis and N. gonorrhoeae) in addition to a number of commensal species that primarily colonize mucosal surfaces in man. Within the genus, there is considerable diversity and apparent redundancy in the components involved in respiration. Here, we identify a unique c-type cytochrome (cN ) that is broadly distributed among commensal Neisseria, but absent in the pathogenic species. Specifically, cN supports nitrite reduction in N. gonorrhoeae strains lacking the cytochromes c5 and CcoP established to be critical to NirK nitrite reductase activity. The c-type cytochrome domain of cN shares high sequence identity with those localized c-terminally in c5 and CcoP and all three domains were shown to donate electrons directly to NirK. Thus, we identify three distinct but paralogous proteins that donate electrons to NirK. We also demonstrate functionality for a N. weaverii NirK variant with a C-terminal c-type heme extension. Taken together, modular domain distribution and gene rearrangement events related to these respiratory electron carriers within Neisseria are concordant with major transitions in the macroevolutionary history of the genus. This work emphasizes the importance of denitrification as a selectable trait that may influence speciation and adaptive diversification within this largely host-restricted bacterial genus.

Overproduction of the MtrCDE efflux pump in Neisseria gonorrhoeae produces unexpected changes in cellular transcription patterns

The global consequence of drug efflux gene overexpression in bacteria has not been specifically analyzed because strains showing high-level expression typically have mutations in genes encoding regulatory proteins that control other genes. Results from a transcriptional profiling study performed with a strain of Neisseria gonorrhoeae that is capable of high-level transcription of the mtrCDE efflux pump operon independently of control by cognate regulatory proteins revealed that its overexpression has ramifications for systems other than drug efflux.

Neisseria gonorrhoeae metalloprotease NGO1686 is required for full piliation, and piliation is required for resistance to H2O2- and neutrophil-mediated killing

The sexually transmitted infection gonorrhea is caused exclusively by the human-specific pathogen Neisseria gonorrhoeae. Type IV pili are an essential virulence factor uniformly expressed on clinical gonococcal isolates and are required for several aspects of gonococcal pathogenesis, including adherence to host tissues, autoagglutination, twitching motility, and the uptake of DNA during transformation. Symptomatic gonococcal infection is characterized by the influx of neutrophils or polymorphonuclear leukocytes (PMNs) to the site of infection. PMNs are a key component of gonococcal pathogenesis, mediating the innate immune response through the use of oxidative and nonoxidative killing mechanisms. The M23B family zinc metallopeptidase NGO1686 is required for gonococci to survive oxidative killing by H2O2- and PMN-mediated killing through unknown mechanisms, but the only known target of NGO1686 is peptidoglycan. We report that the effect of NGO1686 on survival after exposure to H2O2 and PMNs is mediated through its role in elaborating pili and that nonpiliated mutants of N. gonorrhoeae are less resistant to killing by H2O2, LL-37, and PMNs than the corresponding piliated strains. These findings add to the various virulence-associated functions attributable to gonococcal pili and may explain the selection basis for piliation in clinical isolates of N. gonorrhoeae.

IMPORTANCE

Successful infectious agents need to overcome host defense systems to establish infection. We show that the Neisseria pilus, a major virulence factor of this organism, which causes gonorrhea, helps protect the bacterium from two major killing mechanisms used by the host to combat infections. We also show that to express the pilus, an enzyme needs to partially degrade the cell wall of the bacterium.

A critical role for the cccA gene product, cytochrome c2, in diverting electrons from aerobic respiration to denitrification in Neisseria gonorrhoeae

Neisseria gonorrhoeae is a microaerophile that, when oxygen availability is limited, supplements aerobic respiration with a truncated denitrification pathway, nitrite reduction to nitrous oxide. We demonstrate that the cccA gene of Neisseria gonorrhoeae strain F62 (accession number NG0292) is expressed, but the product, cytochrome c2, accumulates to only low levels. Nevertheless, a cccA mutant reduced nitrite at about half the rate of the parent strain. We previously reported that cytochromes c4 and c5 transfer electrons to cytochrome oxidase cbb3 by two independent pathways and that the CcoP subunit of cytochrome oxidase cbb3 transfers electrons to nitrite. We show that mutants defective in either cytochrome c4 or c5 also reduce nitrite more slowly than the parent. By combining mutations in cccA (Δc2), cycA (Δc4), cycB (Δc5), and ccoP (ccoP-C368A), we demonstrate that cytochrome c2 is required for electron transfer from cytochrome c4 via the third heme group of CcoP to the nitrite reductase, AniA, and that cytochrome c5 transfers electrons to nitrite reductase by an independent pathway. We propose that cytochrome c2 forms a complex with cytochrome oxidase. If so, the redox state of cytochrome c2 might regulate electron transfer to nitrite or oxygen. However, our data are more consistent with a mechanism in which cytochrome c2 and the CcoQ subunit of cytochrome oxidase form alternative complexes that preferentially catalyze nitrite and oxygen reduction, respectively. Comparison with the much simpler electron transfer pathway for nitrite reduction in the meningococcus provides fascinating insights into niche adaptation within the pathogenic neisseriae.

Why do bacteria use so many enzymes to scavenge hydrogen peroxide?

Hydrogen peroxide (H(2)O(2)) is continuously formed by the autoxidation of redox enzymes in aerobic cells, and it also enters from the environment, where it can be generated both by chemical processes and by the deliberate actions of competing organisms. Because H(2)O(2) is acutely toxic, bacteria elaborate scavenging enzymes to keep its intracellular concentration at nanomolar levels. Mutants that lack such enzymes grow poorly, suffer from high rates of mutagenesis, or even die. In order to understand how bacteria cope with oxidative stress, it is important to identify the key enzymes involved in H(2)O(2) degradation. Catalases and NADH peroxidase (Ahp) are primary scavengers in many bacteria, and their activities and physiological impacts have been unambiguously demonstrated through phenotypic analysis and through direct measurements of H(2)O(2) clearance in vivo. Yet a wide variety of additional enzymes have been proposed to serve similar roles: thiol peroxidase, bacterioferritin comigratory protein, glutathione peroxidase, cytochrome c peroxidase, and rubrerythrins. Each of these enzymes can degrade H(2)O(2) in vitro, but their contributions in vivo remain unclear. In this review we examine the genetic, genomic, regulatory, and biochemical evidence that each of these is a bonafide scavenger of H(2)O(2) in the cell. We also consider possible reasons that bacteria might require multiple enzymes to catalyze this process, including differences in substrate specificity, compartmentalization, cofactor requirements, kinetic optima, and enzyme stability. It is hoped that the resolution of these issues will lead to an understanding of stress resistance that is more accurate and perceptive.

Transcriptional portrait of Actinobacillus pleuropneumoniae during acute disease--potential strategies for survival and persistence in the host

Background

Gene expression profiles of bacteria in their natural hosts can provide novel insight into the host-pathogen interactions and molecular determinants of bacterial infections. In the present study, the transcriptional profile of the porcine lung pathogen Actinobacillus pleuropneumoniae was monitored during the acute phase of infection in its natural host.

Methodology/Principal Findings

Bacterial expression profiles of A. pleuropneumoniae isolated from lung lesions of 25 infected pigs were compared in samples taken 6, 12, 24 and 48 hours post experimental challenge. Within 6 hours, focal, fibrino hemorrhagic lesions could be observed in the pig lungs, indicating that A. pleuropneumoniae had managed to establish itself successfully in the host. We identified 237 differentially regulated genes likely to encode functions required by the bacteria for colonization and survival in the host. This group was dominated by genes involved in various aspects of energy metabolism, especially anaerobic respiration and carbohydrate metabolism. Remodeling of the bacterial envelope and modifications of posttranslational processing of proteins also appeared to be of importance during early infection. The results suggested that A. pleuropneumoniae is using various strategies to increase its fitness, such as applying Na+ pumps as an alternative way of gaining energy. Furthermore, the transcriptional data provided potential clues as to how A. pleuropneumoniae is able to circumvent host immune factors and survive within the hostile environment of host macrophages. This persistence within macrophages may be related to urease activity, mobilization of various stress responses and active evasion of the host defenses by cell surface sialylation.

Conclusions/Significance

The data presented here highlight the importance of metabolic adjustments to host conditions as virulence factors of infecting microorganisms and help to provide insight into the mechanisms behind the efficient colonization and persistence of A. pleuropneumoniae during acute disease.

Legless pathogens: how bacterial physiology provides the key to understanding pathogenicity

This review argues that knowledge of microbial physiology and metabolism is a prerequisite to understanding mechanisms of pathogenicity. The ability of Neisseria gonorrhoeae to cope with stresses such as those found during infection requires a sialyltransferase to sialylate its lipopolysaccharide using host-derived CMP-NANA in the human bloodstream, the ability to oxidize lactate that is abundant in the human body, outer-membrane lipoproteins that provide the first line of protection against oxidative and nitrosative stress, regulation of NO reduction independently from the nitrite reductase that forms NO, an extra haem group on the C-terminal extension of a cytochrome oxidase subunit, and a respiratory capacity far in excess of metabolic requirements. These properties are all normal components of neisserial physiology; they would all fail rigid definitions of a pathogenicity determinant. In anaerobic cultures of enteric bacteria, duplicate pathways for nitrate reduction to ammonia provide a selective advantage when nitrate is either abundant or scarce. Selection of these alternative pathways is in part regulated by two parallel two-component regulatory systems. NarX-NarL primarily ensures that nitrate is reduced in preference to thermodynamically less favourable terminal electron acceptors, but NarQ-NarP facilitates reduction of limited quantities of nitrate or other, less favourable, terminal electron acceptors in preference to fermentative growth. How enteric bacteria repair damage caused by nitrosative and oxidative damage inflicted by host defences is less well understood. In both N. gonorrhoeae and Escherichia coli, parallel pathways that duplicate particular biochemical functions are far from redundant, but fulfil specific physiological roles.

Tied down: tethering redox proteins to the outer membrane in Neisseria and other genera

Typically, the redox proteins of respiratory chains in Gram-negative bacteria are localized in the cytoplasmic membrane or in the periplasm. An alternative arrangement appears to be widespread within the betaproteobacterial genus Neisseria, wherein several redox proteins are covalently associated with the outer membrane. In the present paper, we discuss the structural properties of these outer membrane redox proteins and the functional consequences of this attachment. Several tethered outer membrane redox proteins of Neisseria contain a weakly conserved repeated structure between the covalent tether and the redox protein globular domain that should enable the redox cofactor-containing domain to extend from the outer membrane, across the periplasm and towards the inner membrane. It is argued that the constraints imposed on the movement and orientation of the globular domains by these tethers favours the formation of electron-transfer complexes for entropic reasons. The attachment to the outer membrane may also affect the exposure of the host to redox proteins with a moonlighting function in the host-microbe interaction, thus affecting the host response to Neisseria infection. We identify putative outer membrane redox proteins from a number of other bacterial genera outside Neisseria, and suggest that this organizational arrangement may be more common than previously recognized.

Neisseria gonorrhoeae virulence factor NG1686 is a bifunctional M23B family metallopeptidase that influences resistance to hydrogen peroxide and colony morphology

Symptomatic gonococcal infection, caused exclusively by the human-specific pathogen Neisseria gonorrhoeae (the gonococcus), is characterized by the influx of polymorphonuclear leukocytes (PMNs) to the site of infection. Although PMNs possess a potent antimicrobial arsenal comprising both oxidative and non-oxidative killing mechanisms, gonococci survive this interaction, suggesting that the gonococcus has evolved many defenses against PMN killing. We previously identified the NG1686 protein as a gonococcal virulence factor that protects against both non-oxidative PMN-mediated killing and oxidative killing by hydrogen peroxide. In this work, we show that deletion of ng1686 affects gonococcal colony morphology but not cell morphology and that overexpression of ng1686 does not confer enhanced survival to hydrogen peroxide on gonococci. NG1686 contains M23B endopeptidase active sites found in proteins that cleave bacterial cell wall peptidoglycan. Strains of N. gonorrhoeae expressing mutant NG1686 proteins with substitutions in many, but not all, conserved metallopeptidase active sites recapitulated the hydrogen peroxide sensitivity and altered colony morphology of the Δng1686 mutant strain. We showed that purified NG1686 protein degrades peptidoglycan in vitro and that mutations in many conserved active site residues abolished its degradative activity. Finally, we demonstrated that NG1686 possesses both dd-carboxypeptidase and endopeptidase activities. We conclude that the NG1686 protein is a M23B peptidase with dual activities that targets the cell wall to affect colony morphology and resistance to hydrogen peroxide and PMN-mediated killing.

Investigation of the electron transport chain to and the catalytic activity of the diheme cytochrome c peroxidase CcpA of Shewanella oneidensis

Bacterial diheme c-type cytochrome peroxidases (BCCPs) catalyze the periplasmic reduction of hydrogen peroxide to water. The gammaproteobacterium Shewanella oneidensis produces the peroxidase CcpA under a number of anaerobic conditions, including dissimilatory iron-reducing conditions. We wanted to understand the function of this protein in the organism and its putative connection to the electron transport chain to ferric iron. CcpA was isolated and tested for peroxidase activity, and its structural conformation was analyzed by X-ray crystallography. CcpA exhibited in vitro peroxidase activity and had a structure typical of diheme peroxidases. It was produced in almost equal amounts under anaerobic and microaerophilic conditions. With 50 mM ferric citrate and 50 μM oxygen in the growth medium, CcpA expression results in a strong selective advantage for the cell, which was detected in competitive growth experiments with wild-type and ΔccpA mutant cells that lack the entire ccpA gene due to a markerless deletion. We were unable to reduce CcpA directly with CymA, MtrA, or FccA, which are known key players in the chain of electron transport to ferric iron and fumarate but identified the small monoheme ScyA as a mediator of electron transport between CymA and BCCP. To our knowledge, this is the first detailed description of a complete chain of electron transport to a periplasmic c-type cytochrome peroxidase. This study furthermore reports the possibility of establishing a specific electron transport chain using c-type cytochromes.

The Composition and Metabolic Phenotype of Neisseria gonorrhoeae Biofilms

Neisseria gonorrhoeae has been shown to form biofilms during cervical infection. Thus, biofilm formation may play an important role in the infection of women. The ability of N. gonorrhoeae to form membrane blebs is crucial to biofilm formation. Blebs contain DNA and outer membrane structures, which have been shown to be major constituents of the biofilm matrix. The organism expresses a DNA thermonuclease that is involved in remodeling of the biofilm matrix. Comparison of the transcriptional profiles of gonococcal biofilms and planktonic runoff indicate that genes involved in anaerobic metabolism and oxidative stress tolerance are more highly expressed in biofilm. The expression of aniA, ccp, and norB, which encode nitrite reductase, cytochrome c peroxidase, and nitric oxide reductase respectively, is required for mature biofilm formation over glass and human cervical cells. In addition, anaerobic respiration occurs in the substratum of gonococcal biofilms and disruption of the norB gene required for anaerobic respiration, results in a severe biofilm attenuation phenotype. It has been demonstrated that accumulation of nitric oxide (NO) contributes to the phenotype of a norB mutant and can retard biofilm formation. However, NO can also enhance biofilm formation, and this is largely dependent on the concentration and donation rate or steady-state kinetics of NO. The majority of the genes involved in gonococcal oxidative stress tolerance are also required for normal biofilm formation, as mutations in the following genes result in attenuated biofilm formation over cervical cells and/or glass: oxyR, gor, prx, mntABC, trxB, and estD. Overall, biofilm formation appears to be an adaptation for coping with the environmental stresses present in the female genitourinary tract. Therefore, this review will discuss the studies, which describe the composition and metabolic phenotype of gonococcal biofilms.

Resistance of Neisseria gonorrhoeae to neutrophils

Infection with the human-specific bacterial pathogen Neisseria gonorrhoeae triggers a potent, local inflammatory response driven by polymorphonuclear leukocytes (neutrophils or PMNs). PMNs are terminally differentiated phagocytic cells that are a vital component of the host innate immune response and are the first responders to bacterial and fungal infections. PMNs possess a diverse arsenal of components to combat microorganisms, including the production of reactive oxygen species and release of degradative enzymes and antimicrobial peptides. Despite numerous PMNs at the site of gonococcal infection, N. gonorrhoeae can be cultured from the PMN-rich exudates of individuals with acute gonorrhea, indicating that some bacteria resist killing by neutrophils. The contribution of PMNs to gonorrheal pathogenesis has been modeled in vivo by human male urethral challenge and murine female genital inoculation and in vitro using isolated primary PMNs or PMN-derived cell lines. These systems reveal that some gonococci survive and replicate within PMNs and suggest that gonococci defend themselves against PMNs in two ways: they express virulence factors that defend against PMNs' oxidative and non-oxidative antimicrobial components, and they modulate the ability of PMNs to phagocytose gonococci and to release antimicrobial components. In this review, we will highlight the varied and complementary approaches used by N. gonorrhoeae to resist clearance by human PMNs, with an emphasis on gonococcal gene products that modulate bacterial-PMN interactions. Understanding how some gonococci survive exposure to PMNs will help guide future initiatives for combating gonorrheal disease.

Organization of the electron transfer chain to oxygen in the obligate human pathogen Neisseria gonorrhoeae: roles for cytochromes c4 and c5, but not cytochrome c2, in oxygen reduction

Although Neisseria gonorrhoeae is a prolific source of eight c-type cytochromes, little is known about how its electron transfer pathways to oxygen are organized. In this study, the roles in the respiratory chain to oxygen of cytochromes c(2), c(4), and c(5), encoded by the genes cccA, cycA, and cycB, respectively, have been investigated. Single mutations in genes for either cytochrome c(4) or c(5) resulted in an increased sensitivity to growth inhibition by excess oxygen and small decreases in the respiratory capacity of the parent, which were complemented by the chromosomal integration of an ectopic, isopropyl-beta-d-thiogalactopyranoside (IPTG)-inducible copy of the cycA or cycB gene. In contrast, a cccA mutant reduced oxygen slightly more rapidly than the parent, suggesting that cccA is expressed but cytochrome c(2) is not involved in electron transfer to cytochrome oxidase. The deletion of cccA increased the sensitivity of the cycB mutant to excess oxygen but decreased the sensitivity of the cycA mutant. Despite many attempts, a double mutant defective in both cytochromes c(4) and c(5) could not be isolated. However, a strain with the ectopically encoded, IPTG-inducible cycB gene with deletions in both cycA and cycB was constructed: the growth and survival of this strain were dependent upon the addition of IPTG, so gonococcal survival is dependent upon the synthesis of either cytochrome c(4) or c(5). These results define the gonococcal electron transfer chain to oxygen in which cytochromes c(4) and c(5), but not cytochrome c(2), provide alternative pathways for electron transfer from the cytochrome bc(1) complex to the terminal oxidase cytochrome cbb(3).

Oxidative stress in Campylobacter jejuni: responses, resistance and regulation

Campylobacter jejuni is a major food-borne human pathogen that paradoxically is an oxygen-sensitive microaerophile, yet must resist the oxidative stresses encountered both in the host and in the environment. Recent studies suggest that, perhaps surprisingly, C. jejuni contains a wide range of enzymes involved in oxidative stress defense, and this review focuses on the properties and roles of these proteins. Although the mechanisms of gene regulation are still poorly understood in C. jejuni, several regulators of the oxidative stress response have been identified and their properties are discussed here. We suggest that future studies should be directed towards identifying the role of additional and less well characterized components involved in oxidative stress resistance, as well as providing a more complete picture of the underlying sensing and regulatory mechanisms.

A periplasmic thioredoxin-like protein plays a role in defense against oxidative stress in Neisseria gonorrhoeae

Thioredoxin-like proteins of the TlpA/ResE/CcmG subfamily are known to face the periplasm in gram-negative bacteria. Using the tlpA gene of Bradyrhizobium japonicum as a query, we identified a locus (NGO1923) in Neisseria gonorrhoeae that encodes a thioredoxin-like protein (NG_TlpA). Bioinformatics analysis indicated that the predicted NG_TlpA protein contained a cleavable signal peptide at the N terminus, and secondary structure analysis identified a thioredoxin fold with a helical insertion (approximately 25 residues), similar to that found in B. japonicum TlpA but absent in cytoplasmic thioredoxins. Biochemical characterization of a recombinant form of NG_TlpA revealed a standard redox potential (E0') of -206 mV. This property and the observation that the oxidized form of the protein exhibited greater thermal stability than the reduced species indicated that NG_TlpA is a reducing thioredoxin and not an oxidizing thiol-disulfide oxidoreductase like DsbA. The thioredoxin activity of NG_TlpA was confirmed in an insulin disulfide reduction assay. A tlpA mutant of N. gonorrhoeae strain 1291 was found to be highly sensitive to oxidative killing by paraquat and hydrogen peroxide, indicating an antioxidant role for the NG_TlpA in this bacterium. The tlpA mutant also exhibited reduced intracellular survival in human primary cervical epithelial cells.

Hydrogen peroxide-producing lactobacilli inhibit gonococci in vitro but not during experimental genital tract infection

Commensal lactobacilli that produce hydrogen peroxide (H(2)O(2)) inhibit Neisseria gonorrhoeae in vitro, and clinical data suggest that they are associated with a reduced risk of gonorrhea. We precolonized mice with Lactobacillus crispatus and then challenged them with N. gonorrhoeae, to measure the effects of H(2)O(2)-producing lactobacilli on gonococcal infection. We found no difference in the duration of infection or the number of gonococci recovered from untreated mice and mice colonized with L. crispatus. A gonococcal catalase mutant and a catalase, cytochrome C peroxidase mutant exhibited greater susceptibility to L. crispatus in vitro than did wild-type bacteria; however, recovery of these mutants from mice was not affected by L. crispatus. We also found no evidence that utilization of lactobacillus-produced lactate by N. gonorrhoeae balances the detrimental effects of H(2)O(2) during infection. We conclude that the association between lactobacilli and gonococci is complex and may be subject to factors that have not been reproduced in vitro.

Transcriptional profiling identifies the metabolic phenotype of gonococcal biofilms

Neisseria gonorrhoeae, the etiologic agent of gonorrhea, is frequently asymptomatic in women, often leading to chronic infections. One factor contributing to this may be biofilm formation. N. gonorrhoeae can form biofilms on glass and plastic surfaces. There is also evidence that biofilm formation may occur during natural cervical infection. To further study the mechanism of gonococcal biofilm formation, we compared transcriptional profiles of N. gonorrhoeae biofilms to planktonic profiles. Biofilm RNA was extracted from N. gonorrhoeae 1291 grown for 48 h in continuous-flow chambers over glass. Planktonic RNA was extracted from the biofilm runoff. In comparing biofilm with planktonic growth, 3.8% of the genome was differentially regulated. Genes that were highly upregulated in biofilms included aniA, norB, and ccp. These genes encode enzymes that are central to anaerobic respiratory metabolism and stress tolerance. Downregulated genes included members of the nuo gene cluster, which encodes the proton-translocating NADH dehydrogenase. Furthermore, it was observed that aniA, ccp, and norB insertional mutants were attenuated for biofilm formation on glass and transformed human cervical epithelial cells. These data suggest that biofilm formation by the gonococcus may represent a response that is linked to the control of nitric oxide steady-state levels during infection of cervical epithelial cells.

Resistance to peroxynitrite in Neisseria gonorrhoeae

Neisseria gonorrhoeae encodes a number of important genes that aid in survival during times of oxidative stress. The same immune cells capable of oxygen-dependent killing mechanisms also have the capacity to generate reactive nitrogen species (RNS) that may function antimicrobially. F62 and eight additional gonococcal strains displayed a high level of resistance to peroxynitrite, while Neisseria meningitidis and Escherichia coli showed a four- to seven-log and a four-log decrease in viability, respectively. Mutation of gonococcal orthologues that are known or suspected to be involved in RNS defence in other bacteria (ahpC, dnrN and msrA) resulted in no loss of viability, suggesting that N. gonorrhoeae has a novel mechanism of resistance to peroxynitrite. Whole-cell extracts of F62 prevented the oxidation of dihydrorhodamine, and decomposition of peroxynitrite was not dependent on ahpC, dnrN or msrA. F62 grown in co-culture with E. coli strain DH10B was shown to protect E. coli viability 10-fold. Also, peroxynitrite treatment of F62 did not result in accumulation of nitrated proteins, suggesting that an active peroxynitrite reductase is responsible for peroxynitrite decomposition rather than a protein sink for amino acid modification.

A strain-specific catalase mutation and mutation of the metal-binding transporter gene mntC attenuate Neisseria gonorrhoeae in vivo but not by increasing susceptibility to oxidative killing by phagocytes

The hallmark of gonorrhea is an intense inflammatory response that is characterized by polymorphonuclear leukocytes (PMNs) with intracellular gonococci. A redundancy of defenses may protect Neisseria gonorrhoeae from phagocyte-derived reactive oxygen species. Here we showed that a gonococcal catalase (kat) mutant in strain MS11 was more sensitive to H(2)O(2) than mutants in cytochrome c peroxidase (ccp), methionine sulfoxide reductase (msrA), or the metal-binding protein (mntC) of the MntABC transporter. kat ccp and kat ccp mntC mutants were significantly more sensitive to H(2)O(2) than mutants in any single factor. None of the mutants showed increased susceptibility to murine PMNs. Recovery of the mntC and kat ccp mntC mutants from the lower genital tract of BALB/c mice, but not the kat or kat ccp mutants, was significantly reduced relative to wild-type bacteria. Interestingly, unlike the MS11 kat mutant, a kat mutant of strain FA1090 was attenuated during competitive infection with wild-type FA1090 bacteria. The FA1090 kat mutant and MS11 mntC mutant were also attenuated in mice that are unable to generate a phagocytic respiratory burst. We conclude that inactivation of three well-characterized antioxidant genes (kat, ccp, and mntC) does not increase gonococcal susceptibility to the phagocytic respiratory burst during infection and that gonococcal catalase and the MntC protein confer an unidentified advantage in vivo. In the case of catalase, this advantage is strain specific. Finally, we also showed that an msrA mutant of strain MS11 demonstrated delayed attenuation in BALB/c but not C57BL/6 mice. Therefore, MsrA/B also appears to play a role in infection that is dependent on host genetic background.

Redox-linked structural changes associated with the formation of a catalytically competent form of the diheme cytochrome c peroxidase from Pseudomonas aeruginosa

A recombinant form of the prototypic diheme bacterial cytochrome c peroxidase (BCCP) from Pseudomonas aeruginosa (PsaCCP) has been expressed in Escherichia coli and purified to homogeneity. This material was used to carry out the first integrated biochemical, spectroscopic and structural investigation of the factors leading to reductive activation of this class of enzymes. A single, tightly bound, Ca2+ ion (K = 3 x 10(10) M-1) found at the domain interface of both the fully oxidized and mixed-valence forms of the enzyme is absolutely required for catalytic activity. Reduction of the electron-transferring (high-potential) heme in the presence of Ca2+ ions triggers substantial structural rearrangements around the active-site (low-potential) heme to allow substrate binding and catalysis. The enzyme also forms a mixed-valence state in the absence of Ca2+ ions, but a combination of electronic absorption, and EPR spectroscopies suggests that under these circumstances the low potential heme remains six-coordinate, unable to bind substrate and therefore catalytically inactive. Our observations strongly suggest that the two mixed-valence forms of native PsaCCP reported previously by Foote and colleagues (Foote, N., Peterson, J., Gadsby, P., Greenwood, C., and Thomson, A. (1985) Biochem. J. 230, 227-237) correspond to the Ca2+-loaded and -depleted forms of the enzyme.

Widespread distribution in pathogenic bacteria of di-iron proteins that repair oxidative and nitrosative damage to iron-sulfur centers

Expression of two genes of unknown function, Staphylococcus aureus scdA and Neisseria gonorrhoeae dnrN, is induced by exposure to oxidative or nitrosative stress. We show that DnrN and ScdA are di-iron proteins that protect their hosts from damage caused by exposure to nitric oxide and to hydrogen peroxide. Loss of FNR-dependent activation of aniA expression and NsrR-dependent repression of norB and dnrN expression on exposure to NO was restored in the gonococcal parent strain but not in a dnrN mutant, suggesting that DnrN is necessary for the repair of NO damage to the gonococcal transcription factors, FNR and NsrR. Restoration of aconitase activity destroyed by exposure of S. aureus to NO or H2O2 required a functional scdA gene. Electron paramagnetic resonance spectra of recombinant ScdA purified from Escherichia coli confirmed the presence of a di-iron center. The recombinant scdA plasmid, but not recombinant plasmids encoding the complete Escherichia coli sufABCDSE or iscRSUAhscBAfdx operons, complemented repair defects of an E. coli ytfE mutant. Analysis of the protein sequence database revealed the importance of the two proteins based on the widespread distribution of highly conserved homologues in both gram-positive and gram-negative bacteria that are human pathogens. We provide in vivo and in vitro evidence that Fe-S clusters damaged by exposure to NO and H2O2 can be repaired by this new protein family, for which we propose the name repair of iron centers, or RIC, proteins.

Delivery of nitric oxide for analysis of the function of cytochrome c'

On delivery of nitric oxide (NO) to protein samples (e.g., cytochrome c'), for spectroscopic experiments it is important to avoid exposure to oxygen and to remove contaminants from the NO gas. We describe a number of techniques for steady-state UV/Vis spectrophotometry and pre-steady-state stopped-flow spectrophotometry analysis of cytochrome c'.

Characterization of two putative cytochrome c peroxidases of Campylobacter jejuni involved in promoting commensal colonization of poultry

Campylobacter jejuni is a leading cause of bacterial gastroenteritis in humans throughout the world, but infection of animals, especially poultry, results in a commensal colonization of the intestines. We previously found that a mutant lacking docA, which encodes a putative cytochrome c peroxidase (CCP), demonstrates up to a 10(5)-fold reduction in colonization of the chick cecum compared to wild-type C. jejuni strain 81-176. Predictions from genomic sequences identified CJJ0382 as a second locus in C. jejuni encoding a CCP, making the bacterium unusual in having two putative CCPs. To understand what advantages are imparted by having two putative CCPs, we compared the colonization requirements of C. jejuni mutants lacking DocA or Cjj0382. Unlike the DeltadocA mutant, a DeltaCJJ0382 mutant demonstrates a maximal 50-fold colonization defect that is dependent on the inoculum dose. The colonization differences of mutants lacking DocA or Cjj0382 suggest that the two predicted CCPs are unlikely to perform redundant functions during in vivo growth. In the characterizations of DocA and Cjj0382, we found that they are stable periplasmic proteins with an apparent heme-dependent peroxidase activity, which are characteristics of bacterial CCPs. However, the peroxidase activities of the proteins do not appear to contribute to resistance to hydrogen peroxide. Instead, we found that resistance to hydrogen peroxide in C. jejuni is mostly attributed to the cytoplasmic catalase KatA. Our data suggest that DocA and Cjj0382 have characteristics of CCPs but likely perform different physiological functions for the bacterium in colonization that are not related to resisting oxidative stress.

The Escherichia coli yhjA gene, encoding a predicted cytochrome c peroxidase, is regulated by FNR and OxyR

The Escherichia coli FNR protein is an oxygen-responsive global transcription factor, and OxyR is a key regulator of the peroxide stress response. Here both FNR and OxyR are shown to regulate expression of the E. coli yhjA gene. The yhjA gene encodes a predicted cytochrome c peroxidase, a bacterial haem-containing protein involved in the peroxide stress response through its ability to convert hydrogen peroxide to water. It is shown that the yhjA gene of E. coli possesses a class II FNR site and an OxyR site upstream of the yhjA transcript start. Expression of yhjA was found to be dependent on this unusual combination of FNR and OxyR under conditions of oxygen starvation. Phenotypic analysis of the yhjA mutant revealed increased sensitivity to exogenous hydrogen peroxide and organic peroxides during growth under anaerobic conditions, consistent with the observed regulation and predicted function of the yhjA gene product.

Molecular characterization and subcellular localization of macrophage infectivity potentiator, a Chlamydia trachomatis lipoprotein

Macrophage infectivity potentiator (MIP) was originally reported to be a chlamydial lipoprotein from experiments showing incorporation of radiolabeled palmitic acid into native and recombinant MIP; inhibition of posttranslational processing of recombinant MIP by globomycin, known to inhibit signal peptidase II; and solubility of native MIP in Triton X-114. However, the detailed structural characterization of the lipid moiety on MIP has never been fully elucidated. In this study, bioinformatics and mass spectrometry analysis, as well as radiolabeling and immunochemical experiments, were conducted to further characterize MIP structure and subcellular localization. In silico analysis showed that the amino acid sequence of MIP is conserved across chlamydial species. A potential signal sequence with a contained lipobox was identified, and a recombinant C20A variant was prepared by replacing the probable lipobox cysteine with an alanine. Both incorporation of U-(14)C-esterified glycerol and [U-(14)C]palmitic acid and posttranslational processing that was inhibitable by globomycin were observed for recombinant wild-type MIP but not for the recombinant C20A MIP variant. The fatty acid contents of native and recombinant MIP were analyzed by gas chromatography-mass spectrometry, and the presence of amide-linked fatty acids in recombinant MIP was investigated by alkaline methanolysis. These results demonstrated a lipid modification in MIP similar to that of other prokaryotic lipoproteins. In addition, MIP was detected in an outer membrane preparation of Chlamydia trachomatis elementary bodies and was shown to be present at the surfaces of elementary bodies by surface biotinylation and surface immunoprecipitation experiments.

Neisseria gonorrhoeae catalase is not required for experimental genital tract infection despite the induction of a localized neutrophil response

Neisseria gonorrhoeae produces several antioxidant defenses, including high levels of catalase, which may facilitate the persistence during an inflammatory response via neutralization of H2O2 produced by phagocytes. In vivo testing of the role of catalase in gonococcal survival is critical since several physiological factors impact interactions between N. gonorrhoeae and polymorphonuclear leukocytes (PMNs). Here we assessed the importance of gonococcal catalase in a surrogate model of female genital tract infection. Female BALB/c mice were treated with 17-beta estradiol to promote susceptibility to N. gonorrhoeae and inoculated intravaginally with wild-type gonococci or a catalase (kat) deletion mutant. A localized PMN influx occurred in an average of 43 and 81% of mice infected with wild-type or kat mutant gonococci, respectively, and PMNs associated with numerous wild-type or catalase-deficient bacteria were observed in vaginal smears. The combined results of six experiments showed a significant difference in the number of days wild-type bacteria were recovered compared to the catalase-deficient gonococci. However, there was much variability between experiments, and we found no correlation between PMN influx, colonization load, and clearance of wild-type or kat mutant bacteria. Estradiol treatment did not impair bacterial uptake, the luminol-dependent chemiluminescence response, or the killing capacity of isolated murine PMNs against N. gonorrhoeae or Staphylococcus aureus. Our data suggest N. gonorrhoeae is not significantly challenged by H2O2 produced by PMNs in the murine lower genital tract; alternatively, redundant defense mechanisms may protect the gonococcus from reactive oxygen species during infection.

The small FNR regulon of Neisseria gonorrhoeae: comparison with the larger Escherichia coli FNR regulon and interaction with the NarQ-NarP regulon

BACKGROUND

Neisseria gonorrhoeae can survive during oxygen starvation by reducing nitrite to nitrous oxide catalysed by the nitrite and nitric oxide reductases, AniA and NorB. The oxygen-sensing transcription factor, FNR, is essential for transcription activation at the aniA promoter, and full activation also requires the two-component regulatory system, NarQ-NarP, and the presence of nitrite. The only other gene known to be activated by the gonococcal FNR is ccp encoding a cytochrome c peroxidase, and no FNR-repressed genes have been reported in the gonococcus. In contrast, FNR acts as both an activator and repressor involved in the control of more than 100 operons in E. coli regulating major changes in the adaptation from aerobic to anaerobic conditions. In this study we have performed a microarray-led investigation of the FNR-mediated responses in N. gonorrhoeae to determine the physiological similarities and differences in the role of FNR in cellular regulation in this species.

RESULTS

Microarray experiments show that N. gonorrhoeae FNR controls a much smaller regulon than its E. coli counterpart; it activates transcription of aniA and thirteen other genes, and represses transcription of six genes that include dnrN and norB. Having previously shown that a single amino acid substitution is sufficient to enable the gonococcal FNR to complement an E. coli fnr mutation, we investigated whether the gonococcal NarQ-NarP can substitute for E. coli NarX-NarL or NarQ-NarP. A plasmid expressing gonococcal narQ-narP was unable to complement E. coli narQP or narXL mutants, and was insensitive to nitrate or nitrite. Mutations that progressively changed the periplasmic nitrate sensing region, the P box, of E. coli NarQ to the sequence of the corresponding region of gonococcal NarQ resulted in loss of transcription activation in response to the availability of either nitrate or nitrite. However, the previously reported ligand-insensitive ability of gonococcal NarQ, the "locked on" phenotype, to activate either E. coli NarL or NarP was confirmed.

CONCLUSION

Despite the sequence similarities between transcription activators of E. coli and N. gonorrhoeae, these results emphasise the fundamental differences in transcription regulation between these two types of pathogenic bacteria.

Expression and characterisation of a major c-type cytochrome encoded by gene kustc0563 from Kuenenia stuttgartiensis as a recombinant protein in Escherichia coli

The purification of small quantities of a major small c-type cytochrome from the anammox bacterium Kuenenia stuttgartiensis has recently been reported. In order to characterise this protein further we have expressed the gene encoding this cytochrome in Escherichia coli and have purified the protein to homogeneity. The protein is directed to the E. coli periplasm using its native signal sequence suggesting that it may be translocated via a Sec-type system in K. stuttgartiensis. The cytochrome has the visible spectroscopic properties typical of a low-spin c-type cytochrome, but these spectroscopic features broaden in high salt solutions. The oxidised cytochrome was able to bind the ligands NO and cyanide. A redox potential of +230 mV suggests that the protein is suitable to act as an electron carrier protein that may be involved in the respiratory chain between hydrazine oxidation and the reduction of nitrite. The predicted protein sequence for the cytochrome suggests it to be a predominantly alpha-helical protein, and this is supported by circular dichroism.

Characterization of the OxyR regulon of Neisseria gonorrhoeae

OxyR regulates the expression of the majority of H(2)O(2) responses in Gram-negative organisms. In a previous study we reported the OxyR-dependent derepression of catalase expression in the human pathogen Neisseria gonorrhoeae. In the present study we used microarray expression profiling of N. gonorrhoeae wild-type strain 1291 and an oxyR mutant strain to define the OxyR regulon. In addition to katA (encoding catalase), only one other locus displayed a greater than two-fold difference in expression in the wild type : oxyR comparison. This locus encodes an operon of two genes, a putative peroxiredoxin/glutaredoxin (Prx) and a putative glutathione oxidoreductase (Gor). Mutant strains were constructed in which each of these genes was inactivated. A previous biochemical study in Neisseria meningitidis had confirmed function of the glutaredoxin/peroxiredoxin. Assay of the wild-type 1291 cell free extract confirmed Gor activity, which was lost in the gor mutant strain. Phenotypic analysis of the prx mutant strain in H(2)O(2) killing assays revealed increased resistance, presumably due to upregulation of alternative defence mechanisms. The oxyR, prx and gor mutant strains were deficient in biofilm formation, and the oxyR and prx strains had decreased survival in cervical epithelial cells, indicating a key role for the OxyR regulon in these processes.

Neisseria gonorrhoeae DNA recombination and repair enzymes protect against oxidative damage caused by hydrogen peroxide

The strict human pathogen Neisseria gonorrhoeae is exposed to oxidative damage during infection. N. gonorrhoeae has many defenses that have been demonstrated to counteract oxidative damage. However, recN is the only DNA repair and recombination gene upregulated in response to hydrogen peroxide (H(2)O(2)) by microarray analysis and subsequently shown to be important for oxidative damage protection. We therefore tested the importance of RecA and DNA recombination and repair enzymes in conferring resistance to H(2)O(2) damage. recA mutants, as well as RecBCD (recB, recC, and recD) and RecF-like pathway mutants (recJ, recO, and recQ), all showed decreased resistance to H(2)O(2). Holliday junction processing mutants (ruvA, ruvC, and recG) showed decreased resistance to H(2)O(2) resistance as well. Finally, we show that RecA protein levels did not increase as a result of H(2)O(2) treatment. We propose that RecA, recombinational DNA repair, and branch migration are all important for H(2)O(2) resistance in N. gonorrhoeae but that constitutive levels of these enzymes are sufficient for providing protection against oxidative damage by H(2)O(2).

Alpha-2,3-sialyltransferase enhances Neisseria gonorrhoeae survival during experimental murine genital tract infection

The addition of host-derived sialic acid to Neisseria gonorrhoeae lipooligosaccharide is hypothesized to be an important mechanism by which gonococci evade host innate defenses. This hypothesis is based primarily on in vitro assays of complement-mediated and phagocytic killing. Here we report that a nonpolar alpha-2,3-sialyltransferase (lst) mutant of N. gonorrhoeae was significantly attenuated in its capacity to colonize the lower genital tract of 17-beta estradiol-treated female BALB/c mice during competitive infection with the wild-type strain. Genetic complementation of the lst mutation restored recovery of the mutant to wild-type levels. Studies with B10.D2-HC(o)H2(d)H(2)-T18c/OSN (C5-deficient) mice showed that attenuation of the lst mutant was not due to increased sensitivity to complement-mediated bacteriolysis, a result that is consistent with recently reported host restrictions in the complement cascade. However, Lst-deficient gonococci were killed more rapidly than sialylated wild-type gonococci following intraperitoneal injection into normal mice, which is consistent with sialylation conferring protection against killing by polymorphonuclear leukocytes (PMNs). As reported for human PMNs, sialylated gonococci were more resistant to killing by murine PMNs, and sialylation led to reduced association with and induction of a weaker respiratory burst in PMNs from estradiol-treated mice. In summary, these studies suggest sialylation confers a survival advantage to N. gonorrhoeae in mice by increasing resistance to PMN killing. This report is the first direct demonstration that alpha-2,3-sialyltransferase contributes to N. gonorrhoeae pathogenesis in an in vivo model. This study also validates the use of experimental murine infection to study certain aspects of gonococcal pathogenesis.

Role of FNR and FNR-regulated, sugar fermentation genes in Neisseria meningitidis infection

While it is generally accepted that anaerobic metabolism is required during infection, supporting experimental data have only been described in a limited number of studies. To provide additional evidence on the role of anaerobic metabolism in bacterial pathogens while invading mammalian hosts, we analysed the effect of the inactivation of FNR, the major regulatory protein involved in the adaptation to oxygen restrictive conditions, and of two of the FNR-regulated genes on the survival of Neisseria meningitidis serogroup B (MenB) in vivo. We found that fnr deletion resulted in more than 1 log reduction in the meningococcal capacity to proliferate both in infant rats and in mice. To identify which of the FNR-regulated genes were responsible for this attenuated phenotype, we defined the FNR regulon by combining DNA microarray analysis and FNR-DNA binding studies. Under oxygen-restricted conditions, FNR positively controlled the transcription of nine transcriptional units, the most upregulated of which were the two operons NMB0388-galM and mapA-pgmbeta implicated in sugar metabolism and fermentation. When galM and mapA were knocked out, the mutants were attenuated by 2 and 3 logs respectively. As the operons are controlled by FNR, from these data we conclude that MenB survival in the host anatomical sites where oxygen is limiting is supported by sugar fermentation.

PerR controls Mn-dependent resistance to oxidative stress in Neisseria gonorrhoeae

In previous studies it has been established that resistance to superoxide by Neisseria gonorrhoeae is dependent on the accumulation of Mn(II) ions involving the ABC transporter, MntABC. A mutant strain lacking the periplasmic binding protein component (MntC) of this transport system is hypersensitive to killing by superoxide anion. In this study the mntC mutant was found to be more sensitive to H2O2 killing than the wild-type. Analysis of regulation of MntC expression revealed that it was de-repressed under low Mn(II) conditions. The N. gonorrhoeae mntABC locus lacks the mntR repressor typically found associated with this locus in other organisms. A search for a candidate regulator of mntABC expression revealed a homologue of PerR, a Mn-dependent peroxide-responsive regulator found in Gram-positive organisms. A perR mutant expressed more MntC protein than wild-type, and expression was independent of Mn(II), consistent with a role for PerR as a repressor of mntABC expression. The PerR regulon of N. gonorrhoeae was defined by microarray analysis and includes ribosomal proteins, TonB-dependent receptors and an alcohol dehydrogenase. Both the mntC and perR mutants had reduced intracellular survival in a human cervical epithelial cell model.

Defenses against oxidative stress in Neisseria gonorrhoeae: a system tailored for a challenging environment

Neisseria gonorrhoeae is a host-adapted pathogen that colonizes primarily the human genitourinary tract. This bacterium encounters reactive oxygen and reactive nitrogen species as a consequence of localized inflammatory responses in the urethra of males and endocervix of females and also of the activity of commensal lactobacilli in the vaginal flora. This review describes recent advances in the understanding of defense systems against oxidative stress in N. gonorrhoeae and shows that while some of its defenses have similarities to the paradigm established with Escherichia coli, there are also some key differences. These differences include the presence of a defense system against superoxide based on manganese ions and a glutathione-dependent system for defense against nitric oxide which is under the control of a novel MerR-like transcriptional regulator. An understanding of the defenses against oxidative stress in N. gonorrhoeae and their regulation may provide new insights into the ways in which this bacterium survives challenges from polymorphonuclear leukocytes and urogenital epithelial cells.

Azurin of pathogenic Neisseria spp. is involved in defense against hydrogen peroxide and survival within cervical epithelial cells

Laz, a lipid-modified azurin of the human pathogens Neisseria gonorrhoeae and Neisseria meningitidis, is involved in defense against oxidative stress and copper toxicity; laz mutant strains are hypersensitive to hydrogen peroxide and copper. The N. gonorrhoeae laz mutant also has decreased survival in an ex vivo primary human ectocervical epithelial assay.

Structure, Mechanism and Physiological Roles of Bacterial Cytochrome c Peroxidases

Cytochrome-c peroxidases (CCPs) are a widespread family of enzymes that catalyse the conversion of hydrogen peroxide (H2O2) to water using haem co-factors. CCPs are found in both eukaryotes and prokaryotes, but the enzymes in each group use a distinct mechanism for catalysis. Eukaryotic CCPs contain a single b-type haem co-factor. Conventional bacterial CCPs (bCCPs) are periplasmic enzymes that contain two covalently bound c-type haems. However, we have identified a sub-group of bCCPs by phylogenetic analysis that contains three haem-binding motifs. Although the structure and mechanism of several bacterial di-haem CCPs has been studied in detail and is well understood, the physiological role of these enzymes is often much less clear, especially in comparison to other peroxidatic enzymes such as catalase and alkyl-hydroperoxide reductase. In this review, the structure, mechanism and possible roles of bCCPs are examined in the context of their periplasmic location, the regulation of their synthesis by oxygen and their particular function in pathogens.