In vitro and in vivo characterization of alkyl hydroperoxide reductase mutant strains of Helicobacter hepaticus

Role of a Putative Alkylhydroperoxidase Rv2159c in the Oxidative Stress Response and Virulence of Mycobacterium tuberculosis

Mycobacterium tuberculosis, which causes tuberculosis, is one of the leading infectious agents worldwide with a high rate of mortality. Following aerosol inhalation, M. tuberculosis primarily infects the alveolar macrophages, which results in a host immune response that gradually activates various antimicrobial mechanisms, including the production of reactive oxygen species (ROS), within the phagocytes to neutralize the bacteria. OxyR is the master regulator of oxidative stress response in several bacterial species. However, due to the absence of a functional oxyR locus in M. tuberculosis, the peroxidase stress is controlled by alkylhydroperoxidases. M. tuberculosis expresses alkylhydroperoxide reductase to counteract the toxic effects of ROS. In the current study, we report the functional characterization of an orthologue of alkylhydroperoxidase family member, Rv2159c, a conserved protein with putative peroxidase activity, during stress response and virulence of M. tuberculosis. We generated a gene knockout mutant of M. tuberculosis Rv2159c (MtbΔ2159) by specialized transduction. The MtbΔ2159 was sensitive to oxidative stress and exposure to toxic transition metals. In a human monocyte (THP-1) cell infection model, MtbΔ2159 showed reduced uptake and intracellular survival and increased expression of pro-inflammatory molecules, including IL-1β, IP-10, and MIP-1α, compared to the wild type M. tuberculosis and Rv2159c-complemented MtbΔ2159 strains. Similarly, in a guinea pig model of pulmonary infection, MtbΔ2159 displayed growth attenuation in the lungs, compared to the wild type M. tuberculosis and Rv2159c-complemented MtbΔ2159 strains. Our study suggests that Rv2159c has a significant role in maintaining the cellular homeostasis during stress and virulence of M. tuberculosis.

Methylation Motifs in Promoter Sequences May Contribute to the Maintenance of a Conserved m5C Methyltransferase in Helicobacter pylori

DNA methylomes of Helicobacter pylori strains are complex due to the large number of DNA methyltransferases (MTases) they possess. H. pylori J99 M.Hpy99III is a 5-methylcytosine (^m5C) MTase that converts GCGC motifs to G^m5CGC. Homologs of M.Hpy99III are found in essentially all H. pylori strains. Most of these homologs are orphan MTases that lack a cognate restriction endonuclease, and their retention in H. pylori strains suggest they have roles in gene regulation. To address this hypothesis, green fluorescent protein (GFP) reporter genes were constructed with six putative promoters that had a GCGC motif in the extended −10 region, and the expression of the reporter genes was compared in wild-type H. pylori G27 and a mutant lacking the M.Hpy99III homolog (M.HpyGIII). The expression of three of the GFP reporter genes was decreased significantly in the mutant lacking M.HpyGIII. In addition, the growth rate of the H. pylori G27 mutant lacking M.HpyGIII was reduced markedly compared to that of the wild type. These findings suggest that the methylation of the GCGC motif in many H. pylori GCGC-containing promoters is required for the robust expression of genes controlled by these promoters, which may account for the universal retention of M.Hpy99III homologs in H. pylori strains.

Oxidative and nitrosative stress defences of Helicobacter and Campylobacter species that counteract mammalian immunity

Helicobacter and Campylobacter species are Gram-negative microaerophilic host-associated heterotrophic bacteria that invade the digestive tract of humans and animals. Campylobacter jejuni is the major worldwide cause of foodborne gastroenteritis in humans, while Helicobacter pylori is ubiquitous in over half of the world's population causing gastric and duodenal ulcers. The colonisation of the gastrointestinal system by Helicobacter and Campylobacter relies on numerous cellular defences to sense the host environment and respond to adverse conditions, including those imposed by the host immunity. An important antimicrobial tool of the mammalian innate immune system is the generation of harmful oxidative and nitrosative stresses to which pathogens are exposed during phagocytosis. This review summarises the regulators, detoxifying enzymes and subversion mechanisms of Helicobacter and Campylobacter that ultimately promote the successful infection of humans.

The sensitive balance between the fully folded and locally unfolded conformations of a model peroxiredoxin

To reduce peroxides, peroxiredoxins (Prxs) require a key "peroxidatic" Cys that, in a substrate-ready fully folded (FF) conformation, is oxidized to sulfenic acid and then, after a local unfolding (LU) of the active site, forms a disulfide bond with a second "resolving" Cys. For Salmonella typhimurium alkyl hydroperoxide reductase C (StAhpC) and some other Prxs, the FF structure is only known for a peroxidatic Cys→Ser variant, which may not accurately represent the wild-type enzyme. Here, we obtain the structure of authentic reduced wild-type StAhpC by dithiothreitol treatment of disulfide form crystals that fortuitously accommodate both the LU and FF conformations. The unique environment of one molecule in the crystal reveals a thermodynamic linkage between the folding of the active site loop and C-terminal regions, and comparisons with the Ser variant show structural and mobility differences from which we infer that the Cys→Ser mutation stabilizes the FF active site. A structure for the C165A variant (a resolving Cys to Ala mutant) in the same crystal form reveals that this mutation destabilizes the folding of the C-terminal region. These structures prove that subtle modifications to Prx structures can substantially influence enzymatic properties. We also present a simple thermodynamic framework for understanding the various mixtures of FF and LU conformations seen in these structures. On the basis of this framework, we rationalize how physiologically relevant regulatory post-translational modifications may modulate activity, and we propose a nonconventional strategy for designing selective Prx inhibitors.

Helicobacter hepaticus NikR controls urease and hydrogenase activities via the NikABDE and HH0418 putative nickel import proteins

Helicobacter hepaticus open reading frame HH0352 was identified as a nickel-responsive regulator NikR. The gene was disrupted by insertion of an erythromycin resistance cassette. The H. hepaticus nikR mutant had five- to sixfold higher urease activity and at least twofold greater hydrogenase activity than the wild-type strain. However, the urease apo-protein levels were similar in both the wild-type and the mutant, suggesting the increase in urease activity in the mutant was due to enhanced Ni-maturation of the urease. Compared with the wild-type strain, the nikR strain had increased cytoplasmic nickel levels. Transcription of nikABDE (putative inner membrane Ni transport system) and hh0418 (putative outer membrane Ni transporter) was nickel- and NikR-repressed. Electrophoretic mobility shift assays (EMSAs) revealed that purified HhNikR could bind to the nikABDE promoter (P(nikA)), but not to the urease or the hydrogenase promoter; NikR-P(nikA) binding was enhanced in the presence of nickel. Also, qRT-PCR and EMSAs indicated that neither nikR nor the exbB-exbD-tonB were under the control of the NikR regulator, in contrast with their Helicobacter pylori homologues. Taken together, our results suggest that HhNikR modulates urease and hydrogenase activities by repressing the nickel transport/nickel internalization systems in H. hepaticus, without direct regulation of the Ni-enzyme genes (the latter is the case for H. pylori). Finally, the nikR strain had a two- to threefold lower growth yield than the parent, suggesting that the regulatory protein might play additional roles in the mouse liver pathogen.

Alkyl hydroperoxide reductase is required for Helicobacter cinaedi intestinal colonization and survival under oxidative stress in BALB/c and BALB/c interleukin-10-/- mice

Helicobacter cinaedi, a common human intestinal bacterium, has been implicated in various enteric and systemic diseases in normal and immunocompromised patients. Protection against oxidative stress is a crucial component of bacterium-host interactions. Alkyl hydroperoxide reductase C (AhpC) is an enzyme responsible for detoxification of peroxides and is important in protection from peroxide-induced stress. H. cinaedi possesses a single ahpC, which was investigated with respect to its role in bacterial survival during oxidative stress. The H. cinaedi ahpC mutant had diminished resistance to organic hydroperoxide toxicity but increased hydrogen peroxide resistance compared with the wild-type (WT) strain. The mutant also exhibited an oxygen-sensitive phenotype and was more susceptible to killing by macrophages than the WT strain. In vivo experiments in BALB/c and BALB/c interleukin-10 (IL-10)(-/-) mice revealed that the cecal colonizing ability of the ahpC mutant was significantly reduced. The mutant also had diminished ability to induce bacterium-specific immune responses in vivo, as shown by immunoglobulin (IgG2a and IgG1) serum levels. Collectively, these data suggest that H. cinaedi ahpC not only contributes to protecting the organism against oxidative stress but also alters its pathogenic properties in vivo.

PerR controls peroxide- and iron-responsive expression of oxidative stress defense genes in Helicobacter hepaticus

Chronic intestinal and hepatic colonization with the microaerophilic murine pathogen Helicobacter hepaticus can lead to a range of inflammatory diseases of the lower digestive tract. Colonization is associated with an active cellular immune response and production of oxygen radicals. During colonization, H. hepaticus needs to cope with and respond to oxidative stress, and here we report on the role of the H. hepaticus PerR-regulator (HH0942) in the expression of the peroxidase-encoding katA (HH0043) and ahpC (HH1564) genes. Transcription of katA and ahpC was induced by hydrogen peroxide, and by iron restriction of growth media. This iron- and hydrogen peroxide-responsive regulation of katA and ahpC was mediated at the transcriptional level, from promoters directly upstream of the genes. Inactivation of the perR gene resulted in constitutive, iron-independent high-level expression of the katA and ahpC transcripts and corresponding proteins. Finally, inactivation of the katA gene resulted in increased sensitivity of H. hepaticus to hydrogen peroxide and reduced aerotolerance. In H. hepaticus, iron metabolism and oxidative stress defense are intimately connected via the PerR regulatory protein. This regulatory pattern resembles that observed in the enteric pathogen Campylobacter jejuni, but contrasts with the pattern observed in the closely related human gastric pathogen Helicobacter pylori.

A pathobiont of the microbiota balances host colonization and intestinal inflammation

The gastrointestinal tract harbors a diverse microbiota that has coevolved with mammalian hosts. Though most associations are symbiotic or commensal, some resident bacteria (termed pathobionts) have the potential to cause disease. Bacterial type VI secretion systems (T6SSs) are one mechanism for forging host-microbial interactions. Here we reveal a protective role for the T6SS of Helicobacter hepaticus, a Gram-negative bacterium of the intestinal microbiota. H. hepaticus mutants with a defective T6SS display increased numbers within intestinal epithelial cells (IECs) and during intestinal colonization. Remarkably, the T6SS directs an anti-inflammatory gene expression profile in IECs, and CD4+ T cells from mice colonized with T6SS mutants produce increased interleukin-17 in response to IECs presenting H. hepaticus antigens. Thus, the H. hepaticus T6SS limits colonization and intestinal inflammation, promoting a balanced relationship with the host. We propose that disruption of such balances contributes to human disorders such as inflammatory bowel disease and colon cancer.

Redundant hydrogen peroxide scavengers contribute to Salmonella virulence and oxidative stress resistance

Salmonella enterica serovar Typhimurium is an intracellular pathogen that can survive and replicate within macrophages. One of the host defense mechanisms that Salmonella encounters during infection is the production of reactive oxygen species by the phagocyte NADPH oxidase. Among them, hydrogen peroxide (H(2)O(2)) can diffuse across bacterial membranes and damage biomolecules. Genome analysis allowed us to identify five genes encoding H(2)O(2) degrading enzymes: three catalases (KatE, KatG, and KatN) and two alkyl hydroperoxide reductases (AhpC and TsaA). Inactivation of the five cognate structural genes yielded the HpxF(-) mutant, which exhibited a high sensitivity to exogenous H(2)O(2) and a severe survival defect within macrophages. When the phagocyte NADPH oxidase was inhibited, its proliferation index increased 3.7-fold. Moreover, the overexpression of katG or tsaA in the HpxF(-) background was sufficient to confer a proliferation index similar to that of the wild type in macrophages and a resistance to millimolar H(2)O(2) in rich medium. The HpxF(-) mutant also showed an attenuated virulence in a mouse model. These data indicate that Salmonella catalases and alkyl hydroperoxide reductases are required to degrade H(2)O(2) and contribute to the virulence. This enzymatic redundancy highlights the evolutionary strategies developed by bacterial pathogens to survive within hostile environments.

The Helicobacter hepaticus hefA gene is involved in resistance to amoxicillin

BACKGROUND

Gastrointestinal infections with pathogenic Helicobacter species are commonly treated with combination therapies, which often include amoxicillin. Although this treatment is effective for eradication of Helicobacter pylori, the few existing reports are less clear about antibiotic susceptibility of other Helicobacter species. In this study we have determined the susceptibility of gastric and enterohepatic Helicobacter species to amoxicillin, and have investigated the mechanism of amoxicillin resistance in Helicobacter hepaticus.

MATERIALS AND METHODS

The minimal inhibitory concentration (MIC) of antimicrobial compounds was determined by E-test and agar/broth dilution assays. The hefA gene of H. hepaticus was inactivated by insertion of a chloramphenicol resistance gene. Transcription was measured by quantitative real-time polymerase chain reaction.

RESULTS

Three gastric Helicobacter species (H. pylori, H. mustelae, and H. acinonychis) were susceptible to amoxicillin (MIC < 0.25 mg/L). In contrast, three enterohepatic Helicobacter species (H. rappini, H. bilis, and H. hepaticus) were resistant to amoxicillin (MIC of 8, 16, and 6-64 mg/L, respectively). There was no detectable beta-lactamase activity in H. hepaticus, and inhibition of beta-lactamases did not change the MIC of amoxicillin of H. hepaticus. A H. hepaticus hefA (hh0224) mutant, encoding a TolC-component of a putative efflux system, resulted in loss of amoxicillin resistance (MIC 0.25 mg/L), and also resulted in increased sensitivity to bile acids. Finally, transcription of the hefA gene was not responsive to amoxicillin, but induced by bile acids.

CONCLUSIONS

Rodents are frequently colonized by a variety of enterohepatic Helicobacter species, and this may affect their global health status and intestinal inflammatory responses. Animal facilities should have treatment strategies for Helicobacter infections, and hence resistance of enterohepatic Helicobacter species to amoxicillin should be considered when designing eradication programs.

Peroxiredoxin-1 from Schistosoma japonicum functions as a scavenger against hydrogen peroxide but not nitric oxide

Three peroxiredoxins (Prxs) are expressed during most of the developmental stages in the schistosome. Prx-1 is localized on the surface of the schistosomula and adults of Schistosoma japonicum, while Prx-2 is localized in the sub-tegumental tissues, parenchyma, vitelline glands, and gut epithelium, but not on the surface of the worms. We applied RNA interference techniques to suppress the specific genes of S. japonicum Prxs. Schistosomula of S. japonicum were cultured together with long-dsRNA encoding Prx-1 and Prx-2 of S. japonicum (the soaking method). The transcription level of each Prx gene was reduced by an RNA interference (RNAi)-mediated effect specifically. Although neither Prx was the essential protein for survival of S. japonicum schistosomula, Prx-1 dsRNA-treated larvae were susceptible to hydrogen peroxide. Moreover, these larvae were also susceptible to t-butyl hydroperoxide and cumene-hydroperoxide. However, the knockdown of neither Prx-1 nor Prx-2 influenced the resistance against nitric oxide generated from DETA/NO. Prx-1 may work as a scavenger against reactive oxygen species (ROS) generated outside of the schistosomes to prevent the oxidation of the bodies and/or the attack by immune cells producing the ROS. These findings suggest that Prx-1 may become a novel target of drugs and vaccines for schistosomiasis.

Nickel enzyme maturation in Helicobacter hepaticus: roles of accessory proteins in hydrogenase and urease activities

Helicobacter hepaticus, a causative agent of chronic hepatitis and hepatocellular carcinoma in mice, possesses a hydrogenase and a urease, both of which are nickel-containing enzymes. Analysis of the genome sequence of H. hepaticus revealed a full set of accessory genes which are required for the nickel maturation of each enzyme in other micro-organisms. Erythromycin-resistant mutants were constructed in four of these genes, hypA, hypB, ureE and ureG. Controls for polar effect were provided for hypA or hypB mutants by disrupting each gene located immediately downstream, i.e. hp0809 or hypC, respectively. Urease and hydrogenase activities were determined for each strain with or without supplemented nickel in the medium. As expected, the ureE and the ureG mutants had negligible urease activity, but they retained normal levels of hydrogenase activity. Urease levels could not be increased by the addition of nickel to the medium. The H. hepaticus hypA and hypB strains were deficient in both urease and hydrogenase activities, suggesting that both gene products act in a similar fashion as their counterparts in H. pylori. However, in contrast with the analogous mutants of H. pylori, the addition of nickel into the growth medium failed to restore either urease or hydrogenase enzyme levels in the H. hepaticus hypA or hypB mutants, indicating a probably unique role for these genes in the mouse liver pathogen.

Antisense RNA modulation of alkyl hydroperoxide reductase levels in Helicobacter pylori correlates with organic peroxide toxicity but not infectivity

Much of the gene content of the human gastric pathogen Helicobacter pylori ( approximately 1.7-Mb genome) is considered essential. This view is based on the completeness of metabolic pathways, infrequency of nutritional auxotrophies, and paucity of pathway redundancies typically found in bacteria with larger genomes. Thus, genetic analysis of gene function is often hampered by lethality. In the absence of controllable promoters, often used to titrate gene function, we investigated the feasibility of an antisense RNA interference strategy. To test the antisense approach, we targeted alkyl hydroperoxide reductase (AhpC), one of the most abundant proteins expressed by H. pylori and one whose function is essential for both in vitro growth and gastric colonization. Here, we show that antisense ahpC (as-ahpC) RNA expression from shuttle vector pDH37::as-ahpC achieved an approximately 72% knockdown of AhpC protein levels, which correlated with increased susceptibilities to hydrogen peroxide, cumene, and tert-butyl hydroperoxides but not with growth efficiency. Compensatory increases in catalase levels were not observed in the knockdowns. Expression of single-copy antisense constructs (expressed under the urease promoter and containing an fd phage terminator) from the rdxA locus of mouse-colonizing strain X47 achieved a 32% knockdown of AhpC protein levels (relative to wild-type X47 levels), which correlated with increased susceptibility to organic peroxides but not with mouse colonization efficiency. Our studies indicate that high levels of AhpC are not required for in vitro growth or for primary gastric colonization. Perhaps AhpC, like catalase, assumes a greater role in combating exogenous peroxides arising from lifelong chronic inflammation. These studies also demonstrate the utility of antisense RNA interference in the evaluation of gene function in H. pylori.