Catalases promote resistance of oxidative stress in Vibrio cholerae

Host-Parasite Interactions and Integrated Management Strategies for Ecytonucleospora Hepatopenaei Infection in Shrimp

BACKGROUND

Ecytonucleospora hepatopenaei (EHP) is a major parasitic pathogen in shrimp causing hepatopancreatic microsporidiosis, which leads to significant growth retardation and global economic losses. This pathogen employs various immune evasion strategies that complicate treatment and management.

PURPOSE

This review examines the complex host-parasite interactions, focusing on the immune evasion mechanisms used by EHP. The study explores how EHP manipulates host immune pathways, including NF-κB, JAK/STAT, Toll, and IMD, to suppress immune responses, inhibit antimicrobial peptide production, and avoid detection, thus ensuring its persistence in the host.

METHODS

The authors reviewed recent research from databases like PubMed, Scopus, and Web of Science, including studies up to 2024. The keywords Ecytonucleospora hepatopenaei, immune evasion, EHP treatment, and associated words with topics were used in this search.

RESULTS

EHP induces oxidative stress, which weakens the host immune system while simultaneously upregulating antioxidant responses to favor its survival. The parasite also alters the gut microbiota and disrupts key cellular processes, such as cell cycle regulation, further enhancing its ability to sustain infection.

CONCLUSION

This review highlights the need for integrated management strategies, including disease-resistant breeding, microbiota modulation, and advanced diagnostics, to combat EHP. By providing an overview of EHP's immune evasion tactics, this study aims to advance knowledge in the field and support efforts to improve shrimp health and aquaculture sustainability.

Vibrio harveyi uses both type III secretion system and quorum sensing for the colonization of the European abalone

The vibriosis of the European abalone, Haliotis tuberculata, is characterized by the rapidity of the infection by the pathogen Vibrio harveyi ORM4, leading to death of animals only after two days. The lethality of the pathogen is linked to the production of the type III secretion system (T3SS) and to genes regulated by quorum sensing (QS). The aim of this study was to investigate the colonization of the European abalone by both virulent and avirulent V. harveyi strains, as well as the involvement of T3SS and QS during infection. Our results emphasize the importance of gills for the bacterial establishment as the bacterial concentration of the avirulent V. harveyi strain significantly decreased from 189.3 ± 98.6 CFU/mg to 0.8 ± 0.5 CFU/mg between 24 and 48 h post-infection (hpi). In opposition, the pathogen V. harveyi ORM4 was able to maintain itself on the gills, with a concentration of 461.9 ± CFU/mg at 48 hpi, which was allowed by the production of T3SS and a functional QS. Following the infection cycle of V. harveyi ORM4 inside H. tuberculata, we also demonstrated that QS is essential for the ability of V. harveyi ORM4 to colonize the abalone hemolymph and to maintain in it. In response to the presence of V. harveyi, an increase in reactive oxygen species production was recorded, while the phagocytosis activity remained unchanged. We also highlighted the involvement of both QS and T3SS to escape the immune system activity, and that an overproduction of T3SS induced hemocyte mortality. This study provides the evidence that both T3SS and QS are essential for the establishment of V. harveyi ORM4 inside the European abalone.

Oxidative stress responses in biofilms

Oxidizing agents are low-molecular-weight molecules that oxidize other substances by accepting electrons from them. They include reactive oxygen species (ROS), such as superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO-), and reactive chlorine species (RCS) including sodium hypochlorite (NaOCl) and its active ingredient hypochlorous acid (HOCl), and chloramines. Bacteria encounter oxidizing agents in many different environments and from diverse sources. Among them, they can be produced endogenously by aerobic respiration or exogenously by the use of disinfectants and cleaning agents, as well as by the mammalian immune system. Furthermore, human activities like industrial effluent pollution, agricultural runoff, and environmental activities like volcanic eruptions and photosynthesis are also sources of oxidants. Despite their antimicrobial effects, bacteria have developed many mechanisms to resist the damage caused by these toxic molecules. Previous research has demonstrated that growing as a biofilm particularly enhances bacterial survival against oxidizing agents. This review aims to summarize the current knowledge on the resistance mechanisms employed by bacterial biofilms against ROS and RCS, focussing on the most important mechanisms, including the formation of biofilms in response to oxidative stressors, the biofilm matrix as a protective barrier, the importance of detoxifying enzymes, and increased protection within multi-species biofilm communities. Understanding the complexity of bacterial responses against oxidative stress will provide valuable insights for potential therapeutic interventions and biofilm control strategies in diverse bacterial species.

Non-O1/Non-O139 Vibrio cholerae-An Underestimated Foodborne Pathogen? An Overview of Its Virulence Genes and Regulatory Systems Involved in Pathogenesis

In recent years, the number of foodborne infections with non-O1 and non-O139 Vibrio cholerae (NOVC) has increased worldwide. These have ranged from sporadic infection cases to localized outbreaks. The majority of case reports describe self-limiting gastroenteritis. However, severe gastroenteritis and even cholera-like symptoms have also been described. All reported diarrheal cases can be traced back to the consumption of contaminated seafood. As climate change alters the habitats and distribution patterns of aquatic bacteria, there is a possibility that the number of infections and outbreaks caused by Vibrio spp. will further increase, especially in countries where raw or undercooked seafood is consumed or clean drinking water is lacking. Against this background, this review article focuses on a possible infection pathway and how NOVC can survive in the human host after oral ingestion, colonize intestinal epithelial cells, express virulence factors causing diarrhea, and is excreted by the human host to return to the environment.

Unraveling the mechanistic insights of sophorolipid-capped gold nanoparticle-induced cell death in Vibrio cholerae

IMPORTANCE

Vibrio cholerae, a Gram-negative bacterium, is the causative agent of a fatal disease, "cholera." Prevention of cholera outbreak is possible by eliminating the bacteria from the environment. However, antimicrobial resistance developed in microorganisms has posed a threat and challenges to its treatment. Application of nanoparticles is a useful and effective option for the elimination of such microorganisms. Metal-based nanopaticles exhibit microbial toxicity through non-specific mechanisms. To prevent resistance development and increase antibacterial efficiency, rational designing of nanoparticles is required. Thus, knowledge on the exact mechanism of action of nanoparticles is highly essential. In this study, we explore the possible mechanisms of antibacterial activity of AuNPs-SL against V. cholerae. We show that the interaction of AuNPs-SL with V. cholerae enhances ROS production and membrane depolarization, change in permeability, and leakage of intracellular content. This action leads to the depletion of cellular ATP level, DNA damage, and subsequent cell death.

Increased iron utilization and oxidative stress tolerance in a Vibrio cholerae flrA mutant confers resistance to amoeba predation

IMPORTANCE

Persistence of V. cholerae in the aquatic environment contributes to the fatal diarrheal disease cholera, which remains a global health burden. In the environment, bacteria face predation pressure by heterotrophic protists such as the free-living amoeba A. castellanii. This study explores how a mutant of V. cholerae adapts to acquire essential nutrients and survive predation. Here, we observed that up-regulation of iron acquisition genes and genes regulating resistance to oxidative stress enhances pathogen fitness. Our data show that V. cholerae can defend predation to overcome nutrient limitation and oxidative stress, resulting in an enhanced survival inside the protozoan hosts.

DegS protease regulates antioxidant capacity and adaptability to oxidative stress environment in Vibrio cholerae

Adaptation to oxidative stress is critical for survival of Vibrio cholerae in aquatic ecosystems and hosts. DegS activates the σE envelope stress response. We have previously revealed that DegS may be involved in regulating the oxidative stress response. In this study, we demonstrated that deletion of the degS gene attenuates the antioxidant capacity of V. cholerae. In addition, our results further revealed that the regulation of antioxidant capacity by DegS in V. cholerae could involve the cAMP-CRP complex, which regulates rpoS. XthA is an exonuclease that repairs oxidatively damaged cells and affects the bacterial antioxidant capacity. qRT-PCR showed that DegS, σE, cAMP, CRP, and RpoS positively regulate xthA gene transcription. XthA overexpression partially compensates for antioxidant deficiency in the degS mutant. These results suggest that DegS affects the antioxidant capacity of V.cholerae by regulating xthA expression via the cAMP-CRP-RpoS pathway. In a mouse intestinal colonization experiment, our data showed that V.cholerae degS, rpoE, and rpoS gene deletions were associated with significantly reduced resistance to oxidative stress and the ability to colonize the mouse intestine. In conclusion, these findings provide new insights into the regulation of antioxidant activity by V.cholerae DegS.

The secondary metabolite hydrogen cyanide protects Pseudomonas aeruginosa against sodium hypochlorite-induced oxidative stress

The high pathogenicity of Pseudomonas aeruginosa is attributed to the production of many virulence factors and its resistance to several antimicrobials. Among them, sodium hypochlorite (NaOCl) is a widely used disinfectant due to its strong antimicrobial effect. However, bacteria develop many mechanisms to survive the damage caused by this agent. Therefore, this study aimed to identify novel mechanisms employed by P. aeruginosa to resist oxidative stress induced by the strong oxidizing agent NaOCl. We analyzed the growth of the P. aeruginosa mutants ΔkatA, ΔkatE, ΔahpC, ΔahpF, ΔmsrA at 1 μg/mL NaOCl, and showed that these known H2O2 resistance mechanisms are also important for the survival of P. aeruginosa under NaOCl stress. We then conducted a screening of the P. aeruginosa PA14 transposon insertion mutant library and identified 48 mutants with increased susceptibility toward NaOCl. Among them were 10 mutants with a disrupted nrdJa, bvlR, hcnA, orn, sucC, cysZ, nuoJ, PA4166, opmQ, or thiC gene, which also exhibited a significant growth defect in the presence of NaOCl. We focussed our follow-up experiments (i.e., growth analyzes and kill-kinetics) on mutants with defect in the synthesis of the secondary metabolite hydrogen cyanide (HCN). We showed that HCN produced by P. aeruginosa contributes to its resistance toward NaOCl as it acts as a scavenger molecule, quenching the toxic effects of NaOCl.

Burkholderia pseudomallei biofilm resists Acanthamoeba sp. grazing and produces 8-O-4'-diferulic acid, a superoxide scavenging metabolite after passage through the amoeba

Burkholderia pseudomallei, an etiological agent of melioidosis is an environmental bacterium that can survive as an intracellular pathogen. The biofilm produced by B. pseudomallei is crucial for cellular pathogenesis of melioidosis. The purpose of this investigation is to explore the role of biofilm in survival of B. pseudomallei during encounters with Acanthamoeba sp. using B. pseudomallei H777 (a biofilm wild type), M10 (a biofilm defect mutant) and C17 (a biofilm-complemented strain). The results demonstrated similar adhesion to amoebae by both the biofilm wild type and biofilm mutant strains. There was higher initial internalisation, but the difference diminished after longer encounter with the amoeba. Interestingly, confocal laser scanning microscopy demonstrated that pre-formed biofilm of B. pseudomallei H777 and C17 were markedly more persistent in the face of Acanthamoeba sp. grazing than that of M10. Metabolomic analysis revealed a significant increased level of 8-O-4'-diferulic acid, a superoxide scavenger metabolite, in B. pseudomallei H777 serially passaged in Acanthamoeba sp. The interaction between B. pseudomallei with a free-living amoeba may indicate the evolutionary pathway that enables the bacterium to withstand superoxide radicals in intracellular environments. This study supports the hypothesis that B. pseudomallei biofilm persists under grazing by amoebae and suggests a strategy of metabolite production that turns this bacterium from saprophyte to intracellular pathogen.

Diverse roles of lung macrophages in the immune response to influenza A virus

Influenza viruses are one of the major causes of human respiratory infections and the newly emerging and re-emerging strains of influenza virus are the cause of seasonal epidemics and occasional pandemics, resulting in a huge threat to global public health systems. As one of the early immune cells can rapidly recognize and respond to influenza viruses in the respiratory, lung macrophages play an important role in controlling the severity of influenza disease by limiting viral replication, modulating the local inflammatory response, and initiating subsequent adaptive immune responses. However, influenza virus reproduction in macrophages is both strain- and macrophage type-dependent, and ineffective replication of some viral strains in mouse macrophages has been observed. This review discusses the function of lung macrophages in influenza virus infection in order to better understand the pathogenesis of the influenza virus.

The Viable but Non-Culturable (VBNC) State in Vibrio Species: Why Studying the VBNC State Now Is More Exciting than Ever

During periods that are not conducive for growth or when facing stressful conditions, Vibrios enter a dormant state called the Viable But Non-Culturable (VBNC) state. In this chapter, I will analyse the role of the VBNC state in Vibrio species survival and pathogenesis and the molecular mechanisms regulating this complex phenomenon. I will emphasise some of the novel findings that make studying the VBNC state now more exciting than ever and its significance in the epidemiology of these pathogens and critical role in food safety.

Intestinal colonization against Vibrio cholerae: host and microbial resistance mechanisms

Vibrio cholerae is a non-invasive enteric pathogen known to cause a major public health problem called cholera. The pathogen inhabits the aquatic environment while outside the human host, it is transmitted into the host easily through ingesting contaminated food and water containing the vibrios, thus causing diarrhoea and vomiting. V. cholerae must resist several layers of colonization resistance mechanisms derived from the host or the gut commensals to successfully survive, grow, and colonize the distal intestinal epithelium, thus causing an infection. The colonization resistance mechanisms derived from the host are not specific to V. cholerae but to all invading pathogens. However, some of the gut commensal-derived colonization resistance may be more specific to the pathogen, making it more challenging to overcome. Consequently, the pathogen has evolved well-coordinated mechanisms that sense and utilize the anti-colonization factors to modulate events that promote its survival and colonization in the gut. This review is aimed at discussing how V. cholerae interacts and resists both host- and microbe-specific colonization resistance mechanisms to cause infection.

Transient Glycolytic Complexation of Arsenate Enhances Resistance in the Enteropathogen Vibrio cholerae

The ubiquitous presence of toxic arsenate (As^V) in the environment has raised mechanisms of resistance in all living organisms. Generally, bacterial detoxification of As^V relies on its reduction to arsenite (As^III) by ArsC, followed by the export of As^III by ArsB. However, how pathogenic species resist this metalloid remains largely unknown. Here, we found that Vibrio cholerae, the etiologic agent of the diarrheal disease cholera, outcompetes other enteropathogens when grown on millimolar concentrations of As^V. To do so, V. cholerae uses, instead of ArsCB, the As^V-inducible vc1068-1071 operon (renamed var for vibrio arsenate resistance), which encodes the arsenate repressor ArsR, an alternative glyceraldehyde-3-phosphate dehydrogenase, a putative phosphatase, and the As^V transporter ArsJ. In addition to Var, V. cholerae induces oxidative stress-related systems to counter reactive oxygen species (ROS) production caused by intracellular As^V. Characterization of the var mutants suggested that these proteins function independently from one another and play critical roles in preventing deleterious effects on the cell membrane potential and growth derived from the accumulation As^V. Mechanistically, we demonstrate that V. cholerae complexes As^V with the glycolytic intermediate 3-phosphoglycerate into 1-arseno-3-phosphoglycerate (1As3PG). We further show that 1As3PG is not transported outside the cell; instead, it is subsequently dissociated to enable extrusion of free As^V through ArsJ. Collectively, we propose the formation of 1As3PG as a transient metabolic storage of As^V to curb the noxious effect of free As^V. This study advances our understanding of As^V resistance in bacteria and underscores new points of vulnerability that might be an attractive target for antimicrobial interventions. IMPORTANCE Even though resistance to arsenate has been extensively investigated in environmental bacteria, how enteric pathogens tolerate this toxic compound remains unknown. Here, we found that the cholera pathogen V. cholerae exhibits increased resistance to arsenate compared to closely related enteric pathogens. Such resistance is promoted not by ArsC-dependent reduction of arsenate to arsenite but by an operon encoding an arsenate transporter (ArsJ), an alternative glyceraldehyde 3-phosphate dehydrogenase (VarG), and a putative, uncharacterized phosphatase (VarH). Mechanistically, we demonstrate that V. cholerae detoxifies arsenate by complexing it with the glycolytic intermediate 3-phosphoglycerate into 1-arseno-3-phosphoglycerate (1As3PG). 1As3PG is not transported outside the cell; instead, it is subsequently dissociated by VarH to enable extrusion of free arsenate through ArsJ. Collectively, this study proposes a novel mechanism for arsenate detoxification, entirely independent of arsenate reduction and arsenite extrusion, that enhances V. cholerae resistance to this metalloid compared to other enteric pathogens.

Thiol-based functional mimicry of phosphorylation of the two-component system response regulator ArcA promotes pathogenesis in enteric pathogens

Pathogenic bacteria can rapidly respond to stresses such as reactive oxygen species (ROS) using reversible redox-sensitive oxidation of cysteine thiol (-SH) groups in regulators. Here, we use proteomics to profile reversible ROS-induced thiol oxidation in Vibrio cholerae, the etiologic agent of cholera, and identify two modified cysteines in ArcA, a regulator of global carbon oxidation that is phosphorylated and activated under low oxygen. ROS abolishes ArcA phosphorylation but induces the formation of an intramolecular disulfide bond that promotes ArcA-ArcA interactions and sustains activity. ArcA cysteines are oxidized in cholera patient stools, and ArcA thiol oxidation drives in vitro ROS resistance, colonization of ROS-rich guts, and environmental survival. In other pathogens, such as Salmonella enterica, oxidation of conserved cysteines of ArcA orthologs also promotes ROS resistance, suggesting a common role for ROS-induced ArcA thiol oxidation in modulating ArcA activity, allowing for a balance of expression of stress- and pathogenesis-related genetic programs.

CBS-derived H2S facilitates host colonization of Vibrio cholerae by promoting the iron-dependent catalase activity of KatB

Sensing and resisting oxidative stress is critical for Vibrio cholerae to survive in either the aquatic environment or the gastrointestinal tract. Previous studies mainly focused on the mechanisms of oxidative stress response regulation that rely on enzymatic antioxidant systems, while functions of non-enzymatic antioxidants are rarely discussed in V. cholerae. For the first time, we investigated the role of hydrogen sulfide (H2S), the simplest thiol compound, in protecting V. cholerae against oxidative stress. We found that degradation of L-cysteine by putative cystathionine β-synthase (CBS) is the major source of endogenous H2S in V. cholerae. Our results indicate that intracellular H2S level has a positive correlation with cbs expression, while the enhanced H2S production can render V. cholerae cells less susceptible to H2O2 in vitro. Using proteome analysis and real-time qPCR assay, we found that cbs expression could stimulate the expression of several enzymatic antioxidants, including reactive oxygen species (ROS) detoxifying enzymes SodB, KatG and AhpC, the DNA protective protein DPS and the protein redox regulator Trx1. Assays of ROS detoxification capacities revealed that CBS-derived H2S could promote catalase activity at the post-translational level, especially for KatB, which serves as an important way that endogenous H2S participates in H2O2 detoxification. The enhancement of catalase activity by H2S is achieved through facilitating the uptake of iron. Adult mice experiments showed that cbs mutant has colonization defect, while either complementation of cbs or exogenous supplement of N-Acetyl-L-Cysteine restores its fitness in the host environment. Herein, we proposed that V. cholerae regulates CBS-dependent H2S production for better survival and proliferation under ROS stress.

The Effect of Visible Light on Cell Envelope Subproteome during Vibrio harveyi Survival at 20 °C in Seawater

A number of Vibrio spp. belong to the well-studied model organisms used to understand the strategies developed by marine bacteria to cope with adverse conditions (starvation, suboptimal temperature, solar radiation, etc.) in their natural environments. Temperature and nutrient availability are considered to be the key factors that influence Vibrio harveyi physiology, morphology, and persistence in aquatic systems. In contrast to the well-studied effects of temperature and starvation on Vibrio survival, little is known about the impact of visible light able to cause photooxidative stress. Here we employ V. harveyi ATCC 14126^T as a model organism to analyze and compare the survival patterns and changes in the protein composition of its cell envelope during the long-term permanence of this bacterium in seawater microcosm at 20 °C in the presence and absence of illumination with visible light. We found that V. harveyi exposure to visible light reduces cell culturability likely inducing the entry into the Viable but Non Culturable state (VBNC), whereas populations maintained in darkness remained culturable for at least 21 days. Despite these differences, the starved cells in both populations underwent morphological changes by reducing their size. Moreover, further proteomic analysis revealed a number of changes in the composition of cell envelope potentially accountable for the different adaptation pattern manifested in the absence and presence of visible light.

A multifaceted cellular damage repair and prevention pathway promotes high-level tolerance to β-lactam antibiotics

Bactericidal antibiotics are powerful agents due to their ability to convert essential bacterial functions into lethal processes. However, many important bacterial pathogens are remarkably tolerant against bactericidal antibiotics due to inducible damage repair responses. The cell wall damage response two-component system VxrAB of the gastrointestinal pathogen Vibrio cholerae promotes high-level β-lactam tolerance and controls a gene network encoding highly diverse functions, including negative control over multiple iron uptake systems. How this system contributes to tolerance is poorly understood. Here, we show that β-lactam antibiotics cause an increase in intracellular free iron levels and collateral oxidative damage, which is exacerbated in the ∆vxrAB mutant. Mutating major iron uptake systems dramatically increases ∆vxrAB tolerance to β-lactams. We propose that VxrAB reduces antibiotic-induced toxic iron and concomitant metabolic perturbations by downregulating iron uptake transporters and show that iron sequestration enhances tolerance against β-lactam therapy in a mouse model of cholera infection. Our results suggest that a microorganism's ability to counteract diverse antibiotic-induced stresses promotes high-level antibiotic tolerance and highlights the complex secondary responses elicited by antibiotics.

Glutaredoxins Play an Important Role in the Redox Homeostasis and Symbiotic Capacity of Azorhizobium caulinodans ORS571

Glutaredoxin (GRX) plays an essential role in the control of the cellular redox state and related pathways in many organisms. There is limited information on GRXs from the model nitrogen (N₂)-fixing bacterium Azorhizobium caulinodans. In the present work, we identified and performed functional analyses of monothiol and dithiol GRXs in A. caulinodans in the free-living state and during symbiosis with Sesbania rostrata. Our data show that monothiol GRXs may be very important for bacterial growth under normal conditions and in response to oxidative stress due to imbalance of the redox state in grx mutants of A. caulinodans. Functional redundancies were also observed within monothiol and dithiol GRXs in terms of different physiological functions. The changes in catalase activity and iron content in grx mutants were assumed to favor the maintenance of bacterial resistance against oxidants, nodulation, and N₂ fixation efficiency in this bacterium. Furthermore, the monothiol GRX12 and dithiol GRX34 play a collective role in symbiotic associations between A. caulinodans and Sesbania rostrata. Our study provided systematic evidence that further investigations are required to understand the importance of glutaredoxins in A. caulinodans and other rhizobia.

On or Off: Life-Changing Decisions Made by Vibrio cholerae Under Stress

Vibrio cholerae, the causative agent of the infectious disease, cholera, is commonly found in brackish waters and infects human hosts via the fecal-oral route. V. cholerae is a master of stress resistance as V. cholerae's dynamic lifestyle across different physical environments constantly exposes it to diverse stressful circumstances. Specifically, V. cholerae has dedicated genetic regulatory networks to sense different environmental cues and respond to these signals. With frequent outbreaks costing a tremendous amount of lives and increased global water temperatures providing more suitable aquatic habitats for V. cholerae, cholera pandemics remain a probable catastrophic threat to humanity. Understanding how V. cholerae copes with different environmental stresses broadens our repertoire of measures against infectious diseases and expands our general knowledge of prokaryotic stress responses. In this review, we summarize the regulatory mechanisms of how V. cholerae fights against stresses in vivo and in vitro.

Pathogenicity and virulence regulation of Vibrio cholerae at the interface of host-gut microbiome interactions

The Gram-negative bacterium Vibrio cholerae is responsible for the severe diarrheal pandemic disease cholera, representing a major global public health concern. This pathogen transitions from aquatic reservoirs into epidemics in human populations, and has evolved numerous mechanisms to sense this transition in order to appropriately regulate its gene expression for infection. At the intersection of pathogen and host in the gastrointestinal tract lies the community of native gut microbes, the gut microbiome. It is increasingly clear that the diversity of species and biochemical activities within the gut microbiome represents a driver of infection outcome, through their ability to manipulate the signals used by V. cholerae to regulate virulence and fitness in vivo. A better mechanistic understanding of how commensal microbial action interacts with V. cholerae pathogenesis may lead to novel prophylactic and therapeutic interventions for cholera. Here, we review a subset of this burgeoning field of research.

Ohr and OhrR Are Critical for Organic Peroxide Resistance and Symbiosis in Azorhizobium caulinodans ORS571

Azorhizobium caulinodans is a symbiotic nitrogen-fixing bacterium that forms both root and stem nodules on Sesbania rostrata. During nodule formation, bacteria have to withstand organic peroxides that are produced by plant. Previous studies have elaborated on resistance to these oxygen radicals in several bacteria; however, to the best of our knowledge, none have investigated this process in A. caulinodans. In this study, we identified and characterised the organic hydroperoxide resistance gene ohr (AZC_2977) and its regulator ohrR (AZC_3555) in A. caulinodans ORS571. Hypersensitivity to organic hydroperoxide was observed in an ohr mutant. While using a lacZ-based reporter system, we revealed that OhrR repressed the expression of ohr. Moreover, electrophoretic mobility shift assays demonstrated that OhrR regulated ohr by direct binding to its promoter region. We showed that this binding was prevented by OhrR oxidation under aerobic conditions, which promoted OhrR dimerization and the activation of ohr. Furthermore, we showed that one of the two conserved cysteine residues in OhrR, Cys₁₁, was critical for the sensitivity to organic hydroperoxides. Plant assays revealed that the inactivation of Ohr decreased the number of stem nodules and nitrogenase activity. Our data demonstrated that Ohr and OhrR are required for protecting A. caulinodans from organic hydroperoxide stress and play an important role in the interaction of the bacterium with plants. The results that were obtained in our study suggested that a thiol-based switch in A. caulinodans might sense host organic peroxide signals and enhance symbiosis.

Vibrio cholerae Virulence Activator ToxR Regulates Manganese Transport and Resistance to Reactive Oxygen Species

Like many other pathogens, Vibrio cholerae, the causative agent of cholera, can modulate its gene expression to combat stresses encountered in both aquatic and host environments, including stress posed by reactive oxygen species (ROS). We previously reported that the virulence activator AphB in V. cholerae is involved in ROS resistance. In this study, we found that another key virulence regulator, ToxR, was important for V. cholerae resistance to hydrogen peroxide. Through a genome-wide transposon screen, we discovered that a deletion in mneA, which encodes a manganese exporter, restored ROS resistance of the toxR mutant. We then showed that ToxR did not affect mneA transcription but that the ToxR-regulated major porin OmpU was critical for ROS resistance. The addition of manganese in culture medium restored ROS resistance in both the toxR and ompU mutants. Furthermore, elemental analysis indicated that the intracellular concentration of manganese in both the toxR and ompU mutants was reduced. This may result in intracellular ROS accumulation in these mutants. Our data suggest that ToxR plays an important role in the resistance to reactive oxygen species through the regulation of manganese transport.

Visual Functions Generating Conscious Seeing

Visual functions are reviewed that coincide with conscious as opposed to unconscious vision. Four stages of vision are identified, going from the fully invisible, to subjectively invisible, unattended, and clearly visible. It is proposed that feature extraction, categorization, and some aspects of visual inference occur during full and subjective invisibility. Functions related to perceptual organization, such as grouping and figure-ground segregation, occur during inattention as well as full visibility. It is argued that perceptual organization is the function that is central to understanding the transition from unconscious to conscious seeing. It is discussed what this implies for theories of consciousness such as Recurrent Processing Theory, Higher Order Thought Theory, Integrated Information Theory, and Global Neuronal Workspace Theory.

The Impact of Protozoan Predation on the Pathogenicity of Vibrio cholerae

In the aquatic environment, Vibrio spp. interact with many living organisms that can serve as a replication niche, including heterotrophic protists, or protozoa. Protozoa engulf bacteria and package them into phagosomes where the cells are exposed to low pH, antimicrobial peptides, reactive oxygen/nitrogen species, proteolytic enzymes, and low concentrations of essential metal ions such as iron. However, some bacteria can resist these digestive processes. For example, Vibrio cholerae and Vibrio harveyi can resist intracellular digestion. In order to survive intracellularly, bacteria have acquired and/or developed specific factors that help them to resist the unfavorable conditions encountered inside of the phagosomes. Many of these intra-phagosomal factors used to kill and digest bacteria are highly conserved between eukaryotic cells and thus are also expressed by the innate immune system in the gastrointestinal tract as the first line of defense against bacterial pathogens. Since pathogenic bacteria have been shown to be hypervirulent after they have passed through protozoa, the resistance to digestion by protist hosts in their natural environment plays a key role in enhancing the infectious potential of pathogenic Vibrio spp. This review will investigate the current knowledge in interactions of bacteria with protozoa and human host to better understand the mechanisms used by both protozoa and human hosts to kill bacteria and the bacterial response to them.

Alkyl hydroperoxide reductase is important for oxidative stress resistance and symbiosis in Azorhizobium caulinodans

Reactive oxygen species (ROS) are not only toxic products of oxygen from aerobic metabolism or stress but also signalling molecules involved in the development of the legume-Rhizobium symbiosis. To assess the importance of alkyl hydroperoxide reductase (AhpCD) in the nitrogen-fixating bacterium Azorhizobium caulinodans, we investigated the phenotypes of the ∆ahpCD strain with regards to ROS resistance and symbiotic interactions with Sesbania rostrata. The ∆ahpCD strain was notably more sensitive than its parent strain to hydrogen peroxide (H2O2) but not to two organic peroxides, in the early log phase. The expression of ahpCD was not controlled by a LysR-type transcriptional activator either in vitro or in vivo. The catalase activity of the ∆ahpCD strain was affected at a relatively low level of H2O2 stress. Furthermore, the ∆ahpCD strain induced a reduced number of stem nodules in S. rostrata with lowering of nitrogenase activity. These data suggest that A. caulinodans AhpCD is not only important for H2O2 detoxification in vitro but also critical for symbiosis with S. rostrata. Functional analysis of AhpCD is worth investigating in other rhizobia to gain a comprehensive view of its contributions to ROS defence and symbiotic association with legumes.

Cyclic di-GMP Increases Catalase Production and Hydrogen Peroxide Tolerance in Vibrio cholerae

Vibrio cholerae is a Gram-negative bacterial pathogen that causes the disease cholera, which affects nearly 1 million people each year. In between outbreaks, V. cholerae resides in fresh and salt water environments, where it is able to persist through changes in temperature, oxygen, and salinity. One key characteristic that promotes environmental persistence of V. cholerae is the ability to form multicellular communities, called biofilms, that often adhere to biotic and abiotic sources. Biofilm formation in V. cholerae is positively regulated by the dinucleotide second messenger cyclic dimeric GMP (c-di-GMP). While most research on the c-di-GMP regulon has focused on biofilm formation or motility, we hypothesized that the c-di-GMP signaling network encompassed a larger set of effector functions than reported. We found that high intracellular c-di-GMP increased catalase activity ∼4-fold relative to strains with unaltered c-di-GMP. Genetic studies demonstrated that c-di-GMP mediated catalase activity was due to increased expression of the catalase-encoding gene katB Moreover, c-di-GMP mediated regulation of catalase activity and katB expression required the c-di-GMP dependent transcription factors VpsT and VpsR. Lastly, we found that high c-di-GMP increased survival after H₂O₂ challenge in a katB-, vpsR-, and vpsT-dependent manner. Our results indicate that antioxidant production is regulated by c-di-GMP uncovering a new node in the growing VpsT and VpsR c-di-GMP signaling network of V. cholerae IMPORTANCE As a result of infection with V. cholerae, patients become dehydrated, leading to death if not properly treated. The aquatic environment is the natural reservoir for V. cholerae, where it can survive alterations in temperature, salinity, and oxygen. The second messenger molecule c-di-GMP is an important signal regulating host and aquatic environmental persistence because it controls whether V. cholerae will form a biofilm or disperse through flagellar motility. In this work, we demonstrate another function of c-di-GMP in V. cholerae biology: promoting tolerance to the reactive oxygen species H₂O₂ through the differential regulation of catalase expression. Our results suggest a mechanism where c-di-GMP simultaneously controls biofilm formation and antioxidant production, which could promote persistence in human and marine environments.

Reactive nitrogen species induced catalases promote a novel nitrosative stress tolerance mechanism in Vibrio cholerae

Vibrio cholerae faces nitrosative stress during successful colonization in intestine. Very little information is available on the nitrosative stress protective mechanisms of V. cholerae. Reports show that NorR regulon control two genes hmpA and nnrS responsible for nitric oxide (NO) detoxification in V. cholerae. In the present study we first time report a novel role of V. cholerae catalases under nitrosative stress. Using zymogram analysis of catalase we showed that KatB and KatG activity were induced within 30 min in V. cholerae in the presence of sodium nitroprusside (SNP), a NO donor compound. Surprisingly, V. cholerae cell survival was found to be decreased under nitrosative stress if catalase activities were blocked by ATz, a catalase inhibitor. Flow cytometry study was conducted to detect reactive oxygen species (ROS) and reactive nitrogen species (RNS) using DHE and DHR123, fluorescent probes respectively. Short exposure of SNP to V. cholerae did not generate ROS but RNS was detectable within 30 min. Total glutathione content was increased in V. cholerae cells under nitrosative stress. Furthermore, Superoxide dismutase (SOD) and Glutathione reductase (GR) activities remained unchanged under nitrosative stress in V. cholerae indicated antioxidant role of NO which could produce peroxynitrite. To investigate the role of catalase induction under nitrosative stress in V. cholerae, we conducted peroxynitrite reductase assay using cell lysates. Interestingly, SNP treated V. cholerae cell lysates showed lowest DHR123 oxidation compared to the control set. The extent of DHR123 oxidation was more in V. cholerae cell lysate when catalases were blocked by ATz.

Bacterioferritin comigratory protein is important in hydrogen peroxide resistance, nodulation, and nitrogen fixation in Azorhizobium caulinodans

Reactive oxygen species are not only harmful for rhizobia but also required for the establishment of symbiotic interactions between rhizobia and their legume hosts. In this work, we first investigated the preliminary role of the bacterioferritin comigratory protein (BCP), a member of the peroxiredoxin family, in the nitrogen-fixing bacterium Azorhizobium caulinodans. Our data revealed that the bcp-deficient strain of A. caulinodans displayed an increased sensitivity to inorganic hydrogen peroxide (H₂O₂) but not to two organic peroxides in a growth-phase-dependent manner. Meanwhile, BCP was found to be involved in catalase activity under relatively low H₂O₂ conditions. Furthermore, nodulation and N₂ fixation were significantly impaired by mutation of the bcp gene in A. caulinodans. Our work initially documented the importance of BCP in the bacterial defence against H₂O₂ in the free-living stage of rhizobia and during their symbiotic interactions with legumes. Molecular signalling in vivo is required to decipher the holistic functions of BCP in A. caulinodans as well as in other rhizobia.

Hypermutation-induced in vivo oxidative stress resistance enhances Vibrio cholerae host adaptation

Bacterial pathogens are highly adaptable organisms, a quality that enables them to overcome changing hostile environments. For example, Vibrio cholerae, the causative agent of cholera, is able to colonize host small intestines and combat host-produced reactive oxygen species (ROS) during infection. To dissect the molecular mechanisms utilized by V. cholerae to overcome ROS in vivo, we performed a whole-genome transposon sequencing analysis (Tn-seq) by comparing gene requirements for colonization using adult mice with and without the treatment of the antioxidant, N-acetyl cysteine. We found that mutants of the methyl-directed mismatch repair (MMR) system, such as MutS, displayed significant colonization advantages in untreated, ROS-rich mice, but not in NAC-treated mice. Further analyses suggest that the accumulation of both catalase-overproducing mutants and rugose colony variants in NAC^- mice was the leading cause of mutS mutant enrichment caused by oxidative stress during infection. We also found that rugose variants could revert back to smooth colonies upon aerobic, in vitro culture. Additionally, the mutation rate of wildtype colonized in NAC^- mice was significantly higher than that in NAC⁺ mice. Taken together, these findings support a paradigm in which V. cholerae employs a temporal adaptive strategy to battle ROS during infection, resulting in enriched phenotypes. Moreover, ΔmutS passage and complementation can be used to model hypermuation in diverse pathogens to identify novel stress resistance mechanisms.

Cholera is a devastating diarrheal disease that is still endemic to many developing nations, with the worst outbreak in history having occurred recently in Yemen. Vibrio cholerae, the causative agent of cholera, transitions from aquatic reservoirs to the human gastrointestinal tract, where it expresses virulence factors to facilitate colonization of the small intestines and to combat host innate immune effectors, such as reactive oxygen species (ROS). We applied a genome-wide transposon screen (Tn-seq) and identified that deletion of mutS, which is part of DNA mismatch repair system, drastically increased colonization in ROS-rich mice. The deletion of mutS led to the accumulation of catalase-overproducing mutants and a high frequency rugose phenotype when exposed to ROS selective pressures in vivo. Additionally, ROS elevated mutation frequency in wildtype, both in vitro and in vivo. Our data imply that V. cholerae may modulate mutation frequency as a temporal adaptive strategy to overcome oxidative stress and to enhance infectivity.

Mechanisms of Bacterial Tolerance and Persistence in the Gastrointestinal and Respiratory Environments

Pathogens that infect the gastrointestinal and respiratory tracts are subjected to intense pressure due to the environmental conditions of the surroundings. This pressure has led to the development of mechanisms of bacterial tolerance or persistence which enable microorganisms to survive in these locations. In this review, we analyze the general stress response (RpoS mediated), reactive oxygen species (ROS) tolerance, energy metabolism, drug efflux pumps, SOS response, quorum sensing (QS) bacterial communication, (p)ppGpp signaling, and toxin-antitoxin (TA) systems of pathogens, such as Escherichia coli, Salmonella spp., Vibrio spp., Helicobacter spp., Campylobacter jejuni, Enterococcus spp., Shigella spp., Yersinia spp., and Clostridium difficile, all of which inhabit the gastrointestinal tract. The following respiratory tract pathogens are also considered: Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii, Burkholderia cenocepacia, and Mycobacterium tuberculosis Knowledge of the molecular mechanisms regulating the bacterial tolerance and persistence phenotypes is essential in the fight against multiresistant pathogens, as it will enable the identification of new targets for developing innovative anti-infective treatments.

Engineering microbial physiology with synthetic polymers: cationic polymers induce biofilm formation in Vibrio cholerae and downregulate the expression of virulence genes

Here we report the first application of non-bactericidal synthetic polymers to modulate the physiology of a bacterial pathogen. Poly(N-[3-(dimethylamino)propyl] methacrylamide) (P1) and poly(N-(3-aminopropyl)methacrylamide) (P2), cationic polymers that bind to the surface of V. cholerae, the infectious agent causing cholera disease, can sequester the pathogen into clusters. Upon clustering, V. cholerae transitions to a sessile lifestyle, characterised by increased biofilm production and the repression of key virulence factors such as the cholera toxin (CTX). Moreover, clustering the pathogen results in the minimisation of adherence and toxicity to intestinal epithelial cells. Our results suggest that the reduction in toxicity is associated with the reduction to the number of free bacteria, but also the downregulation of toxin production. Finally we demonstrate that these polymers can reduce colonisation of zebrafish larvae upon ingestion of water contaminated with V. cholerae. Overall, our results suggest that the physiology of this pathogen can be modulated without the need to genetically manipulate the microorganism and that this modulation is an off-target effect that results from the intrinsic ability of the pathogen to sense and adapt to its environment. We believe these findings pave the way towards a better understanding of the interactions between pathogenic bacteria and polymeric materials and will underpin the development of novel antimicrobial polymers.

Detailed Distribution of Corneal Epithelial Thickness and Correlated Characteristics Measured with SD-OCT in Myopic Eyes

Purpose

To investigate the detailed distribution of corneal epithelial thickness in single sectors and its correlated characteristics in myopic eyes.

Methods

SD-OCT was used to measure the corneal epithelial thickness distribution profile. Differences of corneal epithelial thickness between different parameters and some correlations of characteristics were calculated.

Results

The thickest and thinnest part of epithelium were found at the nasal-inferior sector (P < 0.05) and at the superior side (P < 0.05). respectively. Subjects in the low and moderate myopia groups have thicker epithelial thickness than those in the high myopia group (P < 0.05). Epithelial thickness was 1.39 μm thicker in male subjects than in female subjects (P < 0.001). There was a slight negative correlation between corneal epithelial thickness and age (r = −0.13, P = 0.042). Weak positive correlations were found between corneal epithelial thickness and corneal thickness (r = 0.148, P = 0.031). No correlations were found between corneal epithelial thickness, astigmatism axis, corneal front curvature, and IOP.

Conclusions

The epithelial thickness is not evenly distributed across the cornea. The thickest location of the corneal epithelium is at the nasal-inferior sector. People with high myopia tend to have thinner corneal epithelium than low–moderate myopic patients. The corneal epithelial thickness is likely to be affected by some parameters, such as age, gender, and corneal thickness.

OxyR2 Modulates OxyR1 Activity and Vibrio cholerae Oxidative Stress Response

Bacteria have developed capacities to deal with different stresses and adapt to different environmental niches. The human pathogen Vibrio cholerae, the causative agent of the severe diarrheal disease cholera, utilizes the transcriptional regulator OxyR to activate genes related to oxidative stress resistance, including peroxiredoxin PrxA, in response to hydrogen peroxide. In this study, we identified another OxyR homolog in V. cholerae, which we named OxyR2, and we renamed the previous OxyR OxyR1. We found that OxyR2 is required to activate its divergently transcribed gene ahpC, encoding an alkylhydroperoxide reductase, independently of H₂O₂ A conserved cysteine residue in OxyR2 is critical for this function. Mutation of either oxyR2 or ahpC rendered V. cholerae more resistant to H₂O₂ RNA sequencing analyses indicated that OxyR1-activated oxidative stress-resistant genes were highly expressed in oxyR2 mutants even in the absence of H₂O₂ Further genetic analyses suggest that OxyR2-activated AhpC modulates OxyR1 activity by maintaining low intracellular concentrations of H₂O₂ Furthermore, we showed that ΔoxyR2 and ΔahpC mutants were less fit when anaerobically grown bacteria were exposed to low levels of H₂O₂ or incubated in seawater. These results suggest that OxyR2 and AhpC play important roles in the V. cholerae oxidative stress response.

Type VI secretion system contributes to Enterohemorrhagic Escherichia coli virulence by secreting catalase against host reactive oxygen species (ROS)

Enterohemorrhagic Escherichia coli (EHEC) is one major type of contagious and foodborne pathogens. The type VI secretion system (T6SS) has been shown to be involved in the bacterial pathogenicity and bacteria-bacteria competition. Here, we show that EHEC could secrete a novel effector KatN, a Mn-containing catalase, in a T6SS-dependent manner. Expression of katN is promoted by RpoS and OxyR and repressed by H-NS, and katN contributes to bacterial growth under oxidative stress in vitro. KatN could be secreted into host cell cytosol after EHEC is phagocytized by macrophage, which leads to decreased level of intracellular reactive oxygen species (ROS) and facilitates the intramacrophage survival of EHEC. Finally, animal model results show that the deletion mutant of T6SS was attenuated in virulence compared with the wild type strain, while the deletion mutant of katN had comparable virulence to the wild type strain. Taken together, our findings suggest that EHEC could sense oxidative stress in phagosome and decrease the host cell ROS by secreting catalase KatN to facilitate its survival in the host cells.

Legionella pneumophila OxyR Is a Redundant Transcriptional Regulator That Contributes to Expression Control of the Two-Component CpxRA System

Nominally an environmental organism, Legionella pneumophila is an intracellular parasite of protozoa but is also the causative agent of the pneumonia termed Legionnaires' disease, which results from inhalation of aerosolized bacteria by susceptible humans. Coordination of gene expression by a number of identified regulatory factors, including OxyR, assists L. pneumophila in adapting to the stresses of changing environments. L. pneumophila OxyR (OxyR_Lp) is an ortholog of Escherichia coli OxyR; however, OxyR_Lp was shown elsewhere to be functionally divergent, such that it acts as a transcription regulator independently of the oxidative stress response. In this study, the use of improved gene deletion methods has enabled us to generate an unmarked in-frame deletion of oxyR in L. pneumophila Lack of OxyR_Lp did not affect in vitro growth or intracellular growth in Acanthamoeba castellanii protozoa and U937-derived macrophages. The expression of OxyR_Lp does not appear to be regulated by CpxR, even though purified recombinant CpxR bound a DNA sequence similar to that reported for CpxR elsewhere. Surprisingly, a lack of OxyR_Lp resulted in elevated activity of the promoters located upstream of icmR and the lpg1441-cpxA operon, and OxyR_Lp directly bound to these promoter regions, suggesting that OxyR_Lp is a direct repressor. Interestingly, a strain overexpressing OxyR_Lp demonstrated reduced intracellular growth in A. castellanii but not in U937-derived macrophages, suggesting that balanced expression control of the two-component CpxRA system is necessary for survival in protozoa. Taken together, this study suggests that OxyR_Lp is a functionally redundant transcriptional regulator in L. pneumophila under the conditions evaluated herein.IMPORTANCELegionella pneumophila is an environmental pathogen, with its transmission to the human host dependent upon its ability to replicate in protozoa and survive within its aquatic niche. Understanding the genetic factors that contribute to L. pneumophila survival within each of these unique environments will be key to limiting future point-source outbreaks of Legionnaires' disease. The transcriptional regulator L. pneumophila OxyR (OxyR_Lp) has been previously identified as a potential regulator of virulence traits warranting further investigation. This study demonstrated that oxyR is nonessential for L. pneumophila survival in vitro and in vivo via mutational analysis. While the mechanisms of how OxyR_Lp expression is regulated remain elusive, this study shows that OxyR_Lp negatively regulates the expression of the cpxRA two-component system necessary for intracellular survival in protozoa.

OxyR-activated expression of Dps is important for Vibrio cholerae oxidative stress resistance and pathogenesis

Vibrio cholerae is the causative agent of cholera, a dehydrating diarrheal disease. This Gram-negative pathogen is able to modulate its gene expression in order to combat stresses encountered in both aquatic and host environments, including stress posed by reactive oxygen species (ROS). In order to further the understanding of V. cholerae’s transcriptional response to ROS, we performed an RNA sequencing analysis to determine the transcriptional profile of V. cholerae when exposed to hydrogen hydroperoxide. Of 135 differentially expressed genes, VC0139 was amongst the genes with the largest induction. VC0139 encodes a protein homologous to the DPS (DNA-binding protein from starved cells) protein family, which are widely conserved and are implicated in ROS resistance in other bacteria. Using a promoter reporter assay, we show that during exponential growth, dps is induced by H₂O₂ in a manner dependent on the ROS-sensing transcriptional regulator, OxyR. Upon entry into stationary phase, the major stationary phase regulator RpoS is required to transcribe dps. Deletion of dps impaired V. cholerae resistance to both inorganic and organic hydroperoxides. Furthermore, we show that Dps is involved in resistance to multiple environmental stresses. Finally, we found that Dps is important for V. cholerae adult mouse colonization, but becomes dispensable in the presence of antioxidants. Taken together, our results suggest that Dps plays vital roles in both V. cholerae stress resistance and pathogenesis.

Thiol-based switch mechanism of virulence regulator AphB modulates oxidative stress response in Vibrio cholerae

Bacterial pathogens display versatile gene expression to adapt to changing surroundings. For example, Vibrio cholerae, the causative agent of cholera, utilizes distinct genetic programs to combat reactive oxygen species (ROS) in aquatic environments or during host infection. We previously reported that the virulence activator AphB in V. cholerae is involved in ROS resistance. Here by performing a genetic screen, we show that AphB represses ROS resistance gene ohrA, which is also repressed by another regulator, OhrR. Reduced forms of both AphB and OhrR directly bind to the ohrA promoter and repress its expression, whereas organic hydroperoxides such as cumene hydroperoxide (CHP) deactivate AphB and OhrR. OhrA is critical for V. cholerae adult mouse colonization but is dispensable when the mice are treated with antioxidants. Furthermore, similar to our previous finding that AphB and OhrR exhibit different reduction rates during the shift from oxic to anoxic environments, we found that AphB is also oxidized more slowly than OhrR under peroxide stress or exposure to oxygen. This differential regulation optimizes the expression of ohrA and contributes to V. cholerae's ability to survive in a variety of environmental niches that contain different levels of ROS.

Catalases and PhoB/PhoR system independently contribute to oxidative stress resistance in Vibrio cholerae O1

All cells are subjected to oxidative stress, a condition under which reactive oxygen species (ROS) production exceeds elimination. Bacterial defences against ROS include synthesis of antioxidant enzymes like peroxidases and catalases. Vibrio cholerae can produce two distinct catalases, KatB and KatG, which contribute to ROS homeostasis. In this study, we analysed the mechanism behind katG and katB expression in two V. cholerae O1 pandemic strains, O395 and N16961, of classical and El Tor biotypes, respectively. Both strains express these genes, especially at stationary phase. However, El Tor N16961 produces higher KatB and KatG levels and is much more resistant to peroxide challenge than the classical strain, confirming a direct relationship between catalase activity and oxidative stress resistance. Moreover, we showed that katG and katB expression levels depend on inorganic phosphate (Pi) availability, in contrast to other bacterial species. In N16961, katB and katG expression is reduced under Pi limitation relative to Pi abundance. Total catalase activity in N16961 and its phoB mutant cells was similar, independently of growth conditions, indicating that the PhoB/PhoR system is not required for katB and katG expression. However, N16961 cells from Pi-limited cultures were 50-100-fold more resistant to H2O2 challenge and accumulated less ROS than phoB mutant cells. Together, these findings suggest that, besides KatB and KatG, the PhoB/PhoR system is an important protective factor against ROS in V. cholerae N16961. They also corroborate previous results from our and other groups, suggesting that the PhoB/PhoR system is fundamental for V. cholerae biology.

The Na+-Translocating NADH:Quinone Oxidoreductase Enhances Oxidative Stress in the Cytoplasm of Vibrio cholerae

We searched for a source of reactive oxygen species (ROS) in the cytoplasm of the human pathogen Vibrio cholerae and addressed the mechanism of ROS formation using the dye 2',7'-dichlorofluorescein diacetate (DCFH-DA) in respiring cells. By comparing V. cholerae strains with or without active Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR), this respiratory sodium ion redox pump was identified as a producer of ROS in vivo The amount of cytoplasmic ROS detected in V. cholerae cells producing variants of Na(+)-NQR correlated well with rates of superoxide formation by the corresponding membrane fractions. Membranes from wild-type V. cholerae showed increased superoxide production activity (9.8 ± 0.6 μmol superoxide min(-1) mg(-1) membrane protein) compared to membranes from the mutant lacking Na(+)-NQR (0.18 ± 0.01 μmol min(-1) mg(-1)). Overexpression of plasmid-encoded Na(+)-NQR in the nqr deletion strain resulted in a drastic increase in the formation of superoxide (42.6 ± 2.8 μmol min(-1) mg(-1)). By analyzing a variant of Na(+)-NQR devoid of quinone reduction activity, we identified the reduced flavin adenine dinucleotide (FAD) cofactor of cytoplasmic NqrF subunit as the site for intracellular superoxide formation in V. cholerae The impact of superoxide formation by the Na(+)-NQR on the virulence of V. cholerae is discussed.

IMPORTANCE

In several studies, it was demonstrated that the Na(+)-NQR in V. cholerae affects virulence in a yet unknown manner. We identified the reduced FAD cofactor in the NADH-oxidizing NqrF subunit of the Na(+)-NQR as the site of superoxide formation in the cytoplasm of V. cholerae Our study provides the framework to understand how reactive oxygen species formed during respiration could participate in the regulated expression of virulence factors during the transition from aerobic to microaerophilic (intestinal) habitats. This hypothesis may turn out to be right for many other pathogens which, like V. cholerae, depend on the Na(+)-NQR as the sole electrogenic NADH dehydrogenase.

Interplay between microbial d-amino acids and host d-amino acid oxidase modifies murine mucosal defence and gut microbiota

L-Amino acids are the building blocks for proteins synthesized in ribosomes in all kingdoms of life, but d-amino acids (d-aa) have important non-ribosome-based functions(1). Mammals synthesize d-Ser and d-Asp, primarily in the central nervous system, where d-Ser is critical for neurotransmission(2). Bacteria synthesize a largely distinct set of d-aa, which become integral components of the cell wall and are also released as free d-aa(3,4). However, the impact of free microbial d-aa on host physiology at the host-microbial interface has not been explored. Here, we show that the mouse intestine is rich in free d-aa that are derived from the microbiota. Furthermore, the microbiota induces production of d-amino acid oxidase (DAO) by intestinal epithelial cells, including goblet cells, which secrete the enzyme into the lumen. Oxidative deamination of intestinal d-aa by DAO, which yields the antimicrobial product H2O2, protects the mucosal surface in the small intestine from the cholera pathogen. DAO also modifies the composition of the microbiota and is associated with microbial induction of intestinal sIgA. Collectively, these results identify d-aa and DAO as previously unrecognized mediators of microbe-host interplay and homeostasis on the epithelial surface of the small intestine.

A single gene of a commensal microbe affects host susceptibility to enteric infection

Indigenous microbes inside the host intestine maintain a complex self-regulating community. The mechanisms by which gut microbes interact with intestinal pathogens remain largely unknown. Here we identify a commensal Escherichia coli strain whose expansion predisposes mice to infection by Vibrio cholerae, a human pathogen. We refer to this strain as ‘atypical’ E. coli (atEc) because of its inability to ferment lactose. The atEc strain is resistant to reactive oxygen species (ROS) and proliferates extensively in antibiotic-treated adult mice. V. cholerae infection is more severe in neonatal mice transplanted with atEc compared with those transplanted with a typical E. coli strain. Intestinal ROS levels are decreased in atEc-transplanted mice, favouring proliferation of ROS-sensitive V. cholerae. An atEc mutant defective in ROS degradation fails to facilitate V. cholerae infection when transplanted, suggesting that host infection susceptibility can be regulated by a single gene product of one particular commensal species.

The interactions between gut bacteria and enteric pathogens are poorly understood. Here, Yoon et al. show that subinhibitory antibiotic treatment in a mouse model leads to overgrowth of an E. coli strain carrying a catalase-encoding gene that enhances infection with the human pathogen Vibrio cholerae.

Protective roles of katG-homologous genes against extrinsic peroxides in Vibrio parahaemolyticus

The marine foodborne enteropathogen, Vibrio parahaemolyticus, has four putative catalase genes. Function of the katG-homologous genes, katG1(VPA0768) and katG2(VPA0453), was examined using gene deletion mutants, and compared with those of the katE-homologous genes, katE1(VPA1418) and katE2(VPA0305). Bacterial growth of ΔkatG1 was significantly delayed in the presence of 200-300 μM H2O2, and such inhibition was enhanced when incubation temperature was lowered from 37°C to 22°C. In the stationary phase, the ΔkatG1 strain was more susceptible to the lethal dosage of H2O2 than the ΔkatE1 strain. The minimum inhibitory concentrations and minimum bactericidal concentrations revealed that ΔkatE1/ΔkatE2 strains were more susceptible to H2O2 than the ΔkatG1/ΔkatG2 strains in exponential phase, while ΔkatG1 was more susceptible than the ΔkatE1/ΔkatE2 strains in the starved culture. This study demonstrated the chief antioxidative role of katG1 in the stationary phase and starved culture of V. parahaemolyticus, while katG1 and katG2 were also responsive to H2O2 and cumene hydroperoxide in the exponential phase.

Functions of VPA1418 and VPA0305 Catalase Genes in Growth of Vibrio parahaemolyticus under Oxidative Stress

The marine foodborne enteropathogen Vibrio parahaemolyticus has four putative catalase genes. The functions of two katE-homologous genes, katE1 (VPA1418) and katE2 (VPA0305), in the growth of this bacterium were examined using gene deletion mutants with or without complementary genes. The growth of the mutant strains in static or shaken cultures in a rich medium at 37°C or at low temperatures (12 and 4°C), with or without competition from Escherichia coli, did not differ from that of the parent strain. When 175 μM extrinsic H2O2 was added to the culture medium, bacterial growth of the ΔkatE1 strain was delayed and growth of the ΔkatE1 ΔkatE2 and ΔkatE1 ΔahpC1 double mutant strains was completely inhibited at 37°C for 8 h. The sensitivity of the ΔkatE1 strain to the inhibition of growth by H2O2 was higher at low incubation temperatures (12 and 22°C) than at 37°C. The determined gene expression of these catalase and ahpC genes revealed that katE1 was highly expressed in the wild-type strain at 22°C under H2O2 stress, while the katE2 and ahpC genes may play an alternate or compensatory role in the ΔkatE1 strain. This study demonstrated that katE1 encodes the chief functional catalase for detoxifying extrinsic H2O2 during logarithmic growth and that the function of these genes was influenced by incubation temperature.

Influence of oxyR on Growth, Biofilm Formation, and Mobility of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a common marine food-borne enteropathogen. In this study, we examined the antioxidative activity, growth, biofilm formation, and cell mobility of an oxyR deletion mutant and its genetically complementary strain of V. parahaemolyticus. oxyR is the regulator of catalase and ahpC genes. Protection against extrinsic H2O2 and against the organic peroxides cumene hydroperoxide and tert-butyl hydroperoxide was weaker in the deletion mutant than in its parent strain. Expression of the major functional antioxidative genes, ahpC1 and VPA1418, was markedly decreased in the oxyR mutant. Growth of this mutant on agar medium was significantly inhibited by autoclaved 0.25% glucose and by 0.25% dipotassium hydrogen phosphate, 0.5% monosaccharides (glucose, galactose, xylose, and arabinose), or 114.8 mM phosphates. The inhibition of the growth of this oxyR mutant by extrinsic peroxides, autoclaved sugars, and phosphates was eliminated by the complementary oxyR gene or by the addition of catalase to the autoclaved medium, while no inhibition of growth was observed when filter-sterilized sugars were used. The formation of biofilm and swimming mobility were significantly inhibited in the oxyR mutant relative to that in the wild-type strain. This investigation demonstrates the antioxidative function of oxyR in V. parahaemolyticus and its possible roles in biofilm formation, cell mobility, and the protection of growth in heated rich medium.

Functional RelBE-Family Toxin-Antitoxin Pairs Affect Biofilm Maturation and Intestine Colonization in Vibrio cholerae

Toxin–antitoxin (TA) systems are small genetic elements that typically encode a stable toxin and its labile antitoxin. These cognate pairs are abundant in prokaryotes and have been shown to regulate various cellular functions. Vibrio cholerae, a human pathogen that is the causative agent of cholera, harbors at least thirteen TA loci. While functional HigBA, ParDE have been shown to stabilize plasmids and Phd/Doc to mediate cell death in V. cholerae, the function of seven RelBE-family TA systems is not understood. In this study we investigated the function of the RelBE TA systems in V. cholerae physiology and found that six of the seven relBE loci encoded functional toxins in E. coli. Deletion analyses of each relBE locus indicate that RelBE systems are involved in biofilm formation and reactive oxygen species (ROS) resistance. Interestingly, all seven relBE loci are induced under the standard virulence induction conditions and two of the relBE mutants displayed a colonization defect, which was not due to an effect on virulence gene expression. Although further studies are needed to characterize the mechanism of action, our study reveals that RelBE systems are important for V. cholerae physiology.

Catalase Expression Is Modulated by Vancomycin and Ciprofloxacin and Influences the Formation of Free Radicals in Staphylococcus aureus Cultures

Detection of free radicals in biological systems is challenging due to their short half-lives. We have applied electron spin resonance (ESR) spectroscopy combined with spin traps using the probes PBN (N-tert-butyl-α-phenylnitrone) and DMPO (5,5-dimethyl-1-pyrroline N-oxide) to assess free radical formation in the human pathogen Staphylococcus aureus treated with a bactericidal antibiotic, vancomycin or ciprofloxacin. While we were unable to detect ESR signals in bacterial cells, hydroxyl radicals were observed in the supernatant of bacterial cell cultures. Surprisingly, the strongest signal was detected in broth medium without bacterial cells present and it was mitigated by iron chelation or by addition of catalase, which catalyzes the decomposition of hydrogen peroxide to water and oxygen. This suggests that the signal originates from hydroxyl radicals formed by the Fenton reaction, in which iron is oxidized by hydrogen peroxide. Previously, hydroxyl radicals have been proposed to be generated within bacterial cells in response to bactericidal antibiotics. We found that when S. aureus was exposed to vancomycin or ciprofloxacin, hydroxyl radical formation in the broth was indeed increased compared to the level seen with untreated bacterial cells. However, S. aureus cells express catalase, and the antibiotic-mediated increase in hydroxyl radical formation was correlated with reduced katA expression and catalase activity in the presence of either antibiotic. Therefore, our results show that in S. aureus, bactericidal antibiotics modulate catalase expression, which in turn influences the formation of free radicals in the surrounding broth medium. If similar regulation is found in other bacterial species, it might explain why bactericidal antibiotics are perceived as inducing formation of free radicals.

Bacterial pathogens, virulence mechanism and host defence in marine macroalgae

Macroalgae are important ecosystem engineers in temperate marine waters. The function of macroalgae is intimately linked to the composition and structure of their epibiotic bacterial, communities, and evidence has emerged that bacteria can also have a negative impact on their host by causing disease. A few examples exist where bacteria have been unambiguously linked to macroalgal disease, however in many cases, pathogenicity has not been clearly separated from saprophytic behaviour or secondary colonization after disease initiation. Nevertheless, pathogenic pressure by bacteria might be substantial, as macroalgae have evolved a range of innate and induced defence mechanism that have the potential to control bacterial attacks. The presence and abundance of virulence factors in marine bacteria, which have not previously been recognized as pathogens, also represents an underappreciated, opportunistic potential for disease. Given that virulence expression in opportunistic pathogens is often dependent on environmental conditions, we predict that current and future anthropogenic changes in the marine environment will lead to an increase in the occurrence of macroalgal disease. This review highlights important areas of research that require future attention to understand the link between environmental change, opportunistic pathogens and macroalgal health in the world's oceans.