H-NS is a negative regulator of the two hemolysin/cytotoxin gene clusters in Vibrio anguillarum

Attenuation of a DNA cruciform by a conserved regulator directs T3SS1 mediated virulence in Vibrio parahaemolyticus

Pathogenic Vibrio species account for 3-5 million annual life-threatening human infections. Virulence is driven by bacterial hemolysin and toxin gene expression often positively regulated by the winged helix-turn-helix (wHTH) HlyU transcriptional regulator family and silenced by histone-like nucleoid structural protein (H-NS). In the case of Vibrio parahaemolyticus, HlyU is required for virulence gene expression associated with type 3 Secretion System-1 (T3SS1) although its mechanism of action is not understood. Here, we provide evidence for DNA cruciform attenuation mediated by HlyU binding to support concomitant virulence gene expression. Genetic and biochemical experiments revealed that upon HlyU mediated DNA cruciform attenuation, an intergenic cryptic promoter became accessible allowing for exsA mRNA expression and initiation of an ExsA autoactivation feedback loop at a separate ExsA-dependent promoter. Using a heterologous E. coli expression system, we reconstituted the dual promoter elements which revealed that HlyU binding and DNA cruciform attenuation were strictly required to initiate the ExsA autoactivation loop. The data indicate that HlyU acts to attenuate a transcriptional repressive DNA cruciform to support T3SS1 virulence gene expression and reveals a non-canonical extricating gene regulation mechanism in pathogenic Vibrio species.

The Temperature-Dependent Expression of the High-Pathogenicity Island Encoding Piscibactin in Vibrionaceae Results From the Combined Effect of the AraC-Like Transcriptional Activator PbtA and Regulatory Factors From the Recipient Genome

The high-pathogenicity island irp-HPI is widespread among Vibrionaceae encoding the piscibactin siderophore system. The expression of piscibactin genes in the fish pathogen Vibrio anguillarum is favored by low temperatures. However, information about the regulatory mechanism behind irp-HPI gene expression is scarce. In this work, in-frame deletion mutants of V. anguillarum defective in the putative regulators AraC1 and AraC2, encoded by irp-HPI, and in the global regulators H-NS and ToxRS, were constructed and their effect on irp-HPI gene expression was analyzed at 15 and 25°C. The results proved that only AraC1 (renamed as PbtA) is required for the expression of piscibactin biosynthesis and transport genes. PbtA inactivation led to an inability to grow under iron restriction, a loss of the outer membrane piscibactin transporter FrpA, and a significant decrease in virulence for fish. Inactivation of the global repressor H-NS, which is involved in silencing of horizontally acquired genes, also resulted in a lower transcriptional activity of the frpA promoter. Deletion of toxR-S, however, did not have a relevant effect on the expression of the irp-HPI genes. Therefore, while irp-HPI would not be part of the ToxR regulon, H-NS must exert an indirect effect on piscibactin gene expression. Thus, the temperature-dependent expression of the piscibactin-encoding pathogenicity island described in V. anguillarum is the result of the combined effect of the AraC-like transcriptional activator PbtA, harbored in the island, and other not yet defined regulator(s) encoded by the genome. Furthermore, different expression patterns were detected within different irp-HPI evolutionary lineages, which supports a long-term evolution of the irp-HPI genomic island within Vibrionaceae. The mechanism that modulates piscibactin gene expression could also be involved in global regulation of virulence factors in response to temperature changes.

Spatiotemporal Regulation of Vibrio Exotoxins by HlyU and Other Transcriptional Regulators

After invading a host, bacterial pathogens secrete diverse protein toxins to disrupt host defense systems. To ensure successful infection, however, pathogens must precisely regulate the expression of those exotoxins because uncontrolled toxin production squanders energy. Furthermore, inappropriate toxin secretion can trigger host immune responses that are detrimental to the invading pathogens. Therefore, bacterial pathogens use diverse transcriptional regulators to accurately regulate multiple exotoxin genes based on spatiotemporal conditions. This review covers three major exotoxins in pathogenic Vibrio species and their transcriptional regulation systems. When Vibrio encounters a host, genes encoding cytolysin/hemolysin, multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin, and secreted phospholipases are coordinately regulated by the transcriptional regulator HlyU. At the same time, however, they are distinctly controlled by a variety of other transcriptional regulators. How this coordinated but distinct regulation of exotoxins makes Vibrio species successful pathogens? In addition, anti-virulence strategies that target the coordinating master regulator HlyU and related future research directions are discussed.

Ambient pH Alters the Protein Content of Outer Membrane Vesicles, Driving Host Development in a Beneficial Symbiosis

Outer membrane vesicles (OMVs) are continuously produced by Gram-negative bacteria and are increasingly recognized as ubiquitous mediators of bacterial physiology. In particular, OMVs are powerful effectors in interorganismal interactions, driven largely by their molecular contents. These impacts have been studied extensively in bacterial pathogenesis but have not been well documented within the context of mutualism. Here, we examined the proteomic composition of OMVs from the marine bacterium Vibrio fischeri, which forms a specific mutualism with the Hawaiian bobtail squid, Euprymna scolopes We found that V. fischeri upregulates transcription of its major outer membrane protein, OmpU, during growth at an acidic pH, which V. fischeri experiences when it transitions from its environmental reservoir to host tissues. We used comparative genomics and DNA pulldown analyses to search for regulators of ompU and found that differential expression of ompU is governed by the OmpR, H-NS, and ToxR proteins. This transcriptional control combines with nutritional conditions to govern OmpU levels in OMVs. Under a host-encountered acidic pH, V. fischeri OMVs become more potent stimulators of symbiotic host development in an OmpU-dependent manner. Finally, we found that symbiotic development could be stimulated by OMVs containing a homolog of OmpU from the pathogenic species Vibrio cholerae, connecting the role of a well-described virulence factor with a mutualistic element. This work explores the symbiotic effects of OMV variation, identifies regulatory machinery shared between pathogenic and mutualistic bacteria, and provides evidence of the role that OMVs play in animal-bacterium mutualism.IMPORTANCE Beneficial bacteria communicate with their hosts through a variety of means. These communications are often carried out by a combination of molecules that stimulate responses from the host and are necessary for development of the relationship between these organisms. Naturally produced bacterial outer membrane vesicles (OMVs) contain many of those molecules and can stimulate a wide range of responses from recipient organisms. Here, we describe how a marine bacterium, Vibrio fischeri, changes the makeup of its OMVs under conditions that it experiences as it goes from its free-living lifestyle to associating with its natural host, the Hawaiian bobtail squid. This work improves our understanding of how bacteria change their signaling profile as they begin to associate with their beneficial partner animals.

Disruption of hmgA by DNA Duplication is Responsible for Hyperpigmentation in a Vibrio anguillarum Strain

Vibrio anguillarum 531A, isolated from a diseased fish in the Atlantic Ocean, is a mixture composed of about 95 and 5% of highly pigmented cells (strain 531Ad) and cells with normal levels of pigmentation (strain 531Ac), respectively. Analysis of the V. anguillarum 531Ad DNA region encompassing genes involved in the tyrosine metabolism showed a 410-bp duplication within the hmgA gene that results in a frameshift and early termination of translation of the homogentisate 1,2-dioxygenase. We hypothesized that this mutation results in accumulation of homogentisate that is oxidized and polymerized to produce pyomelanin. Introduction in E. coli of recombinant clones carrying the V. anguillarum hppD (4-hydroxyphenylpyruvate-dioxygenase), and a mutated hmgA produced brown colored colonies. Complementation with a recombinant clone harboring hmgA restored the original color to the colonies confirming that in the absence of homogentisate 1,2-dioxygenase the intermediary in tyrosine catabolism homogentisate accumulates and undergoes nonenzymatic oxidation and polymerization resulting in high amounts of the brown pigment. Whole-genome sequence analysis showed that V. anguillarum 531 Ac and 531Ad differ in the hmgA gene mutation and 23 mutations, most of which locate to intergenic regions and insertion sequences.

Negative regulation of bleomycins biosynthesis by ArsR/SmtB family repressor BlmR in Streptomyces verticillus

Bleomycin, a broad-spectrum antibiotic, has been widely used for various tumor treatments. However, its poor fermentation yield is not satisfactory for industrial production. Here, the ArsR/SmtB family regulator BlmR was characterized as a repressor of bleomycin production. As an autoregulator, BlmR was found to bind to a 12-2-12 imperfect palindrome sequence in its own promoter, and deletion of blmR led to a 34% increase of bleomycin B2 production compared with the wild-type strain. Using reverse transcription and quantitative PCR (RT-qPCR), blmT, which encoded a putative transporter, was identified as the target gene regulated by BlmR. Therefore, high-production strain was constructed by blmT overexpression in a blmR deletion strain, and the bleomycin B2 titer reached to 80 mg/L, which was 1.9-fold higher than the wild-type strain. Moreover, electrophoretic mobility shift assay (EMSA) showed neither metal-binding motifs nor redox switches in BlmR. In order to elucidate the regulatory mechanism, a model of BlmR was constructed by homology modeling and protein-protein docking. The BlmR-DNA complex was generated by protein-DNA docking with the assistance of site-directed mutagenesis and molecular dynamic (MD) simulation, which directly revealed several key amino acid residues needed for the maintenance and stabilization of the interface between BlmR and target DNA. The interface information could provide the configuration reference and seek the potential effectors that could interact with BlmR, thereby extending the regulation role of ArsR/SmtB family members on the improvement of antibiotic production.

Discovery of lahS as a Global Regulator of Environmental Adaptation and Virulence in Aeromonas hydrophila

Aeromonas hydrophila is an important aquatic microorganism that can cause fish hemorrhagic septicemia. In this study, we identified a novel LysR family transcriptional regulator (LahS) in the A. hydrophila Chinese epidemic strain NJ-35 from a library of 947 mutant strains. The deletion of lahS caused bacteria to exhibit significantly decreased hemolytic activity, motility, biofilm formation, protease production, and anti-bacterial competition ability when compared to the wild-type strain. In addition, the determination of the fifty percent lethal dose (LD50) in zebrafish demonstrated that the lahS deletion mutant (ΔlahS) was highly attenuated in virulence, with an approximately 200-fold increase in LD50 observed as compared with that of the wild-type strain. However, the ΔlahS strain exhibited significantly increased antioxidant activity (six-fold). Label-free quantitative proteome analysis resulted in the identification of 34 differentially expressed proteins in the ΔlahS strain. The differentially expressed proteins were involved in flagellum assembly, metabolism, redox reactions, and cell density induction. The data indicated that LahS might act as a global regulator to directly or indirectly regulate various biological processes in A. hydrophila NJ-35, contributing to a greater understanding the pathogenic mechanisms of A. hydrophila.

The Transcriptional Regulator HlyU Positively Regulates Expression of exsA, Leading to Type III Secretion System 1 Activation in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a marine bacterium that is globally recognized as the leading cause of seafood-borne gastroenteritis. V. parahaemolyticus uses various toxins and two type 3 secretion systems (T3SS-1 and T3SS-2) to subvert host cells during infection. We previously determined that V. parahaemolyticus T3SS-1 activity is upregulated by increasing the expression level of the master regulator ExsA under specific growth conditions. In this study, we set out to identify V. parahaemolyticus genes responsible for linking environmental and growth signals to exsA gene expression. Using transposon mutagenesis in combination with a sensitive and quantitative luminescence screen, we identify HlyU and H-NS as two antagonistic regulatory proteins controlling the expression of exsA and, hence, T3SS-1 in V. parahaemolyticus Disruption of hns leads to constitutive unregulated exsA gene expression, consistent with its known role in repressing exsA transcription. In contrast, genetic disruption of hlyU completely abrogated exsA expression and T3SS-1 activity. A V. parahaemolyticushlyU null mutant was significantly deficient for T3SS-1-mediated host cell death during in vitro infection. DNA footprinting studies with purified HlyU revealed a 56-bp protected DNA region within the exsA promoter that contains an inverted repeat sequence. Genetic evidence suggests that HlyU acts as a derepressor, likely by displacing H-NS from the exsA promoter, leading to exsA gene expression and appropriately regulated T3SS-1 activity. Overall, the data implicate HlyU as a critical positive regulator of V. parahaemolyticus T3SS-1-mediated pathogenesis.IMPORTANCE Many Vibrio species are zoonotic pathogens, infecting both animals and humans, resulting in significant morbidity and, in extreme cases, mortality. While many Vibrio species virulence genes are known, their associated regulation is often modestly understood. We set out to identify genetic factors of V. parahaemolyticus that are involved in activating exsA gene expression, a process linked to a type III secretion system involved in host cytotoxicity. We discover that V. parahaemolyticus employs a genetic regulatory switch involving H-NS and HlyU to control exsA promoter activity. While HlyU is a well-known positive regulator of Vibrio species virulence genes, this is the first report linking it to a transcriptional master regulator and type III secretion system paradigm.

Isocitrate dehydrogenase mutation in Vibrio anguillarum results in virulence attenuation and immunoprotection in rainbow trout (Oncorhynchus mykiss)

Background

Vibrio anguillarum is an extracellular bacterial pathogen that is a causative agent of vibriosis in finfish and crustaceans with mortality rates ranging from 30% to 100%. Mutations in central metabolism (glycolysis and the TCA cycle) of intracellular pathogens often result in attenuated virulence due to depletion of required metabolic intermediates; however, it was not known whether mutations in central metabolism would affect virulence in an extracellular pathogen such as V. anguillarum.

Results

Seven central metabolism mutants were created and characterized with regard to growth in minimal and complex media, expression of virulence genes, and virulence in juvenile rainbow trout (Oncorhynchus mykiss). Only the isocitrate dehydrogenase (icd) mutant was attenuated in virulence against rainbow trout challenged by either intraperitoneal injection or immersion. Further, the icd mutant was shown to be immunoprotective against wild type V. anguillarum infection. There was no significant decrease in the expression of the three hemolysin genes detected by qRT-PCR. Additionally, only the icd mutant exhibited a significantly decreased growth yield in complex media. Growth yield was directly related to the abundance of glutamate. A strain with a restored wild type icd gene was created and shown to restore growth to a wild type cell density in complex media and pathogenicity in rainbow trout.

Conclusions

The data strongly suggest that a decreased growth yield, resulting from the inability to synthesize α-ketoglutarate, caused the attenuation despite normal levels of expression of virulence genes. Therefore, the ability of an extracellular pathogen to cause disease is dependent upon the availability of host-supplied nutrients for growth. Additionally, a live vaccine strain could be created from an icd deletion strain.

Electronic supplementary material

The online version of this article (10.1186/s12866-017-1124-1) contains supplementary material, which is available to authorized users.

Iron Acquisition Strategies of Vibrio anguillarum

The hemorrhagic septicemic disease vibriosis caused by Vibrio anguillarum shows noticeable similarities to invasive septicemia in humans, and in this case, the V. anguillarum–host system has the potential to serve as a model for understanding native eukaryotic host–pathogen interactions. Iron acquisition, as a fierce battle occurring between pathogenic V. anguillarum and the fish host, is a pivotal step for virulence. In this article, advances in defining the roles of iron uptake pathways in growth and virulence of V. anguillarum have been summarized, divided into five aspects, including siderophore biosynthesis and secretion, iron uptake, iron release, and regulation of iron uptake. Understanding the molecular mechanisms of iron acquisition will have important implications for the pathogenicity of this organism.

Metal homeostasis in bacteria: the role of ArsR-SmtB family of transcriptional repressors in combating varying metal concentrations in the environment

Bacterial infections cause severe medical problems worldwide, resulting in considerable death and loss of capital. With the ever-increasing rise of antibiotic-resistant bacteria and the lack of development of new antibiotics, research on metal-based antimicrobial therapy has now gained pace. Metal ions are essential for survival, but can be highly toxic to organisms if their concentrations are not strictly controlled. Through evolution, bacteria have acquired complex metal-management systems that allow them to acquire metals that they need for survival in different challenging environments while evading metal toxicity. Metalloproteins that controls these elaborate systems in the cell, and linked to key virulence factors, are promising targets for the anti-bacterial drug development. Among several metal-sensory transcriptional regulators, the ArsR-SmtB family displays greatest diversity with several distinct metal-binding and nonmetal-binding motifs that have been characterized. These prokaryotic metolloregulatory transcriptional repressors represses the expression of operons linked to stress-inducing concentrations of metal ions by directly binding to the regulatory regions of DNA, while derepression results from direct binding of metal ions by these homodimeric proteins. Many bacteria, e.g., Mycobacterium tuberculosis, Bacillus anthracis, etc., have evolved to acquire multiple metal-sensory motifs which clearly demonstrate the importance of regulating concentrations of multiple metal ions. Here, we discussed the mechanisms of how ArsR-SmtB family regulates the intracellular bioavailability of metal ions both inside and outside of the host. Knowledge of the metal-challenges faced by bacterial pathogens and their survival strategies will enable us to develop the next generation drugs.

Spatial organization of DNA sequences directs the assembly of bacterial chromatin by a nucleoid-associated protein

Structural differentiation of bacterial chromatin depends on cooperative binding of abundant nucleoid-associated proteins at numerous genomic DNA sites and stabilization of distinct long-range nucleoprotein structures. Histone-like nucleoid-structuring protein (H-NS) is an abundant DNA-bridging, nucleoid-associated protein that binds to an AT-rich conserved DNA sequence motif and regulates both the shape and the genetic expression of the bacterial chromosome. Although there is ample evidence that the mode of H-NS binding depends on environmental conditions, the role of the spatial organization of H-NS-binding sequences in the assembly of long-range nucleoprotein structures remains unknown. In this study, by using high-resolution atomic force microscopy combined with biochemical assays, we explored the formation of H-NS nucleoprotein complexes on circular DNA molecules having different arrangements of identical sequences containing high-affinity H-NS-binding sites. We provide the first experimental evidence that variable sequence arrangements result in various three-dimensional nucleoprotein structures that differ in their shape and the capacity to constrain supercoils and compact the DNA. We believe that the DNA sequence-directed versatile assembly of periodic higher-order structures reveals a general organizational principle that can be exploited for knowledge-based design of long-range nucleoprotein complexes and purposeful manipulation of the bacterial chromatin architecture.

Role of the Histone-Like Nucleoid Structuring Protein (H-NS) in the Regulation of Virulence Factor Expression and Stress Response in Vibrio vulnificus

Temperature is one of the important parameters regulating the expression of virulence factors in bacteria. The global regulator, a histone-like nucleoid structuring protein (H-NS), is known to play a crucial role in this regulation. In the present study, we first clarified the role of H-NS in the temperature-dependent regulation of virulence factor production in Vibrio vulnificus, including that of the cytolytic toxin (V. vulnificus hemolysin: VVH) and the proteolytic enzyme (V. vulnificus protease: VVP). The expression of hns itself was subjected to temperature regulation, where hns was expressed more at 26 ℃ than at 37 ℃. VVH production and the expression of its gene vvhA were increased by disruption of the hns gene. H-NS appeared to affect the vvhA expression by the well-documented transcriptional silencing mechanism. On the other hand, hns disruption resulted in the reduction of VVP production and the expression of its gene vvpE. H-NS was suggested to positively regulate vvpE expression through the increase in the level of the rpoS mRNA. Moreover, H-NS was found to contribute to the survival of V. vulnificus in stressful environments. When compared to the wild type strain, the hns mutant exhibited reduced survival rates when subjected to acidic pH, hyperosmotic and oxidative stress.

H-NS: an overarching regulator of the Vibrio cholerae life cycle

Vibrio cholerae has become a model organism for studies connecting virulence, pathogen evolution and infectious disease ecology. The coordinate expression of motility, virulence and biofilm enhances its pathogenicity, environmental fitness and fecal-oral transmission. The histone-like nucleoid structuring protein negatively regulates gene expression at multiple phases of the V. cholerae life cycle. Here we discuss: (i) the regulatory and structural implications of H-NS chromatin-binding in the two-chromosome cholera bacterium; (ii) the factors that counteract H-NS repression; and (iii) a model for the regulation of the V. cholerae life cycle that integrates H-NS repression, cyclic diguanylic acid signaling and the general stress response.

Transcription of exsD is repressed directly by H-NS in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a leading cause of seafood-associated diarrhea and gastroenteritis. This bacteria expresses a major virulence determinant called T3SS1. Expression of T3SS1 is tightly regulated by the ExsA-ExsC-ExsD regulatory system. The transcription of exsA and probably exsC is repressed directly by the H-NS protein. In this study, the regulation of exsD by H-NS was investigated using quantitative RT-PCR, primer extension, LacZ fusion, electrophoretic mobility shift, and DNase I footprinting assays. The results showed that His-H-NS protected a single region from 61 bp to 174 bp upstream of exsD against DNase I digestion, and a transcription start site located at 105 bp upstream of exsD was detected and its activity was repressed by H-NS. Therefore, a single H-NS-dependent promoter was transcribed for exsD in V. parahaemolyticus. Thus, all three genes in the ExsA-ExsC-ExsD regulatory system of T3SS1 are directly repressed by H-NS in V. parahaemolyticus.

Contributions of tropodithietic acid and biofilm formation to the probiotic activity of Phaeobacter inhibens

Background

The probiotic bacterium Phaeobacter inhibens strain S4Sm, isolated from the inner shell surface of a healthy oyster, secretes the antibiotic tropodithietic acid (TDA), is an excellent biofilm former, and increases oyster larvae survival when challenged with bacterial pathogens. In this study, we investigated the specific roles of TDA secretion and biofilm formation in the probiotic activity of S4Sm.

Results

Mutations in clpX (ATP-dependent ATPase) and exoP (an exopolysaccharide biosynthesis gene) were created by insertional mutagenesis using homologous recombination. Mutation of clpX resulted in the loss of TDA production, no decline in biofilm formation, and loss of the ability to inhibit the growth of Vibrio tubiashii and Vibrio anguillarum in co-colonization experiments. Mutation of exoP resulted in a ~60 % decline in biofilm formation, no decline in TDA production, and delayed inhibitory activity towards Vibrio pathogens in co-colonization experiments. Both clpX and exoP mutants exhibited reduced ability to protect oyster larvae from death when challenged by Vibrio tubiashii. Complementation of the clpX and exoP mutations restored the wild type phenotype. We also found that pre-colonization of surfaces by S4Sm was critical for this bacterium to inhibit pathogen colonization and growth.

Conclusions

Our observations demonstrate that probiotic activity by P. inhibens S4Sm involves contributions from both biofilm formation and the production of the antibiotic TDA. Further, probiotic activity also requires colonization of surfaces by S4Sm prior to the introduction of the pathogen.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0617-z) contains supplementary material, which is available to authorized users.

Larva of the greater wax moth, Galleria mellonella, is a suitable alternative host for studying virulence of fish pathogenic Vibrio anguillarum

BACKGROUND

Microbial diseases cause considerable economic losses in aquaculture and new infection control measures often rely on a better understanding of pathogenicity. However, disease studies performed in fish hosts often require specialist infrastructure (e.g., aquaria), adherence to strict legislation and do not permit high-throughput approaches; these reasons justify the development of alternative hosts. This study aimed to validate the use of larvae of the greater wax moth (Galleria mellonella) to investigate virulence of the important fish pathogen, Vibrio anguillarum.

RESULTS

Using 11 wild-type isolates of V. anguillarum, these bacteria killed larvae in a dose-dependent manner and replicated inside the haemolymph, but infected larvae were rescued by antibiotic therapy. Crucially, virulence correlated significantly and positively in larva and Atlantic salmon (Salmo salar) infection models. Challenge studies with mutants knocked out for single virulence determinants confirmed conserved roles in larva and fish infections in some cases (pJM1 plasmid, rtxA), but not all (empA, flaA, flaE).

CONCLUSIONS

The G. mellonella model is simple, more ethically acceptable than experiments on vertebrates and, crucially, does not necessitate liquid systems, which reduces infrastructure requirements and biohazard risks associated with contaminated water. The G. mellonella model may aid our understanding of microbial pathogens in aquaculture and lead to the timely introduction of new effective remedies for infectious diseases, while adhering to the principles of replacement, reduction and refinement (3Rs) and considerably reducing the number of vertebrates used in such studies.

RNA-seq analysis identifies new genes regulated by the histone-like nucleoid structuring protein (H-NS) affecting Vibrio cholerae virulence, stress response and chemotaxis

The histone-like nucleoid structuring protein (H-NS) functions as a transcriptional silencer by binding to AT-rich sequences at bacterial promoters. However, H-NS repression can be counteracted by other transcription factors in response to environmental changes. The identification of potential toxic factors, the expression of which is prevented by H-NS could facilitate the discovery of new regulatory proteins that may contribute to the emergence of new pathogenic variants by anti-silencing. Vibrio cholerae hns mutants of the El Tor biotype exhibit altered virulence, motility and environmental stress response phenotypes compared to wild type. We used an RNA-seq analysis approach to determine the basis of the above hns phenotypes and identify new targets of H-NS transcriptional silencing. H-NS affected the expression of 18% of all predicted genes in a growth phase-dependent manner. Loss of H-NS resulted in diminished expression of numerous genes encoding methyl-accepting chemotaxis proteins as well as chemotaxis toward the attractants glycine and serine. Deletion of hns also induced an endogenous envelope stress response resulting in elevated expression of rpoE encoding the extracytoplamic sigma factor E (σ^E). The RNA-seq analysis identified new genes directly repressed by H-NS that can affect virulence and biofilm development in the El Tor biotype cholera bacterium. We show that H-NS and the quorum sensing regulator HapR silence the transcription of the vieSAB three-component regulatory system in El Tor biotype V. cholerae. We also demonstrate that H-NS directly represses the transcription of hlyA (hemolysin), rtxCA (the repeat in toxin or RTX), rtxBDE (RTX transport) and the biosynthesis of indole. Of these genes, H-NS occupancy at the hlyA promoter was diminished by overexpression of the transcription activator HlyU. We discuss the role of H-NS transcriptional silencing in phenotypic differences exhibited by V. cholerae biotypes.

Identification of the target DNA sequence and characterization of DNA binding features of HlyU, and suggestion of a redox switch for hlyA expression in the human pathogen Vibrio cholerae from in silico studies

HlyU, a transcriptional regulator common in many Vibrio species, activates the hemolysin gene hlyA in Vibrio cholerae, the rtxA1 operon in Vibrio vulnificus and the genes of plp-vah1 and rtxACHBDE gene clusters in Vibrio anguillarum. The protein is also proposed to be a potential global virulence regulator for V. cholerae and V. vulnificus. Mechanisms of gene control by HlyU in V. vulnificus and V. anguillarum are reported. However, detailed elucidation of the interaction of HlyU in V. cholerae with its target DNA at the molecular level is not available. Here we report a 17-bp imperfect palindrome sequence, 5′-TAATTCAGACTAAATTA-3′, 173 bp upstream of hlyA promoter, as the binding site of HlyU. This winged helix-turn-helix protein binds necessarily as a dimer with the recognition helices contacting the major grooves and the β-sheet wings, the minor grooves. Such interactions enhance hlyA promoter activity in vivo. Mutations affecting dimerization as well as those in the DNA–protein interface hamper DNA binding and transcription regulation. Molecular dynamic simulations show hydrogen bonding patterns involving residues at the mutation sites and confirmed their importance in DNA binding. On binding to HlyU, DNA deviates by ∼68º from linearity. Dynamics also suggest a possible redox control in HlyU.

Characterization of Plp, a phosphatidylcholine-specific phospholipase and hemolysin of Vibrio anguillarum

BACKGROUND

Vibrio anguillarum is the causative agent of vibriosis in fish. Several extracellular proteins secreted by V. anguillarum have been shown to contribute to virulence. While two hemolysin gene clusters, vah1-plp and rtxACHBDE, have been previously identified and described, the activities of the protein encoded by the plp gene were not known. Here we describe the biochemical activities of the plp-encoded protein and its role in pathogenesis.

RESULTS

The plp gene, one of the components in vah1 cluster, encodes a 416-amino-acid protein (Plp), which has homology to lipolytic enzymes containing the catalytic site amino acid signature SGNH. Hemolytic activity of the plp mutant increased 2-3-fold on sheep blood agar indicating that plp represses vah1; however, hemolytic activity of the plp mutant decreased by 2-3-fold on fish blood agar suggesting that Plp has different effects against erythrocytes from different species. His6-tagged recombinant Plp protein (rPlp) was over-expressed in E. coli. Purified and re-folded active rPlp exhibited phospholipase A2 activity against phosphatidylcholine and no activity against phosphatidylserine, phosphatidylethanolamine, or sphingomyelin. Characterization of rPlp revealed broad optimal activities at pH 5-9 and at temperatures of 30-64°C. Divalent cations and metal chelators did not affect activity of rPlp. We also demonstrated that Plp was secreted using thin layer chromatography and immunoblot analysis. Additionally, rPlp had strong hemolytic activity towards rainbow trout erythrocytes, but not to sheep erythrocytes suggesting that rPlp is optimized for lysis of phosphatidylcholine-rich fish erythrocytes. Further, only the loss of the plp gene had a significant effect on hemolytic activity of culture supernatant on fish erythrocytes, while the loss of rtxA and/or vah1 had little effect. However, V. anguillarum strains with mutations in plp or in plp and vah1 exhibited no significant reduction in virulence compared to the wild type strain when used to infect rainbow trout.

CONCLUSION

The plp gene of V. anguillarum encoding a phospholipase with A2 activity is specific for phosphatidylcholine and, therefore, able to lyse fish erythrocytes, but not sheep erythrocytes. Mutation of plp does not affect the virulence of V. anguillarum in rainbow trout.