Identification and characterization of additional flagellin genes from Vibrio anguillarum

Identification, characterization and complete genome analysis of a Vibrio anguillarum isolated from Sebastes schlegelii

Vibrio anguillarum is an important fish pathogen in mariculture, which can infect fish with great economic losses. In this study, a Vibrio anguillarum isolated from Sebastes schlegelii was named VA1 and was identified and characterized from aspects of morphology, physiological and biochemical characteristics, 16SRNA, virulence genes, drug sensitivity, and extracellular enzyme activity. At the same time, The VA1 was investigated at the genomic level. The results showed that a Gram-negative was isolated from the diseased fish. The VA1 was characterized with uneven surface and visible flagella wrapped in a sheath and microbubble structures. The VA1 was identified as Vibrio anguillarum based on the 16S RNA sequence and physiological and biochemical characteristics. The VA1 carried most of the virulence genes (24/29) and was resistant to penicillin, oxacillin, ampicillin, cefradine, neomycin, pipemidic acid, ofloxacin, and norfloxacin. The pathogenicity of the isolated strain was confirmed by an experimental analysis, and its LD50 was 6.43 × 106 CFU/ml. The VA1 had the ability to secrete gelatinase, protease, and amylase, and it had α-hemolysis. The whole genome size of the VA1 was 4232328bp and the G + C content was 44.95 %, consisting of two circular chromosomes, Chromosome1 and Chromosome2, with no plasmid. There were 1006 predicted protein coding sequences (CDSs). A total of 526 genes were predicted as virulence-related genes which could be classified as type IV pili, flagella, hemolysin, siderophore, and type VI secretion system. Virulence genes and correlation data were supported with the histopathological examination of the affected organs and tissues. 194 genes were predicted as antibiotic resistance genes, including fluoroquinolone antibiotic, aminoglycoside antibiotic, and beta-lactam resistant genes, which agreed with the results of the above drug sensitivity, indicating VA1 to be a multidrug-resistant bacterium. This study provided a theoretical basis for a better understanding of pathogenicity and antibiotic resistance, which might contribute to the prevention of V. anguillarum in the future.

The Vibrio Polar Flagellum: Structure and Regulation

Here we discuss the structure and regulation of the Vibrio flagellum and its role in the virulence of pathogenic species. We will cover some of the novel insights into the structure of this nanomachine that have recently been enabled by cryoelectron tomography. We will also highlight the recent genetic studies that have increased our understanding in flagellar synthesis specifically at the bacterial cell pole, temporal regulation of flagellar genes, and how the flagellum enables directional motility through Run-Reverse-Flick cycles.

Duplicated Flagellins in Pseudomonas Divergently Contribute to Motility and Plant Immune Elicitation

Flagellins are the main constituents of the flagellar filaments that provide bacterial motility, chemotactic ability, and host immune elicitation ability. Although the functions of flagellins have been extensively studied in bacteria with a single flagellin-encoding gene, the function of multiple flagellin-encoding genes in a single bacterial species is largely unknown. Here, the model plant-growth-promoting bacterium Pseudomonas kilonensis F113 was used to decipher the divergent functions of duplicated flagellins. We demonstrate that the two flagellins (FliC-1 and FliC-2) in 12 Pseudomonas strains, including F113, are evolutionarily distinct. Only the fliC-1 gene but not the fliC-2 gene in strain F113 is responsible for flagellar biogenesis, motility, and plant immune elicitation. The transcriptional expression of fliC-2 was significantly lower than that of fliC-1 in medium and in planta, most likely due to variations in promoter activity. In silico prediction revealed that all fliC-2 genes in the 12 Pseudomonas strains have a poorly conserved promoter motif. Compared to the Flg22-2 epitope (relative to FliC-2), Flg22-1 (relative to FliC-1) induced stronger FLAGELLIN SENSING 2 (FLS2)-mediated microbe-associated molecular pattern-triggered immunity and significantly inhibited plant root growth. A change in the 19th amino acid in Flg22-2 reduced its binding affinity to the FLS2/brassinosteroid insensitive 1-associated kinase 1 complex. Also, Flg22-2 epitopes in the other 11 Pseudomonas strains were presumed to have low binding affinity due to the same change in the 19th amino acid. These findings suggest that Pseudomonas has evolved duplicate flagellins, with only FliC-1 contributing to motility and plant immune elicitation. IMPORTANCE Flagellins have emerged as important microbial patterns. This work focuses on flagellin duplication in some plant-associated Pseudomonas. Our findings on the divergence of duplicated flagellins provide a conceptual framework for better understanding the functional determinant flagellin and its peptide in multiple-flagellin plant-growth-promoting rhizobacteria.

FlrA-independent production of flagellar proteins is required for proper flagellation in Shewanella putrefaciens

Flagella are multiprotein complexes whose assembly and positioning require complex spatiotemporal control. Flagellar assembly is thought to be controlled by several transcriptional tiers, which are mediated through various master regulators. Here, we revisited the regulation of flagellar genes in polarly flagellated gammaproteobacteria by the regulators FlrA, RpoN (σ⁵⁴ ) and FliA (σ²⁸ ) in Shewanella putrefaciens CN-32 at the transcript and protein level. We found that a number of regulatory and structural proteins were present in the absence of the main regulators, suggesting that initiation of flagella assembly and motor activation relies on the abundance control of only a few structural key components that are required for the formation of the MS- and C-ring and the flagellar type III secretion system. We identified FlrA-independent promoters driving expression of the regulators of flagellar number and positioning, FlhF and FlhG. Reduction of the gene expression levels from these promoters resulted in the emergence of hyperflagellation. This finding indicates that basal expression is required to adjust the flagellar counter in Shewanella. This is adding a deeper layer to the regulation of flagellar synthesis and assembly.

Shoc2 recognizes bacterial flagellin and mediates antibacterial Erk/Stat signaling in an invertebrate

Flagellin is a key bacterial virulence factor that can stimulate molecular immune signaling in both animals and plants. The detailed mechanisms of recognizing flagellin and mounting an efficient immune response have been uncovered in vertebrates; however, whether invertebrates can discriminate flagellin remains largely unknown. In the present study, the homolog of human SHOC2 leucine rich repeat scaffold protein in kuruma shrimp (Marsupenaeus japonicus), designated MjShoc2, was found to interact with Vibrio anguillarum flagellin A (FlaA) using yeast two-hybrid and pull-down assays. MjShoc2 plays a role in antibacterial response by mediating the FlaA-induced expression of certain antibacterial effectors, including lectin and antimicrobial peptide. FlaA challenge, via MjShoc2, led to phosphorylation of extracellular regulated kinase (Erk), and the subsequent activation of signal transducer and activator of transcription (Stat), ultimately inducing the expression of effectors. Therefore, by establishing the FlaA/MjShoc2/Erk/Stat signaling axis, this study revealed a new antibacterial strategy in shrimp, and provides insights into the flagellin sensing mechanism in invertebrates.

Flagellin sensing has been proven as a general antibacterial strategy. Recognition of bacterial flagellin by the transmembrane receptor toll like receptor 5 (TLR5) leads to the activation of nuclear factor kappa B (NF-κB) pathway and induction of proinflammatory cytokines, while recognition by the intracellular nucleotide-binding leucine-rich (NLR) receptor leads to caspase-activation and cytokines-expression. Although flagellin is an effective immune stimulator that induces antimicrobial peptides in Drosophila and in crustaceans, how an invertebrate host senses flagellin and mounts an immune response is poorly understood. Here, we used the flagellin (FlaA) from Vibrio anguillarum, a pathogen of shrimp, as a bait protein to screen a yeast two-hybrid library derived from kuruma shrimp (Marsupenaeus japonicus). We found a scaffold protein, MjShoc2, able to interact with FlaA. We also found that FlaA could effectively induce the expression of certain recognized antibacterial effectors in shrimp depending on MjShoc2. We revealed that extracellular regulated kinase (Erk) phosphorylation occurred downstream of FlaA/MjShoc2, and led to signal transducer and activator of transcription (Stat) activation, resulting in transcription of certain effectors. Therefore our study provides new insights into the FlaA-induced molecular immunity in invertebrates.

Can only one physiological trait determinate the adverse effect of green fluorescent protein (GFP) incorporation on Vibrio virulence?

Green fluorescent protein (GFP) has been used extensively for in situ animal studies that follow up bacterial infection under epifluorescence microscopy. It is assumed that GFP is acting as a "neutral" protein with no influence on the bacterial physiology. To verify this hypothesis, the virulence of Vibrio splendidus ME9, Vibrio anguillarum NB10, and their respective GFP-tagged strains ME9-GFP and NB10-GFP (transconjugants) was compared in vitro and tested in vivo towards blue mussel (Mytilus edulis) larvae. Results showed that the incorporation of GFP negatively impacted the growth and swimming motility of NB10 in vitro. Correspondingly, the mRNA levels of genes involved in bacterial swimming motility (flaA, flaE, and cheR) were significantly down-regulated in NB10-GFP. As for the strain ME9 on the other hand, GFP incorporation only had a negative effect on swimming motility. However, both the strains NB10-GFP and ME9-GFP showed almost the same virulence as their respective parental strain towards mussel larvae in vivo. Overall, the data presented here demonstrated that incorporation of GFP may cause modifications in cell physiology and highlight the importance of preliminary physiological tests to minimize the negative influence of GFP tagging when it is used to monitor the target localization. The study also supports the idea that the virulence of Vibrio species is determined by complex regulatory networks. Notwithstanding the change of a single physiological trait, especially growth or swimming motility, the GFP-tagged Vibrio strain can thus still be considered usable in studies mainly focusing on the virulence of the strain. KEY POINTS: • The effect of GFP incorporation on physiological trait of Vibrio strains. • The virulence in vibrios could be multifactorial. • The stable virulence of Vibrio strains after GFP incorporation.

Multifunctional transcription factor CytR of Vibrio cholerae is important for pathogenesis

Vibrio cholerae, the Gram-negative facultative pathogen, resides in the aquatic environment and infects humans and causes diarrhoeagenic cholera. Although the environment differs drastically, V. cholerae thrives in both of these conditions aptly and chitinases play a vital role in their persistence and nutrient acquisition. Chitinases also play a role in V. cholerae pathogenesis. Chitinases and its downstream chitin utilization genes are regulated by sensor histidine kinase ChiS, which also plays a significant role in pathogenesis. Recent exploration suggests that CytR, a transcription factor of the LacI family in V. cholerae, also regulates chitinase secretion in environmental conditions. Since chitinases and chitinase regulator ChiS is involved in pathogenesis, CytR might also play a significant role in pathogenicity. However, the role of CytR in pathogenesis is yet to be known. This study explores the regulation of CytR on the activation of ChiS in the presence of mucin and its role in pathogenesis. Therefore, we created a CytR isogenic mutant strain of V. cholerae (CytR¯) and found considerably less β-hexosaminidase enzyme production, which is an indicator of ChiS activity. The CytR¯ strain greatly reduced the expression of chitinases chiA1 and chiA2 in mucin-supplemented media. Electron microscopy showed that the CytR¯ strain was aflagellate. The expression of flagellar-synthesis regulatory genes flrB, flrC and class III flagellar-synthesis genes were reduced in the CytR¯ strain. The isogenic CytR mutant showed less growth compared to the wild-type in mucin-supplemented media as well as demonstrated highly retarded motility and reduced mucin-layer penetration. The CytR mutant revealed decreased adherence to the HT-29 cell line. In animal models, reduced fluid accumulation and colonization were observed during infection with the CytR¯ strain due to reduced expression of ctxB, toxT and tcpA. Collectively these data suggest that CytR plays an important role in V. cholerae pathogenesis.

A localized adaptor protein performs distinct functions at the Caulobacter cell poles

Asymmetric cell division yields two distinct daughter cells by mechanisms that underlie stem cell behavior and cellular diversity in all organisms. The bacterium Caulobacter crescentus is able to orchestrate this complex process with less than 4,000 genes. This article describes a strategy deployed by Caulobacter where a regulatory protein, PopA, is programed to perform distinct roles based on its subcellular address. We demonstrate that, depending on the availability of a second messenger molecule, PopA adopts either a monomer or dimer form. The two oligomeric forms interact with different partners at the two cell poles, playing a critical role in the degradation of a master transcription factor at one pole and flagellar assembly at the other pole.

Asymmetric cell division generates two daughter cells with distinct characteristics and fates. Positioning different regulatory and signaling proteins at the opposing ends of the predivisional cell produces molecularly distinct daughter cells. Here, we report a strategy deployed by the asymmetrically dividing bacterium Caulobacter crescentus where a regulatory protein is programmed to perform distinct functions at the opposing cell poles. We find that the CtrA proteolysis adaptor protein PopA assumes distinct oligomeric states at the two cell poles through asymmetrically distributed c-di-GMP: dimeric at the stalked pole and monomeric at the swarmer pole. Different polar organizing proteins at each cell pole recruit PopA where it interacts with and mediates the function of two molecular machines: the ClpXP degradation machinery at the stalked pole and the flagellar basal body at the swarmer pole. We discovered a binding partner of PopA at the swarmer cell pole that together with PopA regulates the length of the flagella filament. Our work demonstrates how a second messenger provides spatiotemporal cues to change the physical behavior of an effector protein, thereby facilitating asymmetry.

Ligilactobacillus agilis BKN88 possesses thermo-/acid-stable heteropolymeric flagellar filaments

Many flagellated bacteria possess multiple flagellins, but the roles and the compositions of each flagellin are diverse and poorly understood. In Ligilactobacillus agilis BKN88, there are two active flagellin gene paralogues but their function and composition in its flagellar filaments have not been described. The aim of this study is to find the function and composition of the flagellins by employing mutant strains each of which expresses a single flagellin or a modified flagellin. Two single flagellin-expressing strains were both flagellated while the number of flagella per cell in the single flagellin-expressing derivatives was lower than that in the wild type. Nonetheless, these derivative strains were apparently equally motile as the wild type. This indicates that either flagellin is sufficient for cell motility. The immunological activity via Toll-like receptor 5 of the single flagellin-expressing strains or purified single flagellins was readily detectable but mostly variably weaker than that of the wild type. The flagellar filaments of wild type L. agilis BKN88 were more acid-/thermo-stable than those of single flagellin-expressing derivatives. Using a combination of immunoprecipitation and flagellin-specific staining, wild type BKN88 appeared to possess heteropolymeric flagellar filaments consisting of both flagellins and each flagellin appeared to be equally distributed throughout the filaments. The results of this study suggest that the two flagellins together form a more robust filament than either alone and are thus functionally complementary.

Construction of a Vibrio anguillarum flagellin B mutant and analysis of its immuno-stimulation effects on Macrobrachium rosenbergii

Vibrio anguillarum is a globally distributed aquatic pathogen, and its flagellin B (FlaB) protein can evoke innate immune responses in hosts. In order to explore the role of FlaB in V. anguillarum infection, we constructed a FlaB-deficient mutant using overlapping PCR and two-step homologous recombination, and gene sequencing confirmed successful knockout of the FlaB gene. Scanning electron microscopy showed no significant differences in the morphological structure of the flagellum between wild-type and FlaB-deficient strains. The mutant was subsequently injected into the freshwater prawn (Macrobrachium rosenbergii) to explore its pathogenicity in the host, and expression of myeloid differentiation factor 88, prophenoloxidase, catalase, superoxide dismutase and glutathione peroxidase was investigated by real-time PCR. The results showed that deletion of FlaB had little effect on V. anguillarum-induced expression of these immune-related genes (p > 0.05). In general, the FlaB mutant displayed similar flagella morphology and immune characteristics to the wild-type strain, hence we speculated that knockout of FlaB might promote the expression and function of other flagellin proteins. Furthermore, this study provides a rapid and simple method for obtaining stable mutants of V. anguillarum free from foreign plasmid DNA.

What drives changes in the virulence and antibiotic resistance of Vibrio harveyi in the South China Sea?

To understand the driving environmental factors in changes of bacterial virulence and antibiotic resistance, we determined the prevalence, antibiotic resistance and antibiotic resistance and virulence genes of Vibrio harveyi isolated from diseased marine fish in south coastal China. We isolated 2, 52 and 53 V. harveyi strains from Fujian, Hainan and Guangdong, respectively, and identified them by multilocus sequence analysis of 16S rRNA-toxR_Vh -rctB. Nine typical virulence genes were represented at a higher average in Hainan (7.39 ± 0.24) than in Guangdong (6.91 ± 0.28). Five atypical virulence genes were detected in some isolates. In particular, flaC and vvh were detected in more than 60% of isolates. Their average number was significantly higher in Hainan (2.30 ± 0.20) than in Guangdong (1.70 ± 0.10). Multidrug resistance was widespread with an average resistance to 4.57 ± 0.18 of 15 antibiotics. Both the average number of antibiotic resistance and antibiotic resistance genes were higher in Hainan (5.25 ± 0.27 and 1.11 ± 0.15, respectively) than in Guangdong (3.87 ± 0.21 and 0.75 ± 0.10, respectively). This study demonstrated that there were more virulence genes and greater drug resistance in Hainan than in Guangdong, suggesting that warmer temperature and antibiotics pollutants probably enhance antibiotic resistance and bacterial infection.

Insights into flagellar function and mechanism from the squid-vibrio symbiosis

Flagella are essential and multifunctional nanomachines that not only move symbionts towards their tissue colonization site, but also play multiple roles in communicating with the host. Thus, untangling the activities of flagella in reaching, interacting, and signaling the host, as well as in biofilm formation and the establishment of a persistent colonization, is a complex problem. The squid-vibrio system offers a unique model to study the many ways that bacterial flagella can influence a beneficial association and, generally, other bacteria-host interactions. Vibrio fischeri is a bioluminescent bacterium that colonizes the Hawaiian bobtail squid, Euprymna scolopes. Over the last 15 years, the structure, assembly, and functions of V. fischeri flagella, including not only motility and chemotaxis, but also biofilm formation and symbiotic signaling, have been revealed. Here we discuss these discoveries in the perspective of other host-bacteria interactions.

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.

Diversification of Vibrio anguillarum Driven by the Bacteriophage CHOED

Bacteriophages are an important factor in bacterial evolution. Some reports suggest that lytic bacteriophages can select for resistant mutant strains with reduced virulence. The present study explores the role of the CHOED bacteriophage in the diversification and virulence of its host Vibrio anguillarum. Nine phage-resistant strains were analyzed for their phenotype and different virulence factors, showing alterations in their fitness, motility, biofilm formation, lipopolysaccharide profiles and/or protease activity. Seven of the nine phage-resistant strains showed virulence reduction in a Sparus aurata larvae model. However, this is not generalized since two of the resistant strains show equal virulence compared with the parental strain. The genomic analysis of representative resistant strains displayed that the majority of the mutations are specific for each isolate, affecting genes related to lipopolysaccharide biosynthesis, quorum sensing, motility, toxin and membrane transport. The observed mutations were coherent with the phenotypic and virulence differences observed. These results suggest that the CHOED phage acts as a selective pressure on V. anguillarum, allowing proliferation of resistant strains with different genotypes, phenotypes and degrees of virulence, contributing to bacterial diversification.

Vibrio Flagellar Synthesis

Vibrio spp. are highly motile Gram-negative bacteria, ubiquitously found in aquatic environments. Some Vibrios are responsible for disease and morbidity of marine invertebrates and humans, while others are studied for their symbiotic interactions. Vibrio spp. are motile due to synthesis of flagella that rotate and propel the bacteria. Many Vibrio spp. synthesize monotrichous polar flagella (e.g., V. cholerae, V. alginolyticus); however, some synthesize peritrichous or lophotrichous flagella. Flagellar-mediated motility is intimately connected to biological and cellular processes such as chemotaxis, biofilm formation, colonization, and virulence of Vibrio spp. This review focuses on the polar flagellum and its regulation in regard to Vibrio virulence and environmental persistence.

Survival, Virulent Characteristics, and Transcriptomic Analyses of the Pathogenic Vibrio anguillarum Under Starvation Stress

Many bacteria have developed strategies for metamorphosis into sophisticated survival forms to survive extended periods of environmental stress. As a global causative agent of vibriosis in marine fish farming, Vibrio anguillarum (V. anguillarum) can efficiently grow and proliferate under environmental stress, but the specific mechanism is not clear. In the present study, survival, virulent characteristics, and transcriptomic analysis of the V. anguillarum BH1 were performed under starvation stress. The results demonstrated that V. anguillarum was still culturable and showed rippled surface after 6 months of starvation. Starved cells maintained their infectivity in half-smooth tongue sole (Cynoglossus semilaevi). Detection of virulence factors and virulence-associated genes in starved cells showed that the starved strain still produced β-hemolysis on rabbit blood agar, caseinase, dnase, and gelatinase, and possessed empA, vah1, vah2, vah3, vah4, vah5, rtxA, flaA, flaD, flaE, virC, tonB, mreB, toxR, rpoS, and ftsZ virulence-related genes. In addition, we first reported the RNA-seq study for V. anguillarum with and without starvation treatment for a period of 6 months and emphasized the regulation of gene expression at the whole transcriptional level. It indicated that V. anguillarum expressed 3,089 and 3,072 genes in the control group and starvation stress group, respectively. The differently expressed genes (DEGs) of the starved strain were thereby identified, including 251 up-regulated genes and 272 down-regulated genes in comparison with the non-starved strain. Gene Ontology (GO) analysis and Kyto Encyclopedia Genes and Genomes (KEGG) enrichment analysis of DEGs were also analyzed. GO functional classification revealed that among the significantly regulated genes with known function categories, more genes affiliated with signal transducer activity, molecular transducer activity, and cell communication were significantly up-regulated, and more genes affiliated with cellular macromolecule, cellular component, and structural molecule activity were significantly down-regulated. In addition, the DEGs involved in the pathway of two-component system was significantly up-regulated, and the pathways of ribosome and flagellar assembly were significantly down-regulated. This study provides valuable insight into the survival strategies of V. anguillarum and suggests that a portion of the bacterial populations may remain pathogenic while persisting under starvation stress by up-regulating or down-regulating a series of genes.

The FlaG regulator is involved in length control of the polar flagella of Campylobacter jejuni

Campylobacter jejuni cells have bipolar flagella. Both flagella have similar lengths of about one helical turn, or 3.53±0.52 µm. The flagellar filament is composed of two homologous flagellins: FlaA and FlaB. Mutant strains that express either FlaA or FlaB alone produce filaments that are shorter than those of the wild-type. It is reported that the flaG gene could affect filament length in some species of bacteria, but its function remains unknown. We introduced a flaG-deletion mutation into the C. jejuni wild-type strain and flaA- or flaB-deletion mutant strains, and observed their flagella by microscopy. The ΔflaG mutant cells produced long filaments of two helical turns in the wild-type background. The ΔflaAG double mutant cells produced very short FlaB filaments. On the other hand, ΔflaBG double mutant cells produced long FlaA filaments and their morphology was not helical but straight. Furthermore, FlaG was secreted, and a pulldown assay showed that sigma factor 28 was co-precipitated with purified polyhistidine-tagged FlaG. We conclude that FlaG controls flagella length by negatively regulating FlaA filament assembly and discuss the role of FlaA and FlaB flagellins in C. jejuni flagella formation.

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.

Novel Pattern Recognition Receptor Protects Shrimp by Preventing Bacterial Colonization and Promoting Phagocytosis

The recognition of pathogen-associated molecular patterns is accomplished by the recognition modules of pattern recognition receptors (PRRs). Leucine-rich repeats (LRRs) and C-type lectin-like domain (CTLD) represent the two most universal categories of recognition modules. In the current study, we identified a novel soluble and bacteria-inducible PRR comprising LRRs and a CTLD from the hepatopancreas of kuruma shrimp Marsupenaeus japonicus and named it Leulectin. The module arrangement of Leulectin is unique among all organisms. Both modules, together with the whole molecule, protected shrimp against Vibrio infection. By screening the pathogen-associated molecular patterns that shrimp might encounter, Leulectin was found to sense Vibrio flagellin through the LRRs and to recognize LPS through CTLD. The LRR-flagellin interaction was confirmed by pull-down and far-Western assays and was found to rely on the fourth LRR of Leulectin and the N terminus of flagellin. The recognition of LPS was determined by the long loop region of CTLD in a calcium-independent manner. By sensing the flagellin, LRRs could prevent its attachment to shrimp cells, thereby inhibiting Vibrio colonization. With the ability to recognize LPS, CTLD could agglutinate the bacteria and promote hemocytic phagocytosis. Our study clearly showed the division of labor and the synergy between different recognition modules and provided new insights into the concept of pattern recognition and the function of soluble PRRs in the antibacterial response.

The Role of Vibrios in Diseases of Corals

The tissue, skeleton, and secreted mucus of corals supports a highly dynamic and diverse community of microbes, which play a major role in the health status of corals such as the provision of essential nutrients or the metabolism of waste products. However, members of the Vibrio genus are prominent as causative agents of disease in corals. The aim of this chapter is to review our understanding of the spectrum of disease effects displayed by coral-associated vibrios, with a particular emphasis on the few species where detailed studies of pathogenicity have been conducted. The role of Vibrio shilonii in seasonal bleaching of Oculina patagonica and the development of the coral probiotic hypothesis is reviewed, pointing to unanswered questions about this phenomenon. Detailed consideration is given to studies of V. coralliilyticus and related pathogens and changes in the dominance of vibrios associated with coral bleaching. Other Vibrio-associated disease syndromes discussed include yellow band/blotch disease and tissue necrosis in temperate gorgonian corals. The review includes analysis of the role of enzymes, resistance to oxidative stress, and quorum sensing in virulence of coral-associated vibrios. The review concludes that we should probably regard most-possibly all-vibrios as "opportunistic" pathogens which, under certain environmental conditions, are capable of overwhelming the defense mechanisms of appropriate hosts, leading to rapid growth and tissue destruction.

Temperature Change Induces the Expression of vuuA Encoding Vulnibactin Receptor and crp Encoding Cyclic AMP Receptor Protein in Vibrio vulnificus

Upon entering the human body, Vibrio vulnificus, a gram-negative marine bacterium, must withstand a temperature change (TC) from 25 to 37 °C. This bacterium acquires iron mainly via the vulnibactin receptor (VuuA)-mediated iron uptake system (IUS), which is under the positive control of cyclic AMP receptor protein (CRP), a global regulator responsible for catabolite repression. In this study, we examined the effect of TC on the expression of vuuA and crp, and the reciprocal relation between VuuA-mediated IUS and CRP under iron-limited conditions. Iron limitation increased vuuA expression but decreased crp expression. TC resulted in increased vuuA and crp expression. A crp or vuuA mutation reciprocally decreased vuuA or crp expression. TC could increase vuuA or crp expression even in a crp- or vuuA-mutated background. These results indicate that TC increases the expression of both vuuA and crp by facilitating metabolism under iron-limited conditions, and that CRP and VuuA-mediated IUS interact coordinately toward optimal metabolism in V. vulnificus.

Succinate dehydrogenase mutant of Listonella anguillarum protects rainbow trout against vibriosis

Listonella anguillarum is a Gram-negative facultative anaerobic rod causing hemorrhagic septicemia in marine and rarely in freshwater fish. Succinate dehydrogenase (SDH) plays an important role in the tricarboxylic acid (TCA) cycle by oxidizing succinate to fumarate while reducing ubiquinone to ubiquinol. Recent studies indicate that central metabolic pathways, including the TCA cycle, contribute to bacterial virulence. However, the role of SDH in L. anguillarum virulence has not been studied. Here, we report in-frame deletion of the succinate dehydrogenase iron-sulfur protein (SDHB) and its role in L. anguillarum virulence in rainbow trout. To accomplish this goal, upstream and downstream regions of the L. anguillarum sdhB gene were amplified in-frame and cloned into a suicide plasmid. The chromosomal sdhB gene of L. anguillarum was deleted by homologous recombination. Virulence and immunogenicity of the L. anguillarum ΔsdhB mutant (LaΔsdhB) were determined in rainbow trout. Results show that LaΔsdhB was highly attenuated in rainbow trout, and fish immunized with LaΔsdhB displayed high relative survival rate after exposure to wild type L. anguillarum. These findings indicate SDH is important in L. anguillarum virulence in rainbow trout, and LaΔsdhB could be used as an immersion, oral, or injection vaccine to protect rainbow trout against vibriosis.

Complete genome sequence of Vibrio anguillarum strain NB10, a virulent isolate from the Gulf of Bothnia

Vibrio anguillarum causes a fatal hemorrhagic septicemia in marine fish that leads to great economical losses in aquaculture world-wide. Vibrio anguillarum strain NB10 serotype O1 is a Gram-negative, motile, curved rod-shaped bacterium, isolated from a diseased fish on the Swedish coast of the Gulf of Bothnia, and is slightly halophilic. Strain NB10 is a virulent isolate that readily colonizes fish skin and intestinal tissues. Here, the features of this bacterium are described and the annotation and analysis of its complete genome sequence is presented. The genome is 4,373,835 bp in size, consists of two circular chromosomes and one plasmid, and contains 3,783 protein-coding genes and 129 RNA genes.

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.

RpoS and indole signaling control the virulence of Vibrio anguillarum towards gnotobiotic sea bass (Dicentrarchus labrax) larvae

Quorum sensing, bacterial cell-to-cell communication with small signal molecules, controls the virulence of many pathogens. In contrast to other vibrios, neither the VanI/VanR acylhomoserine lactone quorum sensing system, nor the three-channel quorum sensing system affects virulence of the economically important aquatic pathogen Vibrio anguillarum. Indole is another molecule that recently gained attention as a putative signal molecule. The data presented in this study indicate that indole signaling and the alternative sigma factor RpoS have a significant impact on the virulence of V. anguillarum. Deletion of rpoS resulted in increased expression of the indole biosynthesis gene tnaA and in increased production of indole. Both rpoS deletion and the addition of exogenous indole (50–100 µM) resulted in decreased biofilm formation, exopolysaccharide production (a phenotype that is required for pathogenicity) and expression of the exopolysaccharide synthesis gene wbfD. Further, indole inhibitors increased the virulence of the rpoS deletion mutant, suggesting that indole acts downstream of RpoS. Finally, in addition to the phenotypes found to be affected by indole, the rpoS deletion mutant also showed increased motility and decreased sensitivity to oxidative stress.

Host-induced increase in larval sea bass mortality in a gnotobiotic challenge test with Vibrio anguillarum

Vibrio anguillarum is the major cause of haemorrhagic septicaemia, vibriosis, which is a severe disease affecting marine fish. In this work, it was found that the mortality of gnotobiotic sea bass larvae challenged with V. anguillarum was dependent on the number of dead fish in the vials at the moment of challenge. Based on this finding, the effect of dead hosts (homogenised sea bass larvae or brine shrimp) on the virulence of V. anguillarum towards sea bass larvae was further investigated. Addition of homogenised hosts led to significantly increased larval mortality of challenged larvae, and this was observed for 3 different V. anguillarum strains, i.e. 43, NB 10 and HI 610. In contrast, the addition of similar levels of tryptone had no effect on mortality. In line with this, the motility of all 3 V. anguillarum strains was significantly increased by the addition of homogenised hosts but not by tryptone. These results suggest that dead hosts increase infectivity of V. anguillarum, not merely by offering nutrients to the bacteria, but also by increasing virulence-associated activities such as motility.

Quorum sensing positively regulates flagellar motility in pathogenic Vibrio harveyi

Vibrios belonging to the Harveyi clade are among the major pathogens of aquatic organisms. Quorum sensing (QS) is essential for virulence of V. harveyi towards different hosts. However, most virulence factors reported to be controlled by QS to date are negatively regulated by QS, therefore suggesting that their impact on virulence is limited. In this study, we report that QS positively regulates flagellar motility. We found that autoinducer synthase mutants showed significantly lower swimming motility than the wild type, and the swimming motility could be restored by adding synthetic signal molecules. Further, motility of a luxO mutant with inactive QS (LuxO D47E) was significantly lower than that of the wild type and of a luxO mutant with constitutively maximal QS activity (LuxO D47A). Furthermore, we found that the expression of flagellar genes (both early, middle and late genes) was significantly lower in the luxO mutant with inactive QS when compared with wild type and the luxO mutant with maximal QS activity. Motility assays and gene expression also revealed the involvement of the quorum-sensing master regulator LuxR in the QS regulation of motility. Finally, the motility inhibitor phenamil significantly decreased the virulence of V. harveyi towards gnotobiotic brine shrimp larvae.

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.

Contribution of six flagellin genes to the flagellum biogenesis of Vibrio vulnificus and in vivo invasion

Vibrio vulnificus is a halophilic pathogenic bacterium that is motile due to the presence of a single polar flagellum. V. vulnificus possesses a total of six flagellin genes organized into two loci (flaFBA and flaCDE). We proved that all six of the flagellin genes were transcribed, whereas only five (FlaA, -B, -C, -D, and -F) of the six flagellin proteins were detected. To understand roles of the six V. vulnificus flagellins in motility and virulence, mutants with single and multiple flagellin deletions were constructed. Mutations in flaB or flaC or the flaCDE locus resulted in a significant decrease in motility, adhesion, and cytotoxicity, whereas single mutations in the other flagellin genes or the flaFBA locus showed little or no effect. The motility was completely abolished only in the mutant lacking all six flagellin genes (flaFBA flaCDE). Surprisingly, a double mutation of flaB and flaD, a gene sharing 99% identity with the flaB at the amino acid level, resulted in the largest decrease in motility, adhesion, and cytotoxicity except for the mutant in which all six genes were deleted (the hexa mutant). Additionally, the 50% lethal doses (LD50s) of the flaB flaD and the flaFBA flaCDE mutants increased 23- and 91-fold in a mouse model, respectively, and the in vitro and in vivo invasiveness of the mutants was significantly decreased compared to that of the wild type. Taken together, the multiple flagellin subunits differentially contribute to the flagellum biogenesis and the pathogenesis of V. vulnificus, and among the six flagellin genes, flaB, flaD, and flaC were the most influential components.

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

Hemolysins produced by Vibrio anguillarum have been implicated in the development of hemorrhagic septicemia during vibriosis, a fatal fish disease. Previously, two hemolysin gene clusters responsible for the hemolysis and cytotoxicity of V. anguillarum were identified: the vah1-plp gene cluster and the rtxACHBDE gene cluster. In this study, we identified the hns gene, which encodes the H-NS protein and acts as a negative regulator of both gene clusters. The V. anguillarum H-NS protein shares strong homology with other bacterial H-NS proteins. An hns mutant exhibited increased hemolytic activity and cytotoxicity compared to the wild-type strain. Complementation of the hns mutation restored hemolytic activity and cytotoxicity levels to nearly wild-type levels. Furthermore, expression of rtxA, rtxH, rtxB, vah1, and plp increased in the hns mutant and decreased in the hns-complemented mutant strain compared to expression in the wild-type strain. Additionally, experiments using DNase I showed that purified recombinant H-NS protected multiple sites in the promoter regions of both gene clusters. The hns mutant also exhibited significantly attenuated virulence against rainbow trout. Complementation of the hns mutation restored virulence to wild-type levels, suggesting that H-NS regulates many genes that affect fitness and virulence. Previously, we showed that HlyU is a positive regulator of expression for both gene clusters. In this study, we demonstrate that upregulation by hlyU is hns dependent, suggesting that H-NS acts to repress or silence both gene clusters and HlyU acts to relieve that repression or silencing.

Comparative study of four flagellins of Vibrio anguillarum: vaccine potential and adjuvanticity

Vibrio anguillarum is the etiological agent of vibriosis, an aquaculture disease that affects a wide range of farmed fish. The genome of V. anguillarum contains five flagellin genes, i.e. flaA, flaB, flaC, flaD, and flaE. In this study, we analyzed the vaccine potential and adjuvanticity of FlaA, FlaB, FlaD, and FlaE in a model of Japanese flounder (Paralichthys olivaceus). For this purpose, recombinant FlaA, FlaB, FlaD, and FlaE were expressed in and purified from Escherichia coli. In vivo immunogenicity analysis showed that antibodies against rFlaA, rFlaB, rFlaD, and rFlaE were detected in rat antiserum raised against live V. anguillarum, with the highest antibody level being that against rFlaB. When administered into flounder via intraperitoneal injection, rFlaA, rFlaD, and rFlaE induced comparable relative percent survival (RPS) rates, which were significantly lower than that induced by rFlaB. Specific serum antibodies were induced by all flagellins, however, the antibody level induced by rFlaB was significantly higher than those induced by other three flagellins. Compared to sera from fish vaccinated with rFlaA, rFlaD, and rFlaE, serum from fish vaccinated with rFlaB significantly reduced the infectivity of V. anguillarum against host cells. To examine the potential adjuvant effect of the flagellins, flounder were immunized with rEsa1, a D15-like surface antigen that induces protective immunity as a subunit vaccine, in the presence or absence of rFlaA, rFlaB, rFlaD, and rFlaE respectively. The results showed that rFlaE, but not other three flagellins, significantly increased the RPS of rEsa1. Compared to fish vaccinated with rEsa1, fish vaccinated with rEsa1 plus rFlaE exhibited a significantly higher level of serum antibodies and enhanced expression of the genes involved in innate and adaptive immunity. Taken together, these results indicate that FlaA, FlaB, FlaD, and FlaE have different immunological properties and, as a result, differ in vaccine and adjuvant potentials.

Cyclic AMP and cyclic AMP-receptor protein modulate the autoinducer-2-mediated quorum sensing system in Vibrio vulnificus

This study was undertaken to determine whether cyclic AMP (cAMP) or cAMP-receptor protein (CRP) modulates the activity of the autoinducer (AI)-2-mediated quorum sensing (QS) system in response to glucose availability in Vibrio vulnificus. A mutation in crp impaired V. vulnificus growth, decreased AI-2 production, and repressed the expression of smcR encoding the master regulator SmcR (a Vibrio harveyi LuxR homolog) of the AI-2-QS system, and these changes were prevented by in trans complementation of wild-type crp. Furthermore, glucose repressed smcR expression in the presence of CRP but not in its absence. A mutation in cyaA encoding adenylate cyclase, which is required for cAMP synthesis, also impaired V. vulnificus growth and repressed smcR expression, and these changes were recovered by in trans complementation of wild-type cyaA. These results indicate that cAMP or CRP modulates the AI-2-QS system in response to glucose availability in V. vulnificus, demonstrating the presence of a connection between catabolite repression and quorum sensing in V. vulnificus.

Lipopolysaccharide O-antigen prevents phagocytosis of Vibrio anguillarum by rainbow trout (Oncorhynchus mykiss) skin epithelial cells

Colonization of host tissues is a first step taken by many pathogens during the initial stages of infection. Despite the impact of bacterial disease on wild and farmed fish, only a few direct studies have characterized bacterial factors required for colonization of fish tissues. In this study, using live-cell and confocal microscopy, rainbow trout skin epithelial cells, the main structural component of the skin epidermis, were demonstrated to phagocytize bacteria. Mutant analyses showed that the fish pathogen Vibrio anguillarum required the lipopolysaccharide O-antigen to evade phagocytosis and that O-antigen transport required the putative wzm-wzt-wbhA operon, which encodes two ABC polysaccharide transporter proteins and a methyltransferase. Pretreatment of the epithelial cells with mannose prevented phagocytosis of V. anguillarum suggesting that a mannose receptor is involved in the uptake process. In addition, the O-antigen transport mutants could not colonize the skin but they did colonize the intestines of rainbow trout. The O-antigen polysaccharides were also shown to aid resistance to the antimicrobial factors, lysozyme and polymyxin B. In summary, rainbow trout skin epithelial cells play a role in the fish innate immunity by clearing bacteria from the skin epidermis. In defense, V. anguillarum utilizes O-antigen polysaccharides to evade phagocytosis by the epithelial cells allowing it to colonize rapidly fish skin tissues.

Cyclic AMP-receptor protein activates aerobactin receptor IutA expression in Vibrio vulnificus

The ferrophilic bacterium Vibrio vulnificus can utilize the siderophore aerobactin of Escherichia coli for iron acquisition via its specific receptor IutA. This siderophore piracy by V. vulnificus may contribute to its survival and proliferation, especially in mixed bacterial environments. In this study, we examined the effects of glucose, cyclic AMP (cAMP), and cAMP-receptor protein (Crp) on iutA expression in V. vulnificus. Glucose dose-dependently repressed iutA expression. A mutation in cya encoding adenylate cyclase required for cAMP synthesis severely repressed iutA expression, and this change was recovered by in trans complementing cya or the addition of exogenous cAMP. Furthermore, a mutation in crp encoding Crp severely repressed iutA expression, and this change was recovered by complementing crp. Accordingly, glucose deprivation under iron-limited conditions is an environmental signal for iutA expression, and Crp functions as an activator that regulates iutA expression in response to glucose availability.

Modulation of iron-uptake systems by a mutation of luxS encoding an autoinducer-2 synthase in Vibrio vulnificus

Vibrio vulnificus possesses multiple iron-uptake systems which are mediated by VuuA (vulnibactin receptor), IutA (aerobactin receptor) and HupA (heme receptor). In this study, we determined the effect of a mutation of luxS encoding autoinducer-2 (AI-2) synthase on the expressions of the three receptors. A mutation and an in trans complementation of luxS did not affect the growing ability of V. vulnificus in iron-deficient conditions. Nevertheless, the luxS mutation slightly decreased vuuA expression, but slightly increased iutA and hupA expressions in the transcriptional reporter assay or Western blot analysis. These changes were all recovered by the luxS complementation. These results suggest that AI-2-mediated quorum sensing system may be involved in the fine modulation of V. vulnificus iron-uptake systems, positively affecting vuuA expression but negatively affecting iutA and hupA expressions.

The phosphotransferase VanU represses expression of four qrr genes antagonizing VanO-mediated quorum-sensing regulation in Vibrio anguillarum

Vibrio anguillarum utilizes quorum sensing to regulate stress responses required for survival in the aquatic environment. Like other Vibrio species, V. anguillarum contains the gene qrr1, which encodes the ancestral quorum regulatory RNA Qrr1, and phosphorelay quorum-sensing systems that modulate the expression of small regulatory RNAs (sRNAs) that destabilize mRNA encoding the transcriptional regulator VanT. In this study, three additional Qrr sRNAs were identified. All four sRNAs were positively regulated by σ⁵⁴ and the σ⁵⁴-dependent response regulator VanO, and showed a redundant activity. The Qrr sRNAs, together with the RNA chaperone Hfq, destabilized vanT mRNA and modulated expression of VanT-regulated genes. Unexpectedly, expression of all four qrr genes peaked at high cell density, and exogenously added N-acylhomoserine lactone molecules induced expression of the qrr genes at low cell density. The phosphotransferase VanU, which phosphorylates and activates VanO, repressed expression of the Qrr sRNAs and stabilized vanT mRNA. A model is presented proposing that VanU acts as a branch point, aiding cross-regulation between two independent phosphorelay systems that activate or repress expression of the Qrr sRNAs, giving flexibility and precision in modulating VanT expression and inducing a quorum-sensing response to stresses found in a constantly changing aquatic environment.

Vibrio anguillarum as a fish pathogen: virulence factors, diagnosis and prevention

Vibrio anguillarum, also known as Listonella anguillarum, is the causative agent of vibriosis, a deadly haemorrhagic septicaemic disease affecting various marine and fresh/brackish water fish, bivalves and crustaceans. In both aquaculture and larviculture, this disease is responsible for severe economic losses worldwide. Because of its high morbidity and mortality rates, substantial research has been carried out to elucidate the virulence mechanisms of this pathogen and to develop rapid detection techniques and effective disease-prevention strategies. This review summarizes the current state of knowledge pertaining to V. anguillarum, focusing on pathogenesis, known virulence factors, diagnosis, prevention and treatment.

Detection and identification of Vibrio parahaemolyticus by multiplex PCR and DNA-DNA hybridization on a microarray

In this paper, we developed a rapid and accurate method for the detection of Vibrio parahaemolyticus strains, using multiplex PCR and DNA-DNA hybridization. Multiplex PCR was used to simultaneously amplify three diagnostic genes (tlh, tdh and fla) that serve as molecular markers of V. parahaemolyticus. Biotinylated PCR products were hybridized to primers immobilized on a microarray, and detected by chemiluminesce with avidin-conjugated alkaline phosphatase. With this method, forty-five samples were tested. Eight known virulent strains (tlh(+)/tdh(+)/fla(+)) and four known avirulent strains (tlh(+)/tdh(-)/fla(+)) of the V. parahaemolyticus were successfully detected, and no non-specific hybridization and cross-hybridization reaction were found from fifteen closely-related strains (tlh(-)/tdh(-)/fla(+)) of the Vibrio spp. In addition, all the other eighteen strains of non-Vibrio bacteria (tlh(-)/tdh(-)/fla(-)) gave negative results. The DNA microarray successfully distinguished V. parahaemolyticus from other Vibrio spp. The results demonstrated that this was an efficient and robust method for identifying virulent strains of V. parahaemolyticus.

Complete genome sequence of the marine fish pathogen Vibrio anguillarum harboring the pJM1 virulence plasmid and genomic comparison with other virulent strains of V. anguillarum and V. ordalii

We dissected the complete genome sequence of the O1 serotype strain Vibrio anguillarum 775(pJM1) and determined the draft genomic sequences of plasmidless strains of serotype O1 (strain 96F) and O2β (strain RV22) and V. ordalii. All strains harbor two chromosomes, but 775 also harbors the virulence plasmid pJM1, which carries the anguibactin-producing and cognate transport genes, one of the main virulence factors of V. anguillarum. Genomic analysis identified eight genomic islands in chromosome 1 of V. anguillarum 775(pJM1) and two in chromosome 2. Some of them carried potential virulence genes for the biosynthesis of O antigens, hemolysins, and exonucleases as well as others for sugar transport and metabolism. The majority of genes for essential cell functions and pathogenicity are located on chromosome 1. In contrast, chromosome 2 contains a larger fraction (59%) of hypothetical genes than does chromosome 1 (42%). Chromosome 2 also harbors a superintegron, as well as host "addiction" genes that are typically found on plasmids. Unique distinctive properties include homologues of type III secretion system genes in 96F, homologues of V. cholerae zot and ace toxin genes in RV22, and the biofilm formation syp genes in V. ordalii. Mobile genetic elements, some of them possibly originated in the pJM1 plasmid, were very abundant in 775, resulting in the silencing of specific genes, with only few insertions in the 96F and RV22 chromosomes.

Genetic Determinants of Virulence in the Marine Fish Pathogen Vibrio anguillarum

One of the most studied fish pathogens is Vibrio anguillarum. Development of the genetics and biochemistry of the mechanisms of virulence in this fish pathogen together with clinical and ecologic studies has permitted the intensive development of microbiology in fish diseases. It is the intention of this review to compile the exhaustive knowledge accumulated on this bacterium and its interaction with the host fish by reporting a complete analysis of the V. anguillarum virulence factors and the genetics of their complexity.

Regulation of the Vibrio vulnificus vvpE expression by cyclic AMP-receptor protein and quorum-sensing regulator SmcR

In Vibrio vulnificus, cAMP-receptor protein (CRP) and the quorum-sensing regulator SmcR are simultaneously and cooperatively required for the metalloprotease vvpE gene expression, rather than sequentially in a regulatory cascade. However, this study shows a new temporal and functional sequence between the two factors in regulating vvpE expression. A crp mutation inhibited vvpE expression with growth impairment from early stage. In contrast, a smcR mutation inhibited vvpE expression only at the late stage with no effect on growth. A crp-smcR double mutation severely inhibited vvpE expression with growth impairment from early stage. The inhibited vvpE expression was restored only at the early stage by a crp single complementation, but not at all by a smcR complementation. These results indicate that CRP functions as an essential activator, whereas SmcR functions in the presence of CRP for full vvpE expression.