Role of motility in adherence to and invasion of a fish cell line by Vibrio anguillarum

Characterization of the Vibrio anguillarum VaRyhB regulon and role in pathogenesis

Background

The marine Gram-negative bacterium Vibrio anguillarum is one of the major pathogens in aquaculture. Iron uptake is a prerequisite for virulence and is strictly controlled by a global iron uptake regulator, Fur, which acts as a repressor under iron-replete conditions. When iron is depleted, Fur also functions as an activator, playing an important role in pathogenesis. It is unclear whether this upregulation model is mediated by a small RNA, RyhB.

Methods

The small RNA, VaryhB, was deleted in V. anguillarum strain 775, and its regulon was investigated using transcriptomic analysis. The roles of VaRyhB in siderophore synthesis, chemotaxis and motility, and oxidative stress were evaluated using chrome azurol S (CAS) liquid assay, swimming motility assay, and intracellular reactive oxygen species (ROS) assay, respectively. The virulence of VaRyhB was evaluated by challenging turbot larvae intraperitoneally.

Results

The small RNA called VaRyhB identified in V. anguillarum strain 775 is significantly longer than that in Escherichia coli. Transcriptomic analysis revealed that VaRyhB is critical for iron homeostasis under limited iron conditions, and deletion of VaRyhB resulted in lower expression levels of certain genes for siderophore biosynthesis and transport, thereby leading to impaired growth, reduced siderophore production, and decreased pathogenesis. The virulence factor motility is also upregulated by VaRyhB, and reduced motility capability was observed in the ΔVaryhB mutant, which may be another reason resulting in weak pathogenesis. The sensitivity toward H2O2 in the ΔVafur mutant could be restored by the loss of VaRyhB, suggesting that the role of Fur in oxidative stress is mediated by VaRyhB. VaRyhB also functions to inhibit the expression of genes involved in Fe-S assembly and the TCA cycle. In addition, two aspects of the type VI secretion system and molybdenum cofactor biosynthesis were first identified as being regulated by VaRyhB.

Conclusion

In V. anguillarum, the sRNA VaRyhB plays a critical role in the inhibition of genes involved in the TCA cycle, Fe-S assembly, and the type VI secretion system. It is also essential for the activation of siderophore synthesis, chemotaxis and motility, and anaerobic denitrification. Our work provides the first evidence of the VaRyhB regulon and its role in the pathogenesis of V. anguillarum.

Characterization of the ferric uptake regulator VaFur regulon and its role in Vibrio anguillarum pathogenesis

The Gram-negative marine bacterium Vibrio anguillarum is able to cause vibriosis with hemorrhagic septicemia in many fish species, and iron acquisition is a critical step for virulence. Despite the fact that genes specific to certain processes of iron transport have been studied, the iron-regulated circuits of the V. anguillarum strains remain poorly understood. In this study, we showed that in V. anguillarum strain 775, iron could affect the expression of a number of critical metabolic pathways and virulence factors. The global iron uptake regulator VaFur is the major actor to control these processes for the bacterium to respond to different iron conditions. A VaFur binding motif was identified to distinguish directly and indirectly regulated targets. The absence of VaFur resulted in the aberrant expression of most iron acquisition determinants under rich-iron conditions. A similar regulation pattern was also observed in the transcription of genes coding for the type VI secretion system. The expression of peroxidase genes is positively controlled by VaFur to prevent iron toxicity, and the deletion of Vafur caused impaired growth in the presence of iron and H2O2. VaFur also upregulates some virulence factors under limited-iron conditions, including metalloprotease EmpA and motility, which are likely critical for the high virulence of V. anguillarum 775. The deletion of VaFur led to reduced swimming motility and decreased extracellular protease activity under limited-iron conditions, thereby leading to attenuated pathogenicity. Our study provides more evidence to better understand the VaFur regulon and its role in the pathogenesis of V. anguillarum.IMPORTANCEVibriosis, the most common disease caused by marine bacteria belonging to the genus Vibrio, leads to massive mortality of economical aquatic organisms in Asia. Iron is one of the most important trace elements, and its acquisition is a critical battle occurring between the host and the pathogen. However, excess iron is harmful to cells, so iron utilization needs to be strictly controlled to adapt to different conditions. This process is mediated by the global iron uptake regulator Fur, which acts as a repressor when iron is replete. On the other hand, free iron in the host is limited, so the reduced virulence of the Δfur mutant should not be directly caused by abnormally regulated iron uptake. The significance of this work lies in uncovering the mechanism by which the deletion of Fur causes reduced virulence in Vibrio anguillarum and identifying the critical virulence factors that function under limited-iron conditions.

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.

Zebrafish-based platform for emerging bio-contaminants and virus inactivation research

Emerging bio-contaminants such as viruses have affected health and environment settings of every country. Viruses are the minuscule entities resulting in severe contagious diseases like SARS, MERS, Ebola, and avian influenza. Recent epidemic like the SARS-CoV-2, the virus has undergone mutations strengthen them and allowing to escape from the remedies. Comprehensive knowledge of viruses is essential for the development of targeted therapeutic and vaccination treatments. Animal models mimicking human biology like non-human primates, rats, mice, and rabbits offer competitive advantage to assess risk of viral infections, chemical toxins, nanoparticles, and microbes. However, their economic maintenance has always been an issue. Furthermore, the redundancy of experimental results due to aforementioned aspects is also in examine. Hence, exploration for the alternative animal models is crucial for risk assessments. The current review examines zebrafish traits and explores the possibilities to monitor emerging bio-contaminants. Additionally, a comprehensive picture of the bio contaminant and virus particle invasion and abatement mechanisms in zebrafish and human cells is presented. Moreover, a zebrafish model to investigate the emerging viruses such as coronaviridae and poxviridae has been suggested.

Transcriptome profiles of genes related to growth and virulence potential in Vibrio alginolyticus treated with modified clay

Vibrio alginolyticus is a globally distributed opportunistic pathogen that causes different degrees of disease in various marine organisms, such as fish, shrimp and shellfish. At present, vibriosis caused by V. alginolyticus has a wide epidemic range and causes frequent outbreaks, resulting in substantial losses in aquaculture. According to previous studies, modified clay (MC) could effectively flocculate and reduce the density of Vibrio in water, but it is still unknown whether MC inhibits growth and how it affects virulence in bottom flocs. Here, we studied the response mechanism of V. alginolyticus in flocs treated with MC at the transcriptome level and verified the transcriptomic data combined with relevant physiological experiments and reverse transcription quantitative real-time PCR (RT-qPCR) for the first time. It was found that the morphology of Vibrio in the MC flocs changed, the membrane function was damaged, the antioxidant system was activated, and the material and energy metabolism also changed. In addition, MC could inhibit the expression of virulence factors of V. alginolyticus; for example, flagella, pilus, siderophores, quorum sensing, and other related genes were significantly downregulated. In general, MC effectively inhibited the growth of Vibrio and reduced its virulence potential in flocs, which could provide theoretical support for a new model of healthy aquaculture.

Comparative proteomic profiling reveals specific adaption of Vibrio anguillarum to oxidative stress, iron deprivation and humoral components of innate immunity

The gram-negative bacterium Vibrio (Listonella) anguillarum (VA) is the causative agent of vibriosis, a terminal hemorrhagic septicemia affecting the aquacultural industry across the globe. In the current study we used label-free quantitative proteomics to investigate how VA adapts to conditions that mimic defined aspects of vibriosis-related stress such as exposure to oxidative stress (H₂O₂), exposure to humoral factors of innate immunity through incubation with Atlantic salmon serum, and iron deprivation upon supplementation of 2,2'-dipyridyl (DIP) to the growth medium. We also investigated how regulation of virulence factors may be governed by the VA growth phase and availability of nutrients. All experimental conditions explored revealed stress-specific proteomic adaption of VA and only nine proteins were found to be commonly regulated in all conditions. A general observation made for all stress-related conditions was regulation of multiple metabolic pathways. Notably, iron deprivation and exposure to Atlantic salmon serum evoked upregulation of iron acquisition mechanisms. The findings made in the present study represent a source of potential virulence determinants that can be of use in the search for means to understand vibriosis. SIGNIFICANCE: Vibriosis in fish and shellfish caused by V. anguillarum (VA) is responsible for large economic losses in the aquaculture sector across the globe. However, not much is known about the defense mechanism of this pathogen to percept and adapt to the imposed stresses during infection. Analyzing the response of VA to multiple host-related physiochemical stresses, the quantitative proteomic analysis of the present study indicates modulation of several virulence determinants and key defense networks of this pathogen. Our findings provide a theoretical basis to enhance our understanding of VA pathogenesis and can be employed to improve current intervention strategies to control vibriosis in aquaculture.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Roles of virulence regulator ToxR in viable but non-culturable formation by controlling reactive oxygen species resistance in pathogen Vibrio alginolyticus

Under adverse circumstances, bacteria enter the viable but non-culturable (VBNC) state, a dormancy-like state for survival. The altered gene regulation underlying the entry of the VBNC state has not yet been well elucidated. Here, we reported that a subpopulation of cells (23.8 %) in Vibrio alginolyticus cultures enters the VBNC state in response to nutrient limitation at alkaline pH. The proteolysis of pivotal virulence regulator ToxR at these conditions is associated with VBNC formation. Meantime, ToxR abrogation impaired the mobility and the expression of virulence-associated genes, resulting in attenuated virulence in V. alginolyticus. RNA-seq and ChIP-seq analyses of the cells grown in VBNC-inducing conditions revealed that ToxR directly controls the expression of ∼8 genes including ahpC and dps involved in reactive oxygen species (ROS) resistance. ToxR binds to the promoter regions of kdgR, ppiC, ahpC, and dps and further controls their respective expression under oxidative stress conditions. The cells with impaired ToxR accumulated detrimental intracellular ROS. Moreover, these genes contribute to bacterial culturability as their in-frame deletion strains exhibiting severely decreased plate counts and the complementary strain showed rescued viability. Collectively, this study revealed the role of ToxR in switching on the VBNC state by sensing unfavorable environmental signals such as endogenous ROS (hydrogen peroxide, H₂O₂) in V. alginolyticus and provided mechanistic insights into Vibrio lifestyle adaptation in the marine environment.

Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species

Climate change is expected to have a drastic effect on aquaculture worldwide. As we move forward with the agenda to increase and diversify aquaculture production, rising temperatures will have a progressively relevant impact on fish farming, linked to a multitude of issues associated with fish welfare. Temperature affects the physiology of both fish and pathogens, and has the potential to lead to significant increases in disease outbreaks within aquaculture systems, resulting in severe financial impacts. Significant shifts in future temperature regimes are projected for the Mediterranean Sea. We therefore aim to review and discuss the existing knowledge relating to disease outbreaks in the context of climate change in Mediterranean finfish aquaculture. The objective is to describe the effects of temperature on the physiology of both fish and pathogens, and moreover to list and discuss the principal diseases of the three main fish species farmed in the Mediterranean, namely gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), and meagre (Argyrosomus regius). We will attempt to link the pathology of each disease to a specific temperature range, while discussing potential future disease threats associated with the available climate change trends for the Mediterranean Sea.

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.

Salmonid Antibacterial Immunity: An Aquaculture Perspective

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.

The Expression of Virulence Factors in Vibrio anguillarum Is Dually Regulated by Iron Levels and Temperature

Vibrio anguillarum causes a hemorrhagic septicemia that affects cold- and warm-water adapted fish species. The main goal of this work was to determine the temperature-dependent changes in the virulence factors that could explain the virulence properties of V. anguillarum for fish cultivated at different temperatures. We have found that although the optimal growth temperature is around 25°C, the degree of virulence of V. anguillarum RV22 is higher at 15°C. To explain this result, an RNA-Seq analysis was performed to compare the whole transcriptome profile of V. anguillarum RV22 cultured under low-iron availability at either 25 or 15°C, which would mimic the conditions that V. anguillarum finds during colonization of fish cultivated at warm- or cold-water temperatures. The comparative analysis of transcriptomes at high- and low-iron conditions showed profound metabolic adaptations to grow under low iron. These changes were characterized by a down-regulation of the energetic metabolism and the induction of virulence-related factors like biosynthesis of LPS, production of hemolysins and lysozyme, membrane transport, heme uptake, or production of siderophores. However, the expression pattern of virulence factors under iron limitation showed interesting differences at warm and cold temperatures. Chemotaxis, motility, as well as the T6SS1 genes are expressed at higher levels at 25°C than at 15°C. By contrast, hemolysin RTX pore-forming toxin, T6SS2, and the genes associated with exopolysaccharides synthesis were preferentially expressed at 15°C. Notably, at this temperature, the siderophore piscibactin system was strongly up-regulated. In contrast, at 25°C, piscibactin genes were down-regulated and the vanchrobactin siderophore system seems to supply all the necessary iron to the cell. The results showed that V. anguillarum adjusts the expression of virulence factors responding to two environmental signals, iron levels and temperature. Thus, the relative relevance of each virulence factor for each fish species could vary depending on the water temperature. The results give clues about the physiological adaptations that allow V. anguillarum to cause infections in different fishes and could be relevant for vaccine development against fish vibriosis.

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.

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.

The Siderophore Piscibactin Is a Relevant Virulence Factor for Vibrio anguillarum Favored at Low Temperatures

Vibrio anguillarum causes vibriosis, a hemorrhagic septicaemia that affects many cultured marine fish species worldwide. Two catechol siderophores, vanchrobactin and anguibactin, were previously identified in this bacterium. While vanchrobactin is a chromosomally encoded system widespread in all pathogenic and environmental strains, anguibactin is a plasmid-encoded system restricted to serotype O1 strains. In this work, we have characterized, from a serotype O2 strain producing vanchrobactin, a novel genomic island containing a cluster of genes that would encode the synthesis of piscibactin, a siderophore firstly described in the fish pathogen Photobacterium damselae subsp. piscicida. The chemical characterization of this siderophore confirmed that some strains of V. anguillarum produce piscibactin. An in silico analysis of the available genomes showed that this genomic island is present in many of the highly pathogenic V. anguillarum strains lacking the anguibactin system. The construction of single and double biosynthetic mutants for vanchrobactin and piscibactin allowed us to study the contribution of each siderophore to iron uptake, cell fitness, and virulence. Although both siderophores are simultaneously produced, piscibactin constitute a key virulence factor to infect fish, while vanchrobactin seems to have a secondary role in virulence. In addition, a transcriptional analysis of the gene cluster encoding piscibactin in V. anguillarum showed that synthesis of this siderophore is favored at low temperatures, being the transcriptional activity of the biosynthetic genes three-times higher at 18°C than at 25°C. We also show that iron levels and temperature contribute to balance the synthesis of both siderophores.

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.

Hamp1 but not Hamp2 regulates ferroportin in fish with two functionally distinct hepcidin types

Hepcidin is a small cysteine rich peptide that regulates the sole known cellular iron exporter, ferroportin, effectively controlling iron metabolism. Contrary to humans, where a single hepcidin exists, many fish have two functionally distinct hepcidin types, despite having a single ferroportin gene. This raises the question of whether ferroportin is similarly regulated by the iron regulator Hamp1 and the antimicrobial Hamp2. In sea bass (Dicentrarchus labrax), iron overload prompted a downregulation of ferroportin, associated with an upregulation of hamp1, whereas an opposite response was observed during anemia, with no changes in hamp2 in either situation. During infection, ferroportin expression decreased, indicating iron withholding to avoid microbial proliferation. In vivo administration of Hamp1 but not Hamp2 synthetic peptides caused significant reduction in ferroportin expression, indicating that in teleost fish with two hepcidin types, ferroportin activity is mediated through the iron-regulator Hamp1, and not through the dedicated antimicrobial Hamp2. Additionally, in vitro treatment of mouse macrophages with fish Hamp1 but not Hamp2 caused a decrease in ferroportin levels. These results raise questions on the evolution of hepcidin and ferroportin functional partnership and open new possibilities for the pharmaceutical use of selected fish Hamp2 hepcidins during infections, with no impact on iron homeostasis.

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.

Flagellar expression in clinical isolates of non-typeable Haemophilus influenzae

PURPOSE

Haemophilus influenzae is a commensal organism found in the upper respiratory tract of humans. When H. influenzae becomes a pathogen, these bacteria can move out of their commensal niche and cause multiple respiratory tract diseases such as otitis media, sinusitis, conjunctivitis and bronchitis in children, and chronic obstructive pulmonary disease in adults. However, H. influenzae is currently considered a non-flagellate bacterium.

METHODOLOGY AND RESULTS

In this study, 90 clinical isolates of H. influenzae strains (typeable and non-typeable) showed different degrees of the swarm-motility phenotype in vitro.Keys findings. One of these strains, NTHi BUAP96, showed the highest motility rate and its flagella were revealed using transmission electron microscopy and Ryu staining. Moreover, the flagellar genes fliC and flgH exhibited high homology with those of Actinobacillus pleuropneumoniae, Escherichia coli and Shigella flexneri. Furthermore, Western blot analysis, using anti-flagellin heterologous antibodies from E. coli, demonstrated cross-reaction with a protein present in NTHi BUAP96.

CONCLUSION

This study provides, for the first time, information on flagellar expression in H. influenzae, representing an important finding related to its evolution and pathogenic potential.

Comparative Genome Analyses of Vibrio anguillarum Strains Reveal a Link with Pathogenicity Traits

Comparative genome analysis of strains of a pathogenic bacterial species can be a powerful tool to discover acquisition of mobile genetic elements related to virulence. Here, we compared 28 V. anguillarum strains that differed in virulence in fish larval models. By pan-genome analyses, we found that six of nine highly virulent strains had a unique core and accessory genome. In contrast, V. anguillarum strains that were medium to nonvirulent had low genomic diversity. Integration of genomic and phenotypic features provides insights into the evolution of V. anguillarum and can also be important for survey and diagnostic purposes.

Vibrio anguillarum is a marine bacterium that can cause vibriosis in many fish and shellfish species, leading to high mortalities and economic losses in aquaculture. Although putative virulence factors have been identified, the mechanism of pathogenesis of V. anguillarum is not fully understood. Here, we analyzed whole-genome sequences of a collection of V. anguillarum strains and compared them to virulence of the strains as determined in larval challenge assays. Previously identified virulence factors were globally distributed among the strains, with some genetic diversity. However, the pan-genome revealed that six out of nine high-virulence strains possessed a unique accessory genome that was attributed to pathogenic genomic islands, prophage-like elements, virulence factors, and a new set of gene clusters involved in biosynthesis, modification, and transport of polysaccharides. In contrast, V. anguillarum strains that were medium to nonvirulent had a high degree of genomic homogeneity. Finally, we found that a phylogeny based on the core genomes clustered the strains with moderate to no virulence, while six out of nine high-virulence strains represented phylogenetically separate clusters. Hence, we suggest a link between genotype and virulence characteristics of Vibrio anguillarum, which can be used to unravel the molecular evolution of V. anguillarum and can also be important from survey and diagnostic perspectives.

IMPORTANCE Comparative genome analysis of strains of a pathogenic bacterial species can be a powerful tool to discover acquisition of mobile genetic elements related to virulence. Here, we compared 28 V. anguillarum strains that differed in virulence in fish larval models. By pan-genome analyses, we found that six of nine highly virulent strains had a unique core and accessory genome. In contrast, V. anguillarum strains that were medium to nonvirulent had low genomic diversity. Integration of genomic and phenotypic features provides insights into the evolution of V. anguillarum and can also be important for survey and diagnostic purposes.

Chytridiomycosis of Marine Diatoms-The Role of Stress Physiology and Resistance in Parasite-Host Recognition and Accumulation of Defense Molecules

Little is known about the role of chemotaxis in the location and attachment of chytrid zoospores to potential diatom hosts. Hypothesizing that environmental stress parameters affect parasite-host recognition, four chytrid-diatom tandem cultures (Chytridium sp./Navicula sp., Rhizophydium type I/Nitzschia sp., Rhizophydium type IIa/Rhizosolenia sp., Rhizophydium type IIb/Chaetoceros sp.) were used to test the chemotaxis of chytrid zoospores and the presence of potential defense molecules in a non-contact-co-culturing approach. As potential triggers in the chemotaxis experiments, standards of eight carbohydrates, six amino acids, five fatty acids, and three compounds known as compatible solutes were used in individual and mixed solutions, respectively. In all tested cases, the whole-cell extracts of the light-stressed (continuous light exposure combined with 6 h UV radiation) hosts attracted the highest numbers of zoospores (86%), followed by the combined carbohydrate standard solution (76%), while all other compounds acted as weak triggers only. The results of the phytochemical screening, using biomass and supernatant extracts of susceptible and resistant host-diatom cultures, indicated in most of the tested extracts the presence of polyunsaturated fatty acids, phenols, and aldehydes, whereas the bioactivity screenings showed that the zoospores of the chytrid parasites were only significantly affected by the ethanolic supernatant extract of the resistant hosts.

First evidence of polar flagella in Klebsiella pneumoniae isolated from a patient with neonatal sepsis

The genus Klebsiella belongs to the family Enterobacteriaceae, and is currently considered to be non-motile and non-flagellated. In the present work, 25 Klebsiella strains isolated from nosocomial infections were assessed for motility under different growth conditions. One Klebsiella isolate, KpBUAP021, demonstrated a swim-like motility phenotype. The K. pneumoniae genotype was confirmed by 16S rRNA and rpoB gene sequence analysis. Multilocus sequence typing analysis also revealed that the KpBUAP021 strain places it in the ST345 sequence type, and belongs to the phylogenetic Kpl group. Transmission electron microscopy and the Ryu staining technique revealed that KpBUAP021 expresses polar flagella. Finally, the presence of fliC, fliA and flgH genes in this K. pneumoniae strain was confirmed. This report presents the first evidence for flagella-mediated motility in a K. pneumoniae clinical isolate, and represents an important finding related to its evolution and pathogenic potential.

Bacterial Surfaces: Front Lines in Host-Pathogen Interaction

All bacteria are bound by at least one membrane that acts as a barrier between the cell's interior and the outside environment. Surface components within and attached to the cell membrane are essential for ensuring that the overall homeostasis of the cell is maintained. However, many surface components of the bacterial cell also have an indispensable role mediating interactions of the bacteria with their immediate environment and as such are essential to the pathogenesis of infectious disease. During the course of an infection, bacterial pathogens will encounter many different ecological niches where environmental conditions such as salinity, temperature, pH, and the availability of nutrients fluctuate. It is the bacterial cell surface that is at the front-line of these host-pathogen interactions often protecting the bacterium from hostile surroundings but at the same time playing a critical role in the adherence to host tissues promoting colonization and subsequent infection. To deal effectively with the changing environments that pathogens may encounter in different ecological niches within the host many of the surface components of the bacterial cell are subject to phenotypic variation resulting in heterogeneous subpopulations of bacteria within the clonal population. This dynamic phenotypic heterogeneity ensures that at least a small fraction of the population will be adapted for a particular circumstance should it arise. Diversity within the clonal population has often been masked by studies on entire bacterial populations where it was often assumed genes were expressed in a uniform manner. This chapter, therefore, aims to highlight the non-uniformity in certain cell surface structures and will discuss the implication of this heterogeneity in bacterial-host interaction. Some of the recent advances in studying bacterial surface structures at the single cell level will also be reviewed.

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.

Outer membrane protein U (OmpU) mediates adhesion of Vibrio mimicus to host cells via two novel N-terminal motifs

Vibrio mimicus (V.mimicus) is a causative agent of ascites disease in aquatic animals. Our previous studies have demonstrated that the outer membrane protein U (OmpU) from V.mimicus is an immunoprotective antigen with six immunodominant linear B-cell epitopes. Although the N-terminus of OmpU contains potential binding motifs, it remained unclear whether OmpU possesses adhesion function. Here, the adhesive capacity of recombinant OmpU and V.mimicus to epithelioma papulosum cyprinid (EPC) cells was determined by immunofluorescence and adherence assay. The results showed that after co-incubated with rOmpU, an obvious visible green fluorescence could be observed on the EPC cell surface and the nuclei exhibited blue fluorescence; while the control cell surface did not show any signal, only nuclei exhibited blue fluorescence. The average number of wild-type strain adhered to each cell was 32.3 ± 4.5. The average adhesion number of OmpU gene deletion mutant was significantly reduced to 10.8 ± 0.5 (P < 0.01) and restored to 31.3 ± 2.8 by complement strain (P >0.05). Pretreatment of cells with rOmpU reduced the average adhesion number of wild-type strain to 9.7 ± 2.9 (P < 0.01). Likewise, binding was significantly decreased to 8.8 ± 3.2 (P < 0.01) due to blocking role of OmpU antibodies. To determine binding motifs of OmpU, six immunodominant B-cell epitope peptides labeled with FITC were employed in flow cytometry-based binding assay. Two FITC-labeled epitope peptides (aa90-101 and aa173-192) showed strong binding to EPC cells (the fluorescence positive cell rate was 99 ± 0.6% and 98 ± 0.3%, respectively), which could be specifically competed by excess corresponding unlabeled peptides, whereas the remaining four showed a low level of background binding. This is the first demonstration that OmpU possesses adhesion function and its N terminal 90-101 and 173-192 amino acid regions are critical sites for cell surface binding.

Use of gnotobiotic zebrafish to study Vibrio anguillarum pathogenicity

We evaluated the use of the gnotobiotic zebrafish system to study the effects of bacterial infection, and analyzed expression of genes involved in zebrafish innate immunity. Using a GFP-labeled strain of Vibrio anguillarum, we fluorescently monitored colonization of the zebrafish intestinal tract and used gene expression analysis to compare changes in genes involved in innate immunity between nongnotobiotic and gnotobiotic larvae. The experiments performed with the gnotobiotic zebrafish reveal new insights into V. anguillarum pathogenesis. Specifically, an alteration of the host immune system was detected through the suppression of a number of innate immune genes (NFKB, IL1B, TLR4, MPX, and TRF) during the first 3 h post infection. This immunomodulation can be indicative of a "stealth mechanism" of mucus invasion in which the pathogen found a sheltered niche, a typical trait of intracellular pathogens.

Resistance to Antimicrobial Peptides in Vibrios

Vibrios are associated with a broad diversity of hosts that produce antimicrobial peptides (AMPs) as part of their defense against microbial infections. In particular, vibrios colonize epithelia, which function as protective barriers and express AMPs as a first line of chemical defense against pathogens. Recent studies have shown they can also colonize phagocytes, key components of the animal immune system. Phagocytes infiltrate infected tissues and use AMPs to kill the phagocytosed microorganisms intracellularly, or deliver their antimicrobial content extracellularly to circumvent tissue infection. We review here the mechanisms by which vibrios have evolved the capacity to evade or resist the potent antimicrobial defenses of the immune cells or tissues they colonize. Among their strategies to resist killing by AMPs, primarily vibrios use membrane remodeling mechanisms. In particular, some highly resistant strains substitute hexaacylated Lipid A with a diglycine residue to reduce their negative surface charge, thereby lowering their electrostatic interactions with cationic AMPs. As a response to envelope stress, which can be induced by membrane-active agents including AMPs, vibrios also release outer membrane vesicles to create a protective membranous shield that traps extracellular AMPs and prevents interaction of the peptides with their own membranes. Finally, once AMPs have breached the bacterial membrane barriers, vibrios use RND efflux pumps, similar to those of other species, to transport AMPs out of their cytoplasmic space.

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.

The role of flagella in Clostridium difficile pathogenesis: comparison between a non-epidemic and an epidemic strain

Clostridium difficile is a major cause of healthcare-associated infection and inflicts a considerable financial burden on healthcare systems worldwide. Disease symptoms range from self-limiting diarrhoea to fatal pseudomembranous colitis. Whilst C. difficile has two major virulence factors, toxin A and B, it is generally accepted that other virulence components of the bacterium contribute to disease. C. difficile colonises the gut of humans and animals and hence the processes of adherence and colonisation are essential for disease onset. Previously it has been suggested that flagella might be implicated in colonisation. Here we tested this hypothesis by comparing flagellated parental strains to strains in which flagella genes were inactivated using ClosTron technology. Our focus was on a UK-outbreak, PCR-ribotype 027 (B1/NAP1) strain, R20291. We compared the flagellated wild-type to a mutant with a paralyzed flagellum and also to mutants (fliC, fliD and flgE) that no longer produce flagella in vitro and in vivo. Our results with R20291 provide the first strong evidence that by disabling the motor of the flagellum, the structural components of the flagellum rather than active motility, is needed for adherence and colonisation of the intestinal epithelium during infection. Comparison to published data on 630Δerm and our own data on that strain revealed major differences between the strains: the R20291 flagellar mutants adhered less than the parental strain in vitro, whereas we saw the opposite in 630Δerm. We also showed that flagella and motility are not needed for successful colonisation in vivo using strain 630Δerm. Finally we demonstrated that in strain R20291, flagella do play a role in colonisation and adherence and that there are striking differences between C. difficile strains. The latter emphasises the overriding need to characterize more than just one strain before drawing general conclusions concerning specific mechanisms of pathogenesis.

A novel subtilisin-like serine protease of Batrachochytrium dendrobatidis is induced by thyroid hormone and degrades antimicrobial peptides

Batrachochytrium dendrobatidis (B. dendrobatidis), a chytrid fungus, is one of the major contributors to the global amphibian decline. The fungus infects both tadpoles and adult amphibians. Tadpoles are infected in their keratinized mouthparts, and infected adults exhibit hyperkeratosis and loss of righting reflex. Infections of adults may result in death from cardiac arrest in susceptible species. Thyroid hormone plays a key role in amphibian metamorphosis. The occurrence of B. dendrobatidis in tadpoles during metamorphosis may result in exposure of the fungus to host morphogens including TH. This exposure may induce gene expression in the fungus contributing to invasion and colonization of the host. Here, we demonstrate movement of fungal zoospores toward TH. Additionally, expression of a subtilisin-like serine protease is up-regulated in B. dendrobatidis cells exposed to TH. A gene encoding this protease was cloned from B. dendrobatidis and expressed in Escherichia coli. The protein was partially purified and characterized. The similarity between subtilases of human dermatophytes and the B. dendrobatidis subtilisin-like serine protease suggests the importance of this enzyme in B. dendrobatidis pathogenicity. Cleavage of frog skin antimicrobial peptides (AMPs) by this B. dendrobatidis subtilisin-like serine protease suggests a role for this enzyme in fungal survival and colonization.

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.

Establishment and characterization of a new fish cell line from head kidney of half-smooth tongue sole (Cynoglossus semilaevis)

A new cell line (TSHKC) derived from half-smooth tongue sole (Cynoglossus semilaevis) head kidney was developed. The cell line was subcultured for 40 passages over a period of 360 days. The cell line was optimally maintained in minimum essential medium supplemented with HEPES, antibiotics, fetal bovine serum, 2-Mercaptoethanol (2-Me), sodium pyruvate and basic fibroblast growth factor. The suitable growth temperature for TSHKC cells was 24 °C, and microscopically, TSHKC cells were composed of fibroblast-like cells. Chromosome analysis revealed that the TSHKC cell line had a normal diploid karyotype with 2n = 42, contained the heterogametic W chromosome. The TSHKC cell line was found to be susceptible to lymphocystis disease virus. The fluorescent signals were observed in TSHKC when the cells were transfected with green fluorescent protein and red fluorescent protein reporter plasmids.

LitR of Vibrio salmonicida is a salinity-sensitive quorum-sensing regulator of phenotypes involved in host interactions and virulence

Vibrio (Aliivibrio) salmonicida is the causal agent of cold-water vibriosis, a fatal bacterial septicemia primarily of farmed salmonid fish. The molecular mechanisms of invasion, colonization, and growth of V. salmonicida in the host are still largely unknown, and few virulence factors have been identified. Quorum sensing (QS) is a cell-to-cell communication system known to regulate virulence and other activities in several bacterial species. The genome of V. salmonicida LFI1238 encodes products presumably involved in several QS systems. In this study, the gene encoding LitR, a homolog of the master regulator of QS in V. fischeri, was deleted. Compared to the parental strain, the litR mutant showed increased motility, adhesion, cell-to-cell aggregation, and biofilm formation. Furthermore, the litR mutant produced less cryptic bioluminescence, whereas production of acylhomoserine lactones was unaffected. Our results also indicate a salinity-sensitive regulation of LitR. Finally, reduced mortality was observed in Atlantic salmon infected with the litR mutant, implying that the fish were more susceptible to infection with the wild type than with the mutant strain. We hypothesize that LitR inhibits biofilm formation and favors planktonic growth, with the latter being more adapted for pathogenesis in the fish host.

Temperature regulation of virulence factors in the pathogen Vibrio coralliilyticus

Sea surface temperatures (SST) are rising because of global climate change. As a result, pathogenic Vibrio species that infect humans and marine organisms during warmer summer months are of growing concern. Coral reefs, in particular, are already experiencing unprecedented degradation worldwide due in part to infectious disease outbreaks and bleaching episodes that are exacerbated by increasing SST. For example, Vibrio coralliilyticus, a globally distributed bacterium associated with multiple coral diseases, infects corals at temperatures above 27 °C. The mechanisms underlying this temperature-dependent pathogenicity, however, are unknown. In this study, we identify potential virulence mechanisms using whole genome sequencing of V. coralliilyticus ATCC (American Type Culture Collection) BAA-450. Furthermore, we demonstrate direct temperature regulation of numerous virulence factors using proteomic analysis and bioassays. Virulence factors involved in motility, host degradation, secretion, antimicrobial resistance and transcriptional regulation are upregulated at the higher virulent temperature of 27 °C, concurrent with phenotypic changes in motility, antibiotic resistance, hemolysis, cytotoxicity and bioluminescence. These results provide evidence that temperature regulates multiple virulence mechanisms in V. coralliilyticus, independent of abundance. The ecological and biological significance of this temperature-dependent virulence response is reinforced by climate change models that predict tropical SST to consistently exceed 27 °C during the spring, summer and fall seasons. We propose V. coralliilyticus as a model Gram-negative bacterium to study temperature-dependent pathogenicity in Vibrio-related diseases.

Vibrio salmonicida pathogenesis analyzed by experimental challenge of Atlantic salmon (Salmo salar)

Cold-water vibriosis (CV) is a bacterial septicemia of farmed salmonid fish and cod caused by the Gram-negative bacterium Vibrio (Aliivibrio) salmonicida. To study the pathogenesis of this marine pathogen, Atlantic salmon was experimentally infected by immersion challenge with wild type V. salmonicida and the bacterial distribution in different organs was investigated at different time points. V. salmonicida was identified in the blood as early as 2 h after challenge demonstrating a rapid establishment of bacteremia without an initial period of colonization of the host. Two days after immersion challenge, only a few V. salmonicida were identified in the intestines, but the amount increased with time. In prolonged CV cases, V. salmonicida was the dominating bacterium of the gut microbiota causing a release of the pathogen to the water. We hypothesize that V. salmonicida uses the blood volume for proliferation during the infection of the fish and the salmonid intestine as a reservoir that favors survival and transmission. In addition, a motility-deficient V. salmonicida strain led us to investigate the impact of motility in the CV pathogenesis by comparing the virulence properties of the mutant with the wild type LFI1238 strain in both i.p. and immersion challenge experiments. V. salmonicida was shown to be highly dependent on motility to gain access to the fish host. After invasion, motility was no longer required for virulence, but the absence of normal flagellation delayed the disease development.

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.

AlgW regulates multiple Pseudomonas syringae virulence strategies

Gram-negative bacterial pathogens have evolved a number of virulence-promoting strategies including the production of extracellular polysaccharides such as alginate and the injection of effector proteins into host cells. The induction of these virulence mechanisms can be associated with concomitant downregulation of the abundance of proteins that trigger the host immune system, such as bacterial flagellin. In Pseudomonas syringae, we observed that bacterial motility and the abundance of flagellin were significantly reduced under conditions that induce the type III secretion system. To identify genes involved in this negative regulation, we conducted a forward genetic screen with P. syringae pv. maculicola ES4326 using motility as a screening phenotype. We identified the periplasmic protease AlgW as a key negative regulator of flagellin abundance that also positively regulates alginate biosynthesis and the type III secretion system. We also demonstrate that AlgW constitutes a major virulence determinant of P. syringae required to dampen plant immune responses. Our findings support the conclusion that P. syringae co-ordinately regulates virulence strategies through AlgW in order to effectively suppress host immunity.

Identification and cloning of immunogenic Aliivibrio salmonicida Pal-like protein present in profiled outer membrane and secreted subproteome

Aliivibrio salmonicida is the aetiological agent of cold water vibriosis affecting farmed fish species, a disease that today is fully controlled by vaccination. However, the molecular mechanisms behind the successful vaccine are largely unknown. In order to gain insight into the possible mechanisms of A. salmonicida vaccines, we report here the profiles of both the outer membrane and secreted subproteomes of A. salmonicida LFI315. The 2 subproteomes were resolved by 2-dimensional electrophoresis that identified a total of 82 protein entries. Monoclonal antibodies specific to an unidentified protein antigen were utilized in the immunoproteomic analysis of both outer membrane proteins and extracellular proteins. The immunogenic protein was located in both subproteomes and identified as a 20 kDa peptidoglycan-associated lipoprotein (Pal). The identity of the antigen was verified by heterologous expression of the cloned A. salmonicida pal gene (VSAL_I1899). It is likely that the immunogenic Pal-like protein is among the constituents that act as a protective antigen in the successful vaccine used today. In view of this, it may be considered a potentially useful component in future vaccine development and pathogenicity studies.

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.

Deletion of valR, a homolog of Vibrio harveyiś luxR generates an intermediate colony phenotype between opaque/rugose and translucent/smooth in Vibrio alginolyticus

A previous study has shown that Vibrio alginolyticus ZJ-51 undergoes colony phase variation between opaque/rugose (Op) and translucent/smooth (Tr). The AI-2 quorum-sensing master regulator ValR, a homolog to V. harveyi LuxR, was suggested to be involved in the transition. To investigate the role of ValR in the variation and in biofilm formation, an in-frame deletion of valR in both Op and Tr backgrounds was carried out. The mutants in both backgrounds showed an intermediate colony morphotype, where the colonies were less opaque/rugose but not fully translucent/smooth either. They also showed an intermediate level of motility. However, biofilm formation was severely decreased in both mutants and polar flagella were depleted also. Quantitative PCR showed that most of the genes related to flagellar and polysaccharide biosynthesis were upregulated in the mutant of Op background (Delta valR/Op) but downregulated in the mutant of Tr background (Delta valR/Tr) compared with their parental wild-type strains. This suggests that ValR may control biofilm formation by regulating flagellar biosynthesis and affect the expression of the genes involved in colony phase variation in V. alginolyticus.

Expression, purification and antibody preparation of flagellin FlaA from Vibrio alginolyticus strain HY9901

AIMS

The main aims of this study were to clone and express flagellin flaA gene from Vibrio alginolyticus strain HY9901, also to prepare mouse anti-FlaA polyclonal antibody for future pathogen or vaccine study.

METHODS AND RESULTS

The full-length flaA gene was amplified by PCR with designed primers. The open reading frame of flaA gene contains 1131 bp, and its putative protein consists of 376 amino acid residues. Alignment analysis indicated that the FlaA protein was highly conserved. SDS-PAGE indicated that the FlaA protein was successfully expressed in Escherichia coli BL21 (DE3). Then, the recombinant FlaA protein was purified by affinity chromatography, and the mouse anti-FlaA serum was produced. The expression of flaA gene was verified by various immunological methods, including western blotting, enzyme-linked immunosorbent assay (ELISA) and immunogold electron microscopy (IEM).

CONCLUSIONS

Flagellin flaA gene was cloned and identified from V. alginolyticus HY9901, the recombinant FlaA protein was expressed and purified, and high-titre FlaA protein-specific antibody was produced. Western blot analysis revealed that the prepared antiserum not only specifically react to FlaA fusion protein, but also to natural FlaA protein of V. alginolyticus. The expressed FlaA protein was demonstrated, for the first time, as the component of flagella from V. alginolyticus by IEM.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study may offer important insights into the pathogenesis of V. alginolyticus, provide a base for further studies on the diagnosis and evaluation that whether the FlaA protein could be used as an effective vaccine candidate against infection by V. alginolyticus and other Vibrio species. Additionally, the purified FlaA protein and polyclonal antibody can be used for further functional and structural studies.

Role of flagella in virulence of the coral pathogen Vibrio coralliilyticus

A recently available transposition system was utilized to isolate a nonmotile mutant of the coral-bleaching pathogen Vibrio coralliilyticus. The mutation was localized to the fhlA gene, and the mutant lacked flagella. The flhA mutant was unable to exhibit chemotaxis toward coral mucus or to adhere to corals and subsequently cause infection.

In Vitro Elicitation of Intestinal Immune Responses in Teleost Fish: Evidence for a Type IV Hypersensitivity Reaction in Rainbow Trout

In fish the gut immune system has been the subject of few investigations until now. Here, we provide novel morphological and immunological data on the gut isolated from rainbow trout Salmo gairdneri. The pyloric (P) and terminal (T) segments of trout gut, when morphologically examined, evidenced lymphocytes and macrophages (MØ) loosely dispersed in the intestinal mucosa and in the lamina propria in the absence of typical Peyer's patches-like structures. Furthermore, incubation of P and T sections with Candida albicans (Ca) and functional analysis of supernatants generated some interesting results. In fact, active supernatants, when compared with controls, exhibited cytokine-like activities attributable to the presence of interferon (IFN)-gamma and migration inhibiting factor (MIF), respectively. In particular, IFN-gamma-like activity gave rise to an enhancement of Ca phagocytosis by MØ, whereas MIF inhibited MØ migration in agarose. Taken together, these in vitro data suggest that the gut-associated lymphoreticular tissue in fish possesses the appropriate armamentarium to mount a type IV hypersensitivity response when challenged by microbial antigens.

Adaptive response to environmental changes in the fish pathogen Moritella viscosa

The marine psychrophilic bacterium Moritella viscosa is the causative agent of winter ulcer in farmed Atlantic salmon and cod. In this study, the growth requirements of the pathogen were established. The effects of changes in salinity and temperature on growth, surface features and proteomic regulation were also investigated. The genome of this bacterium has not yet been sequenced; therefore, comparative two-dimensional gel electrophoresis (2-DE) was used, coupled with high performance tandem mass spectrometry (MS/MS), to perform cross-species protein identification. Results from this study establish that M. viscosa is a true marine psychrophilic bacterium capable of surviving and proliferating in an oligotrophic and cold environment. Low temperature combined with 3-4% NaCl resulted in significantly higher cell yields and stability compared to high temperature and 1% NaCl. Nine cytoplasmic proteins were shown to be regulated by temperature and 12 by salinity. Several of the regulated proteins indicated a stressful situation at 15 degrees C compared to 4 degrees C, consistent with the growth characteristics observed. Furthermore, temperature and salinity were demonstrated to be important determinants of motility and viscosity of M. viscosa.