Characterization of heme uptake cluster genes in the fish pathogen Vibrio 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.

Remodulation of bacterial transcriptome after acquisition of foreign DNA: the case of irp-HPI high-pathogenicity island in Vibrio anguillarum

The high-pathogenicity island irp-HPI is widespread in Vibrionaceae and encodes the siderophore piscibactin, as well as the regulator PbtA that is essential for its expression. In this work, we aim to study whether PbtA directly interacts with irp-HPI promoters. Furthermore, we hypothesize that PbtA, and thereby the acquisition of irp-HPI island, may also influence the expression of other genes elsewhere in the bacterial genome. To address this question, an RNAseq analysis was conducted to identify differentially expressed genes after pbtA deletion in Vibrio anguillarum RV22 genetic background. The results showed that PbtA not only modulates the irp-HPI genes but also modulates the expression of a plethora of V. anguillarum core genome genes, inducing nitrate, arginine, and sulfate metabolism, T6SS1, and quorum sensing, while repressing lipopolysaccharide (LPS) production, MARTX toxin, and major porins such as OmpV and ChiP. The direct binding of the C-terminal domain of PbtA to piscibactin promoters (PfrpA and PfrpC), quorum sensing (vanT), LPS transporter wza, and T6SS structure- and effector-encoding genes was demonstrated by electrophoretic mobility shift assay (EMSA). The results provide valuable insights into the regulatory mechanisms underlying the expression of irp-HPI island and its impact on Vibrios transcriptome, with implications in pathogenesis.IMPORTANCEHorizontal gene transfer enables bacteria to acquire traits, such as virulence factors, thereby increasing the risk of the emergence of new pathogens. irp-HPI genomic island has a broad dissemination in Vibrionaceae and is present in numerous potentially pathogenic marine bacteria, some of which can infect humans. Previous works showed that certain V. anguillarum strains exhibit an expanded host range plasticity and heightened virulence, a phenomenon linked to the acquisition of the irp-HPI genomic island. The present work shows that this adaptive capability is likely achieved through comprehensive changes in the transcriptome of the bacteria and that these changes are mediated by the master regulator PbtA encoded within the irp-HPI element. Our results shed light on the broad implications of horizontal gene transfer in bacterial evolution, showing that the acquired DNA can directly mediate changes in the expression of the core genome, with profounds implications in pathogenesis.

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

IMPORTANCE

This study highlights diversity in iron acquisition and regulation in bacteria. The mechanisms of iron acquisition and its regulation in Teredinibacter turnerae, as well as its connection to cellulose utilization, a hallmark phenotype of T. turnerae, expand the paradigm of bacterial iron acquisition. Two of the four TonB genes identified in T. turnerae exhibit functional redundancy and play a crucial role in siderophore-mediated iron transport. Unlike typical TonB genes in bacteria, none of the TonB genes in T. turnerae are clearly iron regulated. This unusual regulation could be explained by another important finding in this study, namely, that the two TonB genes involved in iron transport are also essential for cellulose utilization as a carbon source, leading to the expression of TonB genes even under iron-rich conditions.

The Sigma Factor AsbI Is Required for the Expression of Acinetobactin Siderophore Transport Genes in Aeromonas salmonicida

Aeromonas salmonicida subsp. salmonicida (A. salmonicida), a Gram-negative bacterium causing furunculosis in fish, produces the siderophores acinetobactin and amonabactins in order to extract iron from its hosts. While the synthesis and transport of both systems is well understood, the regulation pathways and conditions necessary for the production of each one of these siderophores are not clear. The acinetobactin gene cluster carries a gene (asbI) encoding a putative sigma factor belonging to group 4 σ factors, or, the ExtraCytoplasmic Function (ECF) group. By generating a null asbI mutant, we demonstrate that AsbI is a key regulator that controls acinetobactin acquisition in A. salmonicida, since it directly regulates the expression of the outer membrane transporter gene and other genes necessary for Fe-acinetobactin transport. Furthermore, AsbI regulatory functions are interconnected with other iron-dependent regulators, such as the Fur protein, as well as with other sigma factors in a complex regulatory network.

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

Teredinibacter turnerae is an intracellular bacterial symbiont that resides in the gills of shipworms, wood-eating bivalve mollusks. This bacterium produces a catechol siderophore, turnerbactin, required for the survival of this bacterium under iron limiting conditions. The turnerbactin biosynthetic genes are contained in one of the secondary metabolite clusters conserved among T. turnerae strains. However, Fe(III)-turnerbactin uptake mechanisms are largely unknown. Here, we show that the first gene of the cluster, fttA a homologue of Fe(III)-siderophore TonB-dependent outer membrane receptor (TBDR) genes is indispensable for iron uptake via the endogenous siderophore, turnerbactin, as well as by an exogenous siderophore, amphi-enterobactin, ubiquitously produced by marine vibrios. Furthermore, three TonB clusters containing four tonB genes were identified, and two of these genes, tonB1b and tonB2, functioned not only for iron transport but also for carbohydrate utilization when cellulose was a sole carbon source. Gene expression analysis revealed that none of the tonB genes and other genes in those clusters were clearly regulated by iron concentration while turnerbactin biosynthesis and uptake genes were up-regulated under iron limiting conditions, highlighting the importance of tonB genes even in iron rich conditions, possibly for utilization of carbohydrates derived from cellulose.

Pathological and Ultrastructural Characterization of an Edwardsiella ictaluri Triple hemR Mutant

Enteric septicemia of catfish, which is caused by Edwardsiella ictaluri, is detrimental to farmed Channel Catfish Ictalurus punctatus. The hemin receptor HemR is involved in binding and uptake of heme into bacteria. Here, we explored pathological and ultrastructural changes in catfish fry that were immunized with a triple hemR mutant of E. ictaluri and challenged with wild-type E. ictaluri (EiWT) 28 d after immunization. Following immunization, pathological changes in the triple hemR-immunized fry were less severe compared to the EiWT-exposed control fry. Widely disseminated bacteria and severe necrosis in most organs, especially the kidney and spleen, were detected in both groups at days 4, 5, and 6. Multifocal granulomatous encephalitis with bacteria was seen in hemR-immunized fry at days 21 and 28 and in EiWT-exposed control fry at day 14. Phagocytic cells in the kidney and spleen of EiWT-exposed control fry contained more replicating bacteria compared to hemR-immunized fry. During the EiWT challenge of immunized fry, a robust immune response was observed in the triple hemR-immunized fry compared to the sham-vaccinated group. Many activated phagocytic cells were detected in the kidney and spleen with fragmented or no bacteria in the triple hemR-immunized fry. Our data suggested that virulence of triple hemR was lower and the onset of the lesions was delayed compared to EiWT. Additionally, triple hemR-immunized fry could mount an immune response and had milder lesions compared to the sham control after EiWT exposure.

Identification of Key Functions Required for Production and Utilization of the Siderophore Piscibactin Encoded by the High-Pathogenicity Island irp-HPI in Vibrionaceae

Piscibactin is a widespread siderophore system present in many different bacteria, especially within the Vibrionaceae family. Previous works showed that most functions required for biosynthesis and transport of this siderophore are encoded by the high-pathogenicity island irp-HPI. In the present work, using Vibrio anguillarum as a model, we could identify additional key functions encoded by irp-HPI that are necessary for piscibactin production and transport and that have remained unknown. Allelic exchange mutagenesis, combined with cross-feeding bioassays and LC-MS analysis, were used to demonstrate that Irp4 protein is an essential component for piscibactin synthesis since it is the thioesterase required for nascent piscibactin be released from the NRPS Irp1. We also show that Irp8 is a MFS-type protein essential for piscibactin secretion. In addition, after passage through the outer membrane transporter FrpA, the completion of ferri-piscibactin internalization through the inner membrane would be achieved by the ABC-type transporter FrpBC. The expression of this transporter is coordinated with the expression of FrpA and with the genes encoding biosynthetic functions. Since piscibactin is a major virulence factor of some pathogenic vibrios, the elements of biosynthesis and transport described here could be additional interesting targets for the design of novel antimicrobials against these bacterial pathogens.

A Highly Unstable and Elusive Plasmid That Encodes the Type III Secretion System Is Necessary for Full Virulence in the Marine Fish Pathogen Photobacterium damselae subsp. piscicida

The marine bacterium Photobacterium damselae subsp. piscicida (Pdp) causes photobacteriosis in fish and important financial losses in aquaculture, but knowledge of its virulence factors is still scarce. We here demonstrate that an unstable plasmid (pPHDPT3) that encodes a type III secretion system (T3SS) is highly prevalent in Pdp strains from different geographical origins and fish host species. We found that pPHDPT3 undergoes curing upon in vitro cultivation, and this instability constitutes a generalized feature of pPHDPT3-like plasmids in Pdp strains. pPHDPT3 markers were detected in tissues of naturally-infected moribund fish and in the Pdp colonies grown directly from the fish tissues but were undetectable in a fraction of the colonies produced upon the first passage of the primeval colonies on agar plates. Notably, cured strains exhibited a marked reduction in virulence for fish, demonstrating that pPHDPT3 is a major virulence factor of Pdp. The attempts to stabilize pPHDPT3 by insertion of antibiotic resistance markers by allelic exchange caused an even greater reduction in virulence. We hypothesize that the existence of a high pressure to shed pPHDPT3 plasmid in vitro caused the selection of clones with off-target mutations and gene rearrangements during the process of genetic modification. Collectively, these results show that pPHDPT3 constitutes a novel, hitherto unreported virulence factor of Pdp that shows a high instability in vitro and warn that the picture of Pdp virulence genes has been historically underestimated, since the loss of the T3SS and other plasmid-borne genes may have occurred systematically in laboratories for decades.

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.

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

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

Vibrio neptunius Produces Piscibactin and Amphibactin and Both Siderophores Contribute Significantly to Virulence for Clams

Vibrio neptunius is an inhabitant of mollusc microbiota and an opportunistic pathogen causing disease outbreaks in marine bivalve mollusc species including oysters and clams. Virulence of mollusc pathogenic vibrios is mainly associated with the production of extracellular products. However, siderophore production is a common feature in pathogenic marine bacteria but its role in fitness and virulence of mollusc pathogens remains unknown. We previously found that V. neptunius produces amphibactin, one of the most abundant siderophores in marine microbes. In this work, synthesis of the siderophore piscibactin was identified as the second siderophore produced by V. neptunius. Single and double mutants in biosynthetic genes of each siderophore system, piscibactin and amphibactin, were constructed in V. neptunius and their role in growth ability and virulence was characterized. Although the High Pathogenicity Island encoding piscibactin is a major virulence factor in vibrios pathogenic for fish, the V. neptunius wild type did not cause mortality in turbot. The results showed that amphibactin contributes more than piscibactin to bacterial fitness in vitro. However, infection challenges showed that each siderophore system contributes equally to virulence for molluscs. The V. neptunius strain unable to produce any siderophore was severely impaired to cause vibriosis in clams. Although the inactivation of one of the two siderophore systems (either amphibactin or piscibactin) significantly reduced virulence compared to the wild type strain, the ability to produce both siderophores simultaneously maximised the degree of virulence. Evaluation of the gene expression pattern of each siderophore system showed that they are simultaneously expressed when V. neptunius is cultivated under low iron availability in vitro and ex vivo. Finally, the analysis of the distribution of siderophore systems in genomes of Vibrio spp. pathogenic for molluscs showed that the gene clusters encoding amphibactin and piscibactin are widespread in the Coralliilyticus clade. Thus, siderophore production would constitute a key virulence factor for bivalve molluscs pathogenic vibrios.

The two-component system RstAB regulates production of a polysaccharide capsule with a role in virulence in the marine pathogen Photobacterium damselae subsp. damselae

The marine bacterium Photobacterium damselae subsp. damselae (Pdd) causes disease in marine animals and humans. Previous studies demonstrated that mutation of the two-component system RstAB strongly impacts virulence of this pathogen, but the RstAB regulon has not been thoroughly elucidated. We here compared the transcriptomes of Pdd RM-71 and ΔrstA and ΔrstB derivatives using RNA-seq. In accordance with previous studies, RstAB positively regulated cytotoxins Dly, PhlyP and PhlyC. This analysis also demonstrated a positive regulation of outer membrane proteins, resistance against antimicrobials and potential virulence factors by this system. Remarkably, RstAB positively regulated two hitherto uncharacterised gene clusters involved in the synthesis of a polysaccharide capsule. Presence of a capsular layer in wild-type cells was confirmed by transmission electron microscopy, whereas rstA and rstB mutants were non-capsulated. Mutants for capsule synthesis genes, wza and wzc exhibited acapsular phenotypes, were impaired in resistance against the bactericidal action of fish serum and mucus, and were strongly impaired in virulence for fish, indicating a major role of capsule in virulence. Collectively, this study demonstrates that RstAB is a major positive regulator of key virulence factors including a polysaccharide capsule essential for full virulence in a pathogenic Photobacterium.

The Vibriolysin-Like Protease VnpA and the Collagenase ColA Are Required for Full Virulence of the Bivalve Mollusks Pathogen Vibrio neptunius

Vibrio neptunius is an important pathogen of bivalve mollusks worldwide. Several metalloproteases have been described as virulence factors in species of Vibrio that are pathogenic to bivalves, but little is known about the contribution of these potential virulence factors to Vibrio neptunius pathogenesis. In silico analysis of the genome of V. neptunius strain PP-145.98 led to the identification of two hitherto uncharacterized chromosomal loci encoding a probable vibriolysin-like metalloprotease and a putative collagenase, which were designated VnpA and ColA, respectively. Single defective mutants of each gene were obtained in V. neptunius PP-145.98, and the phospholipase, esterase and collagenase activities were studied and compared with those of the wild-type strain. The results showed that the single inactivation of vnpA resulted in a 3-fold reduction in phospholipase/esterase activity. Inactivation of colA reduced the collagenase activity by 50%. Finally, infection challenges performed in oyster larvae showed that ΔvnpA and ΔcolA—single mutant strains of V. neptunius—are between 2–3-fold less virulent than the wild-type strain. Thus, the present work demonstrates that the production of both VnpA and ColA is required for the full virulence of the bivalve pathogen V. neptunius.

The marine bivalve molluscs pathogen Vibrio neptunius produces the siderophore amphibactin, which is widespread in molluscs microbiota

Amphiphilic siderophores, including amphibactins, are the most abundant siderophores in oceans. Genes putatively encoding the amphibactin system were proposed in some bacteria and homologues of these genes are particularly abundant in multiple bacterial lineages inhabitant of low-iron seawater. However, since no defective mutant strains in any of these genes were studied to date, their role in amphibactin synthesis or uptake was not demonstrated. In this work, an in silico analysis of the genome of the mollusc pathogen Vibrio neptunius leads us to identify a gene cluster (denoted absABDEF) that is predicted to encode an amphibactin-like siderophore and several mutant strains unable to synthesize or use siderophores were constructed. The results showed that genes absABDEF are required for amphibactin synthesis. A comparative chemical analysis of V. neptunius wild type and biosynthesis mutants allowed us to identify a mixture of nine amphibactin forms produced by this bacterium. In addition, the gene abtA is predicted to encode the ferri-amphibactin outer membrane transporter. The prevalence of the amphibactin system in bivalve hemolymph microbiota was also studied. We found that the amphibactin system is widespread in hemolymph microbiota including both commensal and pathogenic bacterial species. Thus, its contribution to bacterial fitness must be more related to environmental persistence than to pathogenicity.

Diverse Horizontally-Acquired Gene Clusters Confer Sucrose Utilization to Different Lineages of the Marine Pathogen Photobacterium damselae subsp. damselae

The ability to metabolize sucrose is a variable trait within the family Vibrionaceae. The marine bacterium Photobacterium damselae subsp. damselae (Pdd), pathogenic for marine animals and humans, is generally described as negative for sucrose utilization (Scr⁻). Previous studies have reported sucrose-utilizing isolates (Scr⁺), but the genetic basis of this variable phenotype remains uncharacterized. Here, we carried out the genome sequencing of five Scr⁺ and two Scr⁻Pdd isolates and conducted a comparative genomics analysis with sixteen additional Pdd genomes sequenced in previous studies. We identified two different versions of a four-gene cluster (scr cluster) exclusive of Scr⁺ isolates encoding a PTS system sucrose-specific IIBC component (scrA), a fructokinase (scrK), a sucrose-6-phosphate hydrolase (scrB), and a sucrose operon repressor (scrR). A scrA deletion mutant did not ferment sucrose and was impaired for growth with sucrose as carbon source. Comparative genomics analyses suggested that scr clusters were acquired by horizontal transfer by different lineages of Pdd and were inserted into a recombination hot-spot in the Pdd genome. The incongruence of phylogenies based on housekeeping genes and on scr genes revealed that phylogenetically diverse gene clusters for sucrose utilization have undergone extensive horizontal transfer among species of Vibrio and Photobacterium.

Exposure of the Opportunistic Marine Pathogen Photobacterium damselae subsp. damselae to Human Body Temperature Is a Stressful Condition That Shapes the Transcriptome, Viability, Cell Morphology, and Virulence

Photobacterium damselae subsp. damselae (Pdd), an important pathogen for marine animals, is also an opportunistic human pathogen that can cause fatal necrotizing fasciitis. The regulatory changes triggered by the temperature shift experienced by this marine pathogen upon entering the human body, are completely unknown. Here we report an RNA-seq approach combined with phenotypical assays to study the response of Pdd to cultivation at 37°C in comparison to 25°C. We found that cultivation of a Pdd highly virulent strain for fish and mice, RM-71, at 37°C, initially enhanced bacterial growth in comparison to 25°C as evidenced by the increase in optical density. However, cells were found to undergo a progressive loss of viability after 6 h cultivation at 37°C, and no viable cells could be detected from 30 h cultures at 37°C. In contrast, at 25°C, viable cell counts achieved the highest values at 30 h cultivation. Cells grown at 25°C showed normal rod morphology by scanning electron microscopy analysis whereas cells grown at 37°C exhibited chain-like structures and aberrant long shapes suggesting a defect in daughter cell separation and in septum formation. Cells grown at 37°C also exhibited reduced tolerance to benzylpenicillin. Using a RNA-seq approach we discovered that growth at 37°C triggered a heat-shock response, whereas genes involved in motility and virulence were repressed including iron acquisition systems, the type two secretion system, and damselysin toxin, a major virulence factor of Pdd. Human isolates did not exhibit advantage growing at 37°C compared to fish isolates, and comparative genomics did not reveal gene markers specific of human isolates, suggesting that any Pdd genotype existing in the marine environment might potentially cause disease in humans. Altogether, these data indicate that the potential of Pdd to cause disease in humans is an accidental condition rather than a selected trait, and that human body temperature constitutes a stressful condition for Pdd. This study provides the first transcriptome profile of Pdd exposed at human body temperature, and unveils a number of candidate molecular targets for prevention and control of human infections caused by this pathogen.

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.

Thermal stress responses of Sodalis glossinidius, an indigenous bacterial symbiont of hematophagous tsetse flies

Tsetse flies (Diptera: Glossinidae) house a taxonomically diverse microbiota that includes environmentally acquired bacteria, maternally transmitted symbiotic bacteria, and pathogenic African trypanosomes. Sodalis glossinidius, which is a facultative symbiont that resides intra and extracellularly within multiple tsetse tissues, has been implicated as a mediator of trypanosome infection establishment in the fly’s gut. Tsetse’s gut-associated population of Sodalis are subjected to marked temperature fluctuations each time their ectothermic fly host imbibes vertebrate blood. The molecular mechanisms that Sodalis employs to deal with this heat stress are unknown. In this study, we examined the thermal tolerance and heat shock response of Sodalis. When grown on BHI agar plates, the bacterium exhibited the most prolific growth at 25^oC, and did not grow at temperatures above 30^oC. Growth on BHI agar plates at 31°C was dependent on either the addition of blood to the agar or reduction in oxygen levels. Sodalis was viable in liquid cultures for 24 hours at 30^oC, but began to die upon further exposure. The rate of death increased with increased temperature. Similarly, Sodalis was able to survive for 48 hours within tsetse flies housed at 30^oC, while a higher temperature (37^oC) was lethal. Sodalis’ genome contains homologues of the heat shock chaperone protein-encoding genes dnaK, dnaJ, and grpE, and their expression was up-regulated in thermally stressed Sodalis, both in vitro and in vivo within tsetse fly midguts. Arrested growth of E. coli dnaK, dnaJ, or grpE mutants under thermal stress was reversed when the cells were transformed with a low copy plasmid that encoded the Sodalis homologues of these genes. The information contained in this study provides insight into how arthropod vector enteric commensals, many of which mediate their host’s ability to transmit pathogens, mitigate heat shock associated with the ingestion of a blood meal.

Microorganisms associated with insects must cope with fluctuating temperatures. Because symbiotic bacteria influence the biology of their host, how they respond to temperature changes will have an impact on the host and other microorganisms in the host. The tsetse fly and its symbionts represent an important model system for studying thermal tolerance because the fly feeds exclusively on vertebrate blood and is thus exposed to dramatic temperature shifts. Tsetse flies house a microbial community that can consist of symbiotic and environmentally acquired bacteria, viruses, and parasitic African trypanosomes. This work, which makes use of tsetse’s commensal endosymbiont, Sodalis glossinidius, is significance because it represents the only examination of thermal tolerance mechanisms in a bacterium that resides indigenously within an arthropod disease vector. A better understanding of the biology of thermal tolerance in Sodalis provides insight into thermal stress survival in other insect symbionts and may yield information to help control vector-borne disease.

The Outer Membrane Protein FstC of Aeromonas salmonicida subsp. salmonicida Acts as Receptor for Amonabactin Siderophores and Displays a Wide Ligand Plasticity. Structure-Activity Relationships of Synthetic Amonabactin Analogues

Amonabactins are a group of four related catecholate siderophores produced by several species of the genus Aeromonas, including A. hydrophila and the fish pathogen A. salmonicida subsp. salmonicida. Although the gene cluster encoding amonabactin biosynthesis also contains a gene that could encode the ferri-siderophore receptor (fstC), to date there is no experimental evidence to explain its role. In this work, we report the identification of the amonabactins' outer membrane receptor and the determination of the minimal structural parts of these siderophores involved in the molecular recognition by their cognate receptor. The four natural amonabactin forms (P750, T789, P693, and T732) and some mono and biscatecholate amonabactin analogues were chemically synthesized, and their siderophore activity on A. salmonicida FstC(+) and FstC(-) strains was evaluated. The results showed that each amonabactin form has quite different growth promotion activity, with P750 and T789 the most active. The outer membrane receptor FstC recognizes more efficiently biscatecholate siderophores in which the length of the linker between the two iron-binding catecholamide units is 15 atoms (P750 and T789) instead of 12 atoms (P693 and T732). Analysis of the siderophore activity of synthetic analogues indicated that the presence of Phe or Trp residues is not required for siderophore recognition. The results together point toward evidence that the amonabactin receptor FstC admits a high degree of ligand plasticity. We also showed that FstC is present in most Aeromonas species, including relevant human and animal pathogens as A. hydrophila. From the results obtained, we concluded that the ferri-amonabactin uptake pathway involving the outer membrane transporter FstC possesses a considerable functional plasticity that could be exploited for delivery of antimicrobial compounds into the cell. This would allow the use of the siderophore-based iron uptake mechanisms to combat infections caused by species of the genus Aeromonas.

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.

The RstAB System Impacts Virulence, Motility, Cell Morphology, Penicillin Tolerance and Production of Type II Secretion System-Dependent Factors in the Fish and Human Pathogen Photobacterium damselae subsp. damselae

The RstB histidine kinase of the two component system RstAB positively regulates the expression of damselysin (Dly), phobalysin P (PhlyP) and phobalysin C (PhlyC) cytotoxins in the fish and human pathogen Photobacterium damselae subsp. damselae, a marine bacterium of the family Vibrionaceae. However, the function of the predicted cognate response regulator RstA has not been studied so far, and the role of the RstAB system in other cell functions and phenotypes remain uninvestigated. Here, we analyzed the effect of rstA and rstB mutations in cell fitness and in diverse virulence-related features. Both rstA and rstB mutants were severely impaired in virulence for sea bream and sea bass fish. Mutants in rstA and rstB genes were impaired in hemolysis and in Dly-dependent phospholipase activity but had intact PlpV-dependent phospholipase and ColP-dependent gelatinase activities. rstA and rstB mutants grown at 0.5% NaCl exhibited impaired swimming motility, enlarged cell size and impaired ability to separate after cell division, whereas at 1% NaCl the mutants exhibited normal phenotypes. Mutation of any of the two genes also impacted tolerance to benzylpenicillin. Notably, rstA and rstB mutants showed impaired secretion of a number of type II secretion system (T2SS)-dependent proteins, which included the three major cytotoxins Dly, PhlyP and PhlyC, as well as a putative delta-endotoxin and three additional uncharacterized proteins which might constitute novel virulence factors of this pathogenic bacterium. The analysis of the T2SS-dependent secretome of P. damselae subsp. damselae also led to the identification of RstAB-independent potential virulence factors as lipoproteins, sialidases and proteases. The RstAB regulon included plasmid, chromosome I and chromosome II-encoded genes that showed a differential distribution among isolates of this subspecies. This study establishes RstAB as a major regulator of virulence and diverse cellular functions in P. damselae subsp. damselae.

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.

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.

Chromosome-Encoded Hemolysin, Phospholipase, and Collagenase in Plasmidless Isolates of Photobacterium damselae subsp. damselae Contribute to Virulence for Fish

Photobacterium damselae subsp. damselae is a pathogen of marine animals, including fish of importance in aquaculture. The virulence plasmid pPHDD1, characteristic of highly hemolytic isolates, encodes the hemolysins damselysin (Dly) and phobalysin (PhlyP). Strains lacking pPHDD1 constitute the vast majority of the isolates from fish outbreaks, but genetic studies to identify virulence factors in plasmidless strains are scarce. Here, we show that the chromosome I-encoded hemolysin PhlyC plays roles in virulence and cell toxicity in pPHDD1-negative isolates of this pathogen. By combining the analyses of whole genomes and of gene deletion mutants, we identified two hitherto uncharacterized chromosomal loci encoding a phospholipase (PlpV) and a collagenase (ColP). PlpV was ubiquitous in the subspecies and exerted hemolytic activity against fish erythrocytes, which was enhanced in the presence of lecithin. ColP was restricted to a fraction of the isolates and was responsible for the collagen-degrading activity in this subspecies. Consistent with the presence of signal peptides in PlpV and ColP sequences, mutants for the type II secretion system (T2SS) genes epsL and pilD exhibited impairments in phospholipase and collagenase activities. Sea bass virulence experiments and cell culture assays demonstrated major contributions of PhlyC and PlpV to virulence and toxicity.IMPORTANCE This study constitutes genetic and genomic analyses of plasmidless strains of an emerging pathogen in marine aquaculture, Photobacterium damselae subsp. damselae To date, studies on the genetic basis of virulence were restricted to the pPHDD1 plasmid-encoded toxins Dly and PhlyP. However, the vast majority of the recent isolates of this pathogen from fish farm outbreaks lack this plasmid. Here we demonstrate that the plasmidless strains produce two hitherto uncharacterized ubiquitous toxins encoded in chromosome I, namely, the hemolysin PhlyC and the phospholipase PlpV. We report the main roles of these two toxins in fish virulence and in cell toxicity. Our results constitute the basis for a better understanding of the virulence of a widespread marine pathogen.

rstB Regulates Expression of the Photobacterium damselae subsp. damselae Major Virulence Factors Damselysin, Phobalysin P and Phobalysin C

The marine pathogenic bacterium Photobacterium damselae subsp. damselae causes septicemia in marine animals and in humans. The pPHDD1 plasmid-encoded hemolysins damselysin (Dly) and phobalysin P (PhlyP), and the chromosome-encoded hemolysin phobalysin C (PhlyC) constitute its main virulence factors. However, the mechanisms by which expression of these three hemolysins is regulated remain unknown. Here we report the isolation of a mini-Tn10 transposon mutant which showed a strong impairment in its hemolytic activity. The transposon disrupted a putative sensor histidine kinase gene vda_000600 (rstB), which together with vda_000601 (rstA) is predicted to encode a putative two-component regulatory system. This system showed to be homologous to the Vibrio cholerae CarSR/VprAB and Escherichia coli RstAB systems. Reconstruction of the mutant by allelic exchange of rstB showed equal impairment in hemolysis, and complementation with a plasmid expressing rstAB restored hemolysis to wild-type levels. Remarkably, we demonstrated by promoter expression analyses that the reduced hemolysis in the rstB mutant was accompanied by a strong decrease in transcription activities of the three hemolysin genes dly (damselysin), hlyA_pl (phobalysin P) and hlyA_ch (phobalysin C). Thus, RstB, encoded in the small chromosome, regulates plasmid and chromosomal virulence genes. We also found that reduced expression of the three virulence genes correlated with a strong decrease in virulence in a sea bass model, demonstrating that RstB constitutes a master regulator of the three P. damselae subsp. damselae hemolysins and plays critical roles in the pathogenicity of this bacterium. This study represents the first evidence of a direct role of a RstAB-like system in the regulation of bacterial toxins.

Iron assimilation and siderophore production by Vibrio ordalii strains isolated from diseased Atlantic salmon Salmo salar in Chile

Vibrio ordalii is the causative agent of vibriosis in several cultured salmonid species worldwide. Despite its impact on aquaculture, relatively little information is available about its virulence factors. The present study demonstrates for the first time that V. ordalii possesses different systems of iron acquisition, one involving siderophore synthesis and another one that uses direct binding of heme to use iron. Using 6 strains of V. ordalii from Atlantic salmon Salmo salar and the V. ordalii type strain, we could demonstrate that all strains could grow in presence of the chelating agent 2,2'-dipyridyl and produced siderophores in solid and liquid media. Cross-feeding assays among V. ordalii strains evidenced variability in the siderophores produced. Bioassays and PCR data suggest that V. ordalii could produce a siderophore with a structure similar to piscibactin, although the production of a second siderophore in certain strains cannot be discarded. Furthermore, all strains were able to use hemin and hemoglobin as the only iron sources, although the cell yield was higher when using hemoglobin. A hemin-binding assay indicated the presence of constitutive heme-binding molecules at the cell surface of V. ordalii. Virulence tests using rainbow trout as a model of infection revealed a clear relationship between iron-uptake ability and pathogenicity in V. ordalii.

Phobalysin, a Small β-Pore-Forming Toxin of Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae, an important pathogen of marine animals, may also cause septicemia or hyperaggressive necrotizing fasciitis in humans. We previously showed that hemolysin genes are critical for virulence of this organism in mice and fish. In the present study, we characterized the hlyA gene product, a putative small β-pore-forming toxin, and termed it phobalysin P (PhlyP), for "photobacterial lysin encoded on a plasmid." PhlyP formed stable oligomers and small membrane pores, causing efflux of K(+), with no significant leakage of lactate dehydrogenase but entry of vital dyes. The latter feature distinguished PhlyP from the related Vibrio cholerae cytolysin. Attack by PhlyP provoked a loss of cellular ATP, attenuated translation, and caused profound morphological changes in epithelial cells. In coculture experiments with epithelial cells, Photobacterium damselae subsp. damselae led to rapid hemolysin-dependent membrane permeabilization. Unexpectedly, hemolysins also promoted the association of P. damselae subsp. damselae with epithelial cells. The collective observations of this study suggest that membrane-damaging toxins commonly enhance bacterial adherence.

A Transmissible Plasmid-Borne Pathogenicity Island Confers Piscibactin Biosynthesis in the Fish Pathogen Photobacterium damselae subsp. piscicida

The fish pathogen Photobacterium damselae subsp. piscicida produces the siderophore piscibactin. A gene cluster that resembles the Yersinia high-pathogenicity island (HPI) encodes piscibactin biosynthesis. Here, we report that this HPI-like cluster is part of a hitherto-uncharacterized 68-kb plasmid dubbed pPHDP70. This plasmid lacks homologs of genes that mediate conjugation, but we found that it could be transferred at low frequencies from P. damselae subsp. piscicida to a mollusk pathogenic Vibrio alginolyticus strain and to other Gram-negative bacteria, likely dependent on the conjugative functions of the coresident plasmid pPHDP60. Following its conjugative transfer, pPHDP70 restored the capacity of a vibrioferrin mutant of V. alginolyticus to grow under low-iron conditions, and piscibactin became detectable in its supernatant. Thus, pPHDP70 appears to harbor all the genes required for piscibactin biosynthesis and transport. P. damselae subsp. piscicida strains cured of pPHDP70 no longer produced piscibactin, had impaired growth under iron-limited conditions, and exhibited markedly decreased virulence in fish. Collectively, our findings highlight the importance of pPHDP70, with its capacity for piscibactin-mediated iron acquisition, in the virulence of P. damselae subsp. piscicida. Horizontal transmission of this plasmid-borne piscibactin synthesis gene cluster in the marine environment may facilitate the emergence of new pathogens.

Photobacterium damselae subsp. damselae major virulence factors Dly, plasmid-encoded HlyA, and chromosome-encoded HlyA are secreted via the type II secretion system

Photobacterium damselae subsp. damselae is a marine bacterium that causes septicemia in marine animals and in humans. Previously, we had determined a major role of pPHDD1 plasmid-encoded Dly (damselysin) and HlyA (HlyApl) and the chromosome-encoded HlyA (HlyAch) hemolysins in virulence. However, the mechanisms by which these toxins are secreted remain unknown. In this study, we found that a mini-Tn10 transposon mutant in a plasmidless strain showing an impaired hemolytic phenotype contained an insertion in epsL, a component of a type II secretion system (T2SS). Reconstruction of the mutant by allelic exchange confirmed the specific involvement of epsL in HlyAch secretion. In addition, mutation of epsL in a pPHDD1-harboring strain caused an almost complete abolition of hemolytic activity against sheep erythrocytes, indicating that epsL plays a major role in secretion of the plasmid-encoded HlyApl and Dly. This was further demonstrated by analysis of different combinations of hemolysin gene mutants and by strain-strain complementation assays. We also found that mutation of the putative prepilin peptidase gene pilD severely affected hemolysis, which dropped at levels inferior to those of epsL mutants. Promoter expression analyses suggested that impairment of hemolysin secretion in epsL and pilD mutants might constitute a signal that affects hemolysin and T2SS gene expression at the transcriptional level. In addition, single epsL and pilD mutations caused a drastic decrease in virulence for mice, demonstrating a major role of T2SS and pilD in P. damselae subsp. damselae virulence.

Evidence for horizontal gene transfer, gene duplication and genetic variation as driving forces of the diversity of haemolytic phenotypes in Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae, a marine bacterium that causes infections in marine animals and in humans, produces up to three different haemolysins involved in virulence, which include the pPHDD1 plasmid-encoded damselysin (Dly) and HlyApl , and the chromosome-encoded HlyAch . We screened 45 isolates from different origins, and found a correlation between their haemolytic phenotypes and the differential haemolysin gene content. All highly and medium haemolytic strains harboured pPHDD1, with amino acid substitutions in HlyApl and HlyAch being the cause of the medium haemolytic phenotypes in some pPHDD1-harbouring strains. Weakly haemolytic strains contained only hlyAch , whereas nonhaemolytic isolates, in addition to lacking pPHDD1, either lacked hlyAch or contained a hlyAch pseudogene. Sequence analysis of the genomic context of hlyAch uncovered an unexpected genetic diversity, suggesting that hlyAch is located in an unstable chromosomal region. Phylogenetic analysis suggested that hlyApl and hlyAch originated by gene duplication within P. damselae subsp. damselae following acquisition by horizontal transfer. These observations together with the differential distribution of pPHDD1 plasmid among strains suggest that horizontal gene transfer has played a main role in shaping the haemolysin gene baggage in this pathogen.

Synergistic and additive effects of chromosomal and plasmid-encoded hemolysins contribute to hemolysis and virulence in Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae causes infections and fatal disease in marine animals and in humans. Highly hemolytic strains produce damselysin (Dly) and plasmid-encoded HlyA (HlyA(pl)). These hemolysins are encoded by plasmid pPHDD1 and contribute to hemolysis and virulence for fish and mice. In this study, we report that all the hemolytic strains produce a hitherto uncharacterized chromosome-encoded HlyA (HlyAch). Hemolysis was completely abolished in a single hlyAch mutant of a plasmidless strain and in a dly hlyApl hlyAch triple mutant. We found that Dly, HlyA(pl), and HlyAch are needed for full hemolytic values in strains harboring pPHDD1, and these values are the result of the additive effects between HlyApl and HlyAch, on the one hand, and of the synergistic effect of Dly with HlyApl and HlyAch, on the other hand. Interestingly, Dly-producing strains produced synergistic effects with strains lacking Dly production but secreting HlyA, constituting a case of the CAMP (Christie, Atkins, and Munch-Petersen) reaction. Environmental factors such as iron starvation and salt concentration were found to regulate the expression of the three hemolysins. We found that the contributions, in terms of the individual and combined effects, of the three hemolysins to hemolysis and virulence varied depending on the animal species tested. While Dly and HlyApl were found to be main contributors in the virulence for mice, we observed that the contribution of hemolysins to virulence for fish was mainly based on the synergistic effects between Dly and either of the two HlyA hemolysins rather than on their individual effects.

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.

The haem-uptake gene cluster in Vibrio fischeri is regulated by Fur and contributes to symbiotic colonization

Although it is accepted that bacteria-colonizing host tissues are commonly faced with iron-limiting conditions and that pathogenic bacteria often utilize iron from host-derived haem-based compounds, the mechanisms of iron acquisition by beneficial symbiotic bacteria are less clear. The bacterium Vibrio fischeri mutualistically colonizes the light organ of the squid Euprymna scolopes. Genome sequence analysis of V. fischeri revealed a putative haem-uptake gene cluster, and through mutant analysis we confirmed this cluster is important for haemin use by V. fischeri in culture. LacZ reporter assays demonstrated Fur-dependent transcriptional regulation of cluster promoter activity in culture. GFP-based reporter assays revealed that gene cluster promoter activity is induced in symbiotic V. fischeri as early as 14 h post inoculation, although colonization assays with the haem uptake mutant suggested an inability to uptake haem does not begin to limit colonization until later stages of the symbiosis. Our data indicate that the squid light organ is a low iron environment and that haem-based sources of iron are used by symbiotic V. fischeri cells. These findings provide important additional information on the availability of iron during symbiotic colonization of E. scolopes by V. fischeri, as well as the role of haem uptake in non-pathogenic host-microbe interactions.

The Photobacterium damselae subsp. damselae hemolysins damselysin and HlyA are encoded within a new virulence plasmid

Photobacterium damselae subsp. damselae (formerly Vibrio damsela) is a marine bacterium that causes infections and fatal disease in a wide range of marine animals and in humans. Highly hemolytic strains produce damselysin (Dly), a cytolysin encoded by the dly gene that is lethal for mice and has hemolytic activity. We found that Dly is encoded in the highly hemolytic strain RM-71 within a 153,429-bp conjugative plasmid that we dubbed pPHDD1. In addition to Dly, pPHDD1 also encodes a homologue of the pore-forming toxin HlyA. We found a direct correlation between presence of pPHDD1 and a strong hemolytic phenotype in a collection of P. damselae subsp. damselae isolates. Hemolysis was strongly reduced in a double dly hlyA mutant, demonstrating the role of the two pPHDD1-encoded genes in hemolysis. Interestingly, although single hlyA and dly mutants showed different levels of hemolysis reduction depending on the erythrocyte source, hemolysis was not abolished in any of the single mutants, suggesting that the hemolytic phenotype is the result of the additive effect of Dly and HlyA. We found that pPHDD1-encoded dly and hlyA genes are necessary for full virulence for mice and fish. Our results suggest that pPHDD1 can be considered as a driving force for the emergence of a highly hemolytic lineage of P. damselae subsp. damselae.

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.

The ABC-transporter hutCD genes of Photobacterium damselae subsp. piscicida are essential for haem utilization as iron source and are expressed during infection in fish

The marine fish pathogen Photobacterium damselae subsp. piscicida utilizes haem compounds as the sole iron source. In a previous work, we characterized a gene cluster with ten potential haem uptake and utilization genes. Two of these genes, hutC and hutD, which are iron-regulated, conform a putative inner membrane haem ABC transporter. In this study, we constructed an insertional mutant, leading to the inactivation of hutCD genes. Reverse transcriptase-PCR analyses demonstrated that an insertion between the hutB and hutC genes abolished transcription of the downstream hutC and hutD genes. The hutCD mutant was unable to utilize haem as the sole iron source, demonstrating that the putative ABC-transporter proteins HutC and HutD are essential for haem utilization as an iron source in P. damselae subsp. piscicida. In addition, reverse transcriptase-PCR assays conducted with RNA samples isolated from experimentally infected fish revealed the presence of hutCD transcripts. The results demonstrate for the first time that haem uptake genes of a fish pathogen are expressed during the infective process in fish.

Anguibactin- versus vanchrobactin-mediated iron uptake in Vibrio anguillarum: evolution and ecology of a fish pathogen

Vibrio anguillarum is a marine bacterium that is present in many marine aquatic environments and that is the main cause of vibriosis in diverse wild and cultured fish species. Two siderophore-mediated iron uptake systems have been described in V. anguillarum. One, mediated by the siderophore anguibactin, is encoded by the pJM1-type plasmids and is restricted to serotype O1 strains. The second one is mediated by the vanchrobactin siderophore and is widespread in many strains belonging to different serotypes. Both siderophores belong to the catecholate group of siderophores, sharing a 2,3-dihydroxybenzoic acid moiety. Vanchrobactin biosynthesis and transport genes are present in all strains examined although the siderophore is not produced in serotype O1 strains harbouring a pJM1-type plasmid. In these strains the insertion of an IS element in the main vanchrobactin biosynthetic gene vabF leads to the fact that only anguibactin is produced. From our current knowledge we can presume that vanchrobactin is the ancestral siderophore in this species and that the anguibactin-mediated system was later acquired during evolution, likely by horizontal transfer. The role of these two different iron uptake mechanisms in the biology, evolution and ecology of V. anguillarum is discussed although they are still far from being completely understood.

Structure and heme binding properties of Escherichia coli O157:H7 ChuX

For many pathogenic microorganisms, iron acquisition from host heme sources stimulates growth, multiplication, ultimately enabling successful survival and colonization. In gram-negative Escherichia coli O157:H7, Shigella dysenteriae and Yersinia enterocolitica the genes encoded within the heme utilization operon enable the effective uptake and utilization of heme as an iron source. While the complement of proteins responsible for heme internalization has been determined in these organisms, the fate of heme once it has reached the cytoplasm has only recently begun to be resolved. Here we report the first crystal structure of ChuX, a member of the conserved heme utilization operon from pathogenic E. coli O157:H7 determined at 2.05 A resolution. ChuX forms a dimer which remarkably given low sequence homology, displays a very similar fold to the monomer structure of ChuS and HemS, two other heme utilization proteins. Absorption spectral analysis of heme reconstituted ChuX demonstrates that ChuX binds heme in a 1:1 manner implying that each ChuX homodimer has the potential to coordinate two heme molecules in contrast to ChuS and HemS where only one heme molecule is bound. Resonance Raman spectroscopy indicates that the heme of ferric ChuX is composed of a mixture of coordination states: 5-coordinate and high-spin, 6-coordinate and low-spin, and 6-coordinate and high-spin. In contrast, the reduced ferrous form displays mainly a 5-coordinate and high-spin state with a minor contribution from a 6-coordinate and low-spin state. The nu(Fe-CO) and nu(C-O) frequencies of ChuX-bound CO fall on the correlation line expected for histidine-coordinated hemoproteins indicating that the fifth axial ligand of the ferrous heme is the imidazole ring of a histidine residue. Based on sequence and structural comparisons, we designed a number of site-directed mutations in ChuX to probe the heme binding sites and dimer interface. Spectral analysis of ChuX and mutants suggests involvement of H65 and H98 in heme coordination as mutations of both residues were required to abolish the formation of the hexacoordination state of heme-bound ChuX.

Localization and characterization of VVA0331, a 489-kDa RTX-like protein, in Vibrio vulnificus YJ016

Vibrio vulnificus YJ016 contains three genes encoding proteins homologous to repeats-in-toxin proteins. One of these genes, vva0331, possesses a long open reading frame of 13,971 bp in length and resides on the small chromosome between two gene clusters encoding a type I secretion system and several regulatory proteins, respectively. Bioinformatic analysis revealed that VVA0331 consist of nineteen 87-amino acid repeats, two Arg-Gly-Asp motifs, four cysteine residues, an outer membrane protein domain, a polysaccharide-binding site and several motifs related to cell adhesions. These features are distinct from those of typical repeat-in-toxins and autotransporter adhesins. Real-time quantitative PCR analysis indicates that vva0331 gene expression is activated at 30 degrees C and regulated by iron. In addition, VVA0331 is present primarily in a secreted form as determined by cell fractionation assay and Western blot analysis. No significant difference in Hep2 cell adherence, cytotoxicity, and virulence was observed between the wild type and vva0331 mutant strains. In contrast, these strains exhibited apparently different outer membrane protein profiles, and antiserum raised against C-terminal region of VVA0331 reacted with an 85-kDa outer membrane protein of V. vulnificus YJ016.

FvtA is the receptor for the siderophore vanchrobactin in Vibrio anguillarum: utility as a route of entry for vanchrobactin analogues

Some strains of Vibrio anguillarum, the causative agent of vibriosis in a variety of marine animals, produce a catechol-type siderophore named vanchrobactin. The biosynthetic pathway and regulation of vanchrobactin are quite well understood. However, aspects concerning its entry into the cell have remained uncharacterized. In the present study we characterized two genes, fvtA and orf13, encoding potential TonB-dependent ferric-vanchrobactin receptors in serotype O2 V. anguillarum strain RV22. We found that an fvtA mutant was defective for growth under iron limitation conditions and for utilization of vanchrobactin, suggesting that fvtA encodes the vanchrobactin receptor of V. anguillarum. Interestingly, an orf13 mutant was not significantly affected, and results of reverse transcriptase PCR, as well as analysis of outer membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggested that this gene is not expressed. Furthermore, fatA, a plasmid gene coding for the anguibactin receptor in plasmid pJM1-harboring strains, is also present in the chromosome of RV22, although it is inactivated by insertion of transposases. In addition, we found that FvtA is the route of entry for vanchrobactin analogues, and there is evidence that it recognizes primarily the catechol-iron center. These analogues are potential candidate vectors for a Trojan horse strategy aimed at generating antimicrobial compounds exploiting the same route of entry for native siderophores. We found that fvtA and vanchrobactin biosynthesis genes are ubiquitous in both vanchrobactin- and anguibactin-producing V. anguillarum strains, which reinforces the utility of the vanchrobactin route of entry for the design of future strategies for the control of vibriosis.

The genome sequence of the fish pathogen Aliivibrio salmonicida strain LFI1238 shows extensive evidence of gene decay

Background

The fish pathogen Aliivibrio salmonicida is the causative agent of cold-water vibriosis in marine aquaculture. The Gram-negative bacterium causes tissue degradation, hemolysis and sepsis in vivo.

Results

In total, 4 286 protein coding sequences were identified, and the 4.6 Mb genome of A. salmonicida has a six partite architecture with two chromosomes and four plasmids. Sequence analysis revealed a highly fragmented genome structure caused by the insertion of an extensive number of insertion sequence (IS) elements. The IS elements can be related to important evolutionary events such as gene acquisition, gene loss and chromosomal rearrangements. New A. salmonicida functional capabilities that may have been aquired through horizontal DNA transfer include genes involved in iron-acquisition, and protein secretion and play potential roles in pathogenicity. On the other hand, the degeneration of 370 genes and consequent loss of specific functions suggest that A. salmonicida has a reduced metabolic and physiological capacity in comparison to related Vibrionaceae species.

Conclusion

Most prominent is the loss of several genes involved in the utilisation of the polysaccharide chitin. In particular, the disruption of three extracellular chitinases responsible for enzymatic breakdown of chitin makes A. salmonicida unable to grow on the polymer form of chitin. These, and other losses could restrict the variety of carrier organisms A. salmonicida can attach to, and associate with. Gene acquisition and gene loss may be related to the emergence of A. salmonicida as a fish pathogen.

Heme uptake by Microscilla marina and evidence for heme uptake systems in the genomes of diverse marine bacteria

The ability to acquire diverse and abundant forms of iron would be expected to confer a survival advantage in the marine environment, where iron is scarce. Marine bacteria are known to use siderophores and inorganic iron, but their ability to use heme, an abundant intracellular iron form, has only been examined preliminarily. Microscilla marina, a cultured relative of a bacterial group frequently found on marine particulates, was used as a model organism to examine heme uptake. Searches of the genome revealed analogs to known heme transport proteins, and reverse transcription-quantitative PCR analysis of these genes showed that they were expressed and upregulated under iron stress and during growth on heme. M. marina was found to take up heme-bound iron and could grow on heme as a sole iron source, supporting the genetic evidence for heme transport. Similar putative heme transport components were identified in the genomes of diverse marine bacteria. These systems were found in the genomes of many bacteria thought to be particle associated but were lacking in known free-living organisms (e.g., Pelagibacter ubique and marine cyanobacteria). This distribution of transporters is consistent with the hydrophobic, light-sensitive nature of heme, suggesting that it is primarily available on phytoplankton or detritus or in nutrient-rich environments.

Identification of iron regulated genes in the fish pathogen Aeromonas salmonicida subsp. salmonicida: genetic diversity and evidence of conserved iron uptake systems

In this study we applied the Fur titration assay (FURTA) to isolate iron regulated genes in Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis in fish. We have identified genes for siderophore biosynthesis and for ferri-siderophore transport, some of which have been previously described in this species. A gene for a hitherto uncharacterized functional hemin receptor HutE was identified, but its inactivation did not affect significantly the use of hemin as sole iron source, suggesting that redundant genes encoding hemin receptors exist in the A. salmonicida genome. Additional FURTA positive clones contained genes encoding a Flavoprotein, a transcriptional regulator protein, a DNA binding protein Fis, as well as genes encoding putative DapD and Flp/Fap-pilin proteins. A screening of gene distribution demonstrated that all the analyzed strains shared genes for siderophore biosynthesis and transport and for heme utilization, indicating that these two systems of iron acquisition are a conserved trait in this subspecies, whereas other genes are not common to all the isolates. Thus, the Fur regulon of A. salmonicida subsp. salmonicida includes both conserved and differentially occurring genes, suggesting that the genetic diversity within this fish pathogen might be higher than expected.