Iron uptake by Pasteurella piscicida and its role in pathogenicity for fish

An ex vivo Approach in European Seabass Leucocytes Supports the in vitro Regulation by Postbiotics of Aip56 Gene Expression of Photobacterium damselae subsp. piscicida

Shewanella putrefaciens Pdp11 (SpPdp11) is a probiotic strain assayed in aquaculture; however, its postbiotic potential is unknown. Postbiotics are bacterial metabolites, including extracellular products (ECPs) that improve host physiology and immunity. Their production and composition can be affected by different factors such as the growing conditions of the probiotics. Photobacterium damselae subsp. piscicida strain Lg 41/01 (Phdp) is one of the most important pathogens in marine aquaculture. The major virulent factor of this bacterium is the exotoxin aip56, responsible for inducing apoptosis of fish leucocytes. Viable SpPdp11 cells have been reported to increase resistance to challenges with Phdp. This work aimed to evaluate the effect of two ECPs, T2348-ECP and FM1548-ECP, obtained from SpPdp11 grown under different culture conditions that previously demonstrated to exert different degradative and non-cytotoxic activities, as well as the effect on pathogens biofilm formation. These SpPdp11-ECPs were then analyzed by their effect on the viability, phagocytosis, respiratory burst and apoptogenic activity against European sea bass leucocytes infected or not with Phdp supernatant. Both ECPs, T2348-ECP and FM1548-ECP, were not cytotoxic against leucocytes and significantly reduced their apoptosis. Phagocytosis and respiratory burst of leucocytes were significantly reduced by incubation with Phdp supernatant, and not influenced by incubation with T2348-ECP or FM1548-ECP. However, both activities were significantly increased after leucocyte incubation with combined T2348-ECP and FM1548-ECP with Phdp supernatant, compared to those incubated only with Phdp supernatant. Finally, both T2348-ECP and FM1548-ECP significantly reduced the relative in vitro expression of the Phdp aip56 encoding gene.

The role of marine pollution on the emergence of fish bacterial diseases

Marine pollution and bacterial disease outbreaks are two closely related dilemmas that impact marine fish production from fisheries and mariculture. Oil, heavy metals, agrochemicals, sewage, medical wastes, plastics, algal blooms, atmospheric pollutants, mariculture-related pollutants, as well as thermal and noise pollution are the most threatening marine pollutants. The release of these pollutants into the marine aquatic environment leads to significant ecological degradation and a range of non-infectious disorders in fish. Marine pollutants trigger numerous fish bacterial diseases by increasing microbial multiplication in the aquatic environment and suppressing fish immune defense mechanisms. The greater part of these microorganisms is naturally occurring in the aquatic environment. Most disease outbreaks are caused by opportunistic bacterial agents that attack stressed fish. Some infections are more serious and occur in the absence of environmental stressors. Gram-negative bacteria are the most frequent causes of these epizootics, while gram-positive bacterial agents rank second on the critical pathogens list. Vibrio spp., Photobacterium damselae subsp. Piscicida, Tenacibaculum maritimum, Edwardsiella spp., Streptococcus spp., Renibacterium salmoninarum, Pseudomonas spp., Aeromonas spp., and Mycobacterium spp. Are the most dangerous pathogens that attack fish in polluted marine aquatic environments. Effective management strategies and stringent regulations are required to prevent or mitigate the impacts of marine pollutants on aquatic animal health. This review will increase stakeholder awareness about marine pollutants and their impacts on aquatic animal health. It will support competent authorities in developing effective management strategies to mitigate marine pollution, promote the sustainability of commercial marine fisheries, and protect aquatic animal health.

Flavobacterium columnare ferric iron uptake systems are required for virulence

Flavobacterium columnare, which causes columnaris disease, is one of the costliest pathogens in the freshwater fish-farming industry. The virulence mechanisms of F. columnare are not well understood and current methods to control columnaris outbreaks are inadequate. Iron is an essential nutrient needed for metabolic processes and is often required for bacterial virulence. F. columnare produces siderophores that bind ferric iron for transport into the cell. The genes needed for siderophore production have been identified, but other components involved in F. columnare iron uptake have not been studied in detail. We identified the genes encoding the predicted secreted heme-binding protein HmuY, the outer membrane iron receptors FhuA, FhuE, and FecA, and components of an ATP binding cassette (ABC) transporter predicted to transport ferric iron across the cytoplasmic membrane. Deletion mutants were constructed and examined for growth defects under iron-limited conditions and for virulence against zebrafish and rainbow trout. Mutants with deletions in genes encoding outer membrane receptors, and ABC transporter components exhibited growth defects under iron-limited conditions. Mutants lacking multiple outer membrane receptors, the ABC transporter, or HmuY retained virulence against zebrafish and rainbow trout mirroring that exhibited by the wild type. Some mutants predicted to be deficient in multiple steps of iron uptake exhibited decreased virulence. Survivors of exposure to such mutants were partially protected against later infection by wild-type F. columnare.

Short-Term Immune Responses of Gilthead Seabream (Sparus aurata) Juveniles against Photobacterium damselae subsp. piscicida

Photobacteriosis is a septicaemic bacterial disease affecting several marine species around the globe, resulting in significant economic losses. Although many studies have been performed related to the pathogen virulence and resistance factors, information regarding the host defence mechanisms activated once an infection takes place is still scarce. The present study was designed to understand innate immune responses of farmed juvenile gilthead seabream (Sparus aurata) after Photobacterium damselae subsp. piscicida (Phdp) infection. Therefore, two groups of seabream juveniles were intraperitoneally injected with 100 µL of PBS (placebo) or 100 µL of exponentially growing Phdp (1 × 10⁶ CFU/mL; infected). The blood, plasma, liver, and head kidney of six fish from each treatment were sampled immediately before infection and 3, 6, 9, 24 and 48 h after infection for the broad screening of fish immune and oxidative stress responses. Infected animals presented marked anaemia, neutrophilia and monocytosis, conditions that are correlated with an increased expression of genes related to inflammation and phagocytic activity. Similar studies with different fish species and bacteria can be useful for the definition of health biomarkers that might help fish farmers to prevent the occurrence of such diseases.

Comparative Genomics Analyses Support the Reclassification of Bisgaard Taxon 40 as Mergibacter gen. nov., With Mergibacter septicus sp. nov. as Type Species: Novel Insights Into the Phylogeny and Virulence Factors of a Pasteurellaceae Family Member Associated With Mortality Events in Seabirds

The Pasteurellaceae family has been associated with fatal diseases in numerous avian species. Several new taxa within this family, including Bisgaard taxon 40, have been recently described in wild birds, but their genomic characteristics and pathogenicity are not well understood. We isolated Bisgaard taxon 40 from four species of seabirds, including one sampled during a mass, multi-species mortality event in Florida, United States. Here, we present a comprehensive phenotypic and genetic characterization of Bisgaard taxon 40 and comparative genomic analysis with reference strains from the Pasteurellaceae family, aiming at determining its phylogenetic position, antimicrobial susceptibility profile, and identifying putative virulence factors. In silico multilocus sequence-based and whole-genome-based phylogenetic analysis clustered all Bisgaard taxon 40 strains together on a distinct branch separated from the other members of the Pasteurellaceae family, indicating that Bisgaard taxon 40 could represent a new genus. These findings were further supported by protein similarity analyses using the concatenation of 31 conserved proteins and other taxonomic approaches such as the percentage of conserved protein test. Additionally, several putative virulence factors were identified, including those associated with adhesion (capsule, ompA, ompH) and colonization (exbD, fur, galU, galE, lpxA, lpxC, and kdsA) of the host and a cytolethal distending toxin (cdt), which may have played a role in disease development leading to the mortality event. Considerably low minimum inhibitory concentrations (MICs) were found for all the drugs tested, in concordance with the absence of antimicrobial resistance genes in these genomes. The novel findings of this study highlight genomic and phenotypic characteristics of this bacterium, providing insights into genome evolution and pathogenicity. We propose a reclassification of these organisms within the Pasteurellaceae family, designated as Mergibacter gen. nov., with Mergibacter septicus sp. nov. as the type species. The type strain is Mergibacter septicus A25201^T (=DSM 112696).

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.

Vaccines and immune protection of principal Mediterranean marine fish species

This review describes and summarizes the knowledge on established and experimental vaccines developed against viral and bacterial pathologies affecting the most important farmed marine finfish species present in the Mediterranean area, namely European seabass Dicentrarchus labrax, sea bream Sparus aurata, turbot Psetta maxima and meagre Argyrosomus regius. The diseases that have been recorded in seabass, sea bream and meagre are caused by bacteria Vibrio anguillarum, Photobacterium damselae, Tenacibaculum maritimum as well as by viruses such as Viral Encephalopathy and Retinopathy/Viral Nervous Necrosis and Lymphocystic disease. The main pathologies of turbot are instead bacteriosis provoked by Tenacibaculum maritimum, Aeromonas sp. and Vibrio anguillarum, and virosis by viral hemorrhagic septicaemia virus. Some vaccines have been optimized and are now regularly available for the majority of the above-mentioned pathogens. A measurable immune protection has been conferred principally against Vibrio anguillarum, Photobacterium damselae sub. piscicida and VER/VNN.

Outer membrane protein FrpA, the siderophore piscibactin receptor of Photobacterium damselae subsp. piscicida, as a subunit vaccine against photobacteriosis in sole (Solea senegalensis)

Photobacteriosis caused by Photobacterium damselae subsp. piscicida (Pdp) remains one of the main infectious diseases affecting cultured fish in Mediterranean countries. Diverse vaccine formulations based in the use of inactivated bacterial cells have been used with unsatisfactory results, especially in newly cultured species like sole (Solea senegalensis). In this work, we describe the use of the outer membrane receptor (FrpA) of the siderophore piscibactin produced by Pdp as a novel subunit vaccine against photobacteriosis. FrpA has been cloned and expressed in Escherichia coli under an arabinose-inducible promoter. A recombinant protein (rFrpA) containing the pelB localization signal and a His tag was constructed to obtain a pure native form of the protein from E. coli outer membranes. The immunogenicity of rFrpA, and its protective effect against photobacteriosis, was tested by i.p. injection of 30  μg of the protein, mixed with Freund's adjuvant, in sole fingerlings with two immunizations separated by 30 days. Results showed that using either pure rFrpA or whole cells as immobilized antigens in ELISA assays, rFrpA induces the production of specific antibodies in sole. An experimental infection using fish vaccinated with rFrpA or formalin-killed whole cells of Pdp showed that both groups were protected against Pdp infection at similar levels, with no significant differences, reaching RPS values of 73% and 79%, respectively. Thus, FrpA constitutes a promising antigen candidate for the development of novel more effective vaccines against fish photobacteriosis.

Influence of the Genetic Makeup of Common Carp on the Expression of Iron-related Genes During Trypanoplasma Borreli Infection

Introduction

Genes related to iron metabolism play an important role in inflammatory response. The objective of this study was to investigate the role of ferritin, transferrin receptors 1a and 1b, and transferrin genes in the response to blood parasite infection in common carp (Cyprinuscarpio L.).

Material and Methods

Two genetically distinct carp groups were used: R3 carp, which are established as being sensitive to parasitic infection, and SA carp (Cyprinus carpio haematopterus) of wild origin. An established challenge model with Trypanoplasma borreli was applied. Challenged carp were sampled to determine their expression levels of transferrin receptors 1a and 1b, ferritin, and transferrin mRNA. Mortality and serum iron concentration were also measured.

Results

The study revealed contrasting differences in the expression profiles of all key iron regulatory genes except the transferrin gene. In the case of other parameters, significant differences were also observed.

Conclusion

Our results demonstrate that the level of parasitic infection depends on the blood iron status. This parameter was related to the origin of the fish.

Fish pathogen bacteria: Adhesion, parameters influencing virulence and interaction with host cells

Wild fisheries are declining due to over-fishing, climate change, pollution and marine habitat destructions among other factors, and, concomitantly, aquaculture is increasing significantly around the world. Fish infections caused by pathogenic bacteria are quite common in aquaculture, although their seriousness depends on the season. Drug-supplemented feeds are often used to keep farmed fish free from the diseases caused by such bacteria. However, given that bacteria can survive well in aquatic environments independently of their hosts, bacterial diseases have become major impediments to aquaculture development. On the other hand, the indiscriminate uses of antimicrobial agents has led to resistant strains and the need to switch to other antibiotics, although it seems that an integrated approach that considers not only the pathogen but also the host and the environment will be the most effective method in the long-term to improve aquatic animal health. This review covers the mechanisms of bacterial pathogenicity and details the foundations underlying the interactions occurring between pathogenic bacteria and the fish host in the aquatic environment, as well as the factors that influence virulence. Understanding and linking the different phenomena that occur from adhesion to colonization of the host will offer novel and useful means to help design suitable therapeutic strategies for disease prevention and treatment.

Transcription of IVIAT and Virulence Genes in Photobacterium damselae Subsp. piscicida Infecting Solea senegalensis

Photobacterium damselae subsp. piscicida (Phdp) is responsible for disease outbreaks in marine aquaculture worldwide. Solea senegalensis, a valuable fish species for aquaculture in the south of Europe, is frequently affected by this pathogen. It is well established that bacteria respond to environmental signals and, in the case of pathogens, this ability may determine the outcome of their interaction with the host. Determination of gene expression under in vivo conditions constitutes a valuable tool in the assessment of microbial pathogenesis. Considering that different hosts may represent different environments for the pathogen, expression of Phdp virulence and in vivo induced antigen (IVIAT) genes during S. senegalensis infection has been determined in the present work. Increased transcription of genes encoding proteins involved in iron acquisition (Irp1, Irp2, HutB and HutD), oxidative stress defence (AhpC and Sod), adhesion (PDP_0080), toxins (AIP56) and metabolism (Impdh, Shmt and AlaRS) were detected in Phdp infecting S. senegalensis head kidney or liver. The highest increases corresponded to genes involved in survival under iron limiting conditions and oxidative stress, indicating their essential role during infection of sole. Results obtained give insight into Phdp virulence strategies and contribute to the identification of promising targets for the control of photobacteriosis.

Development of a real-time PCR assay for rapid detection and quantification of Photobacterium damselae subsp. piscicida in fish tissues

The availability of a rapid and accurate method for the diagnosis of Photobacterium damselae subsp. piscicida (Phdp), able to discriminate its strictly correlated subsp. damselae (Phdd), formally known as Vibrio damsela, is essential for managing fish pasteurellosis outbreaks in farmed fish. A single-step, high-sensitivity real-time PCR assay for simultaneous detection and quantification of P. damselae was designed targeting partial of the sequence of the bamB gene and tested for specificity and sensitivity on laboratory-generated samples as well as on experimentally infected seabream tissue samples. With a limit of detection (LOD) of one copy in pure bacterial DNA, the sensitivity was higher than all methods previously reported. Validation in target and non-target bacterial species proved the assay was able to discriminate Phdd-Phdp subspecies from diverse hosts/geographical origins and between non-target species. In addition, two SNPs in the target amplicon region determine two distinctive qPCR dissociation curves distinguishing between Phdp-Phdd. This is the first time that a molecular method for P. damselae diagnosis combines detection, quantification and subspecies identification in one step. The assay holds the potential to improve the knowledge of infection dynamics and the development of better strategies to control an important fish disease.

Transcription of immune related genes in Solea senegalensis vaccinated against Photobacterium damselae subsp. piscicida. Identification of surrogates of protection

Solea senegalensis is a flatfish with a great potential for aquaculture, but infectious diseases restrict its production, being this fish species highly susceptible to Photobacterium damselae subsp. piscicida (Phdp) infections. A better understanding of the mechanisms related to fish immune response is crucial for the development of effective approaches in disease management. In the present work, transcriptional changes of immune related genes have been evaluated in farmed S. senegalensis specimens vaccinated against Phdp by intraperitoneal injection (IP) and immersion (IM). IP fish showed higher antibody levels and increased transcription of genes encoding lysozyme C1, complement factors involved in the classical pathway and components involved in the opsonization and the limitation of free iron availability, all of them facilitating the faster elimination of the pathogen and promoting higher RPS after the infection with Phdp. The results of this study seem to support a different intensity of the specimens immune response in the head kidney. Analysis of the immune response in 15 day post-challenged fish showed up-regulation of genes involved in all stages of S. senegalensis immune response, but especially those genes encoding proteins related to the innate response such as complement, lysozyme and iron homeostasis in the head kidney. On the other hand, liver transcription was higher for genes related to inflammation, apoptosis and cell mediated cytotoxicity (CMC). Furthermore, comparison of the differential response of S. senegalensis genes in vaccinated and unvaccinated fish to Phdp infection allowed the identification of a potential biosignature, consisting in 10 genes, as a surrogate of protection and therefore, as indicator of vaccine success against fotobacteriosis after IP vaccination. These results provide important insights into the S. senegalensis protection against Phdp induced by vaccination.

Use of in vivo induced technology to identify antigens expressed by Photobacterium damselae subsp. piscicida during infection of Senegalese sole (Solea senegalensis)

Photobacterium damselae subsp. piscicida (Phdp), the causative agent of photobacteriosis, is an important pathogen in marine aquaculture that affects many different fish species worldwide, including Solea senegalensis, an important fish species for aquaculture in the south of Europe. Bacteria express different repertoires of proteins in response to environmental conditions and when invading a host, sense in vivo environment and adapt by changing the expression of specific proteins. In the case of pathogens, identification of genes with up-regulated expression in vivo compared to in vitro conditions might give an insight into the genes relevant to the bacterial virulence. In the present work, in vivo induced antigen technology (IVIAT) has been used to search for Phdp genes only expressed or up-regulated in infected S. senegalensis. An expression library from Phdp was assayed against pooled sera from convalescent S. senegalensis specimens and 18 clones were positive, indicating that proteins encoded are expressed by Phdp during S. senegalensis infection and are immunogenic for this fish species. In addition, five proteins were reactive against adsorbed sera, indicating their in vivo induced character. Inosine-5'-monophosphate dehydrogenase, serine hydroxy methyltransferase and alanyl-tRNA synthethase, involved in aminoacid and nucleotide metabolism, the protein with antioxidant activity alkyl hydroperoxide reductase and a non-ribosomal peptide synthetase responsible for the synthesis of the siderophore piscibactin have been identified as antigens induced in Phdp during S. senegalensis infection. Proteins induced during in vivo growth of Phdp represent promising targets for the development of novel antimicrobial or prophylactic agents in the treatment and prevention of photobacteriosis.

Experimental infection of octopus vulgaris (Cuvier, 1797) with Photobacterium damsela subsp. piscicida. Immunohistochemical tracking of antigen and tissue responses

Adult common octopus individuals were intramuscularly infected with Photobacterium damsela subsp. piscicida in order to investigate if this species is sensitive to this common and important fish pathogen. The fate of the bacterial antigens and the tissue responses of Octopus vulgaris were studied employing immunohistochemical techniques. Strong reaction at the site of injection was evident from day 2 post-infection that continued until day 14. Great numbers of hemocytes that were attracted at the site of infection were involved in phagocytosis of bacteria. Very early in the infection, a transition of cells to fibroblasts and an effort to isolate the infection was observed. During the course of the study, very large necrotic cells were seen at the site of infection, whereas during the later stages hemocytes with phagocytosed bacteria were observed in well-defined pockets inside the muscle tissue. None of the internal organs tested for the presence of the bacterium were positive with the exception of the digestive gland where antigen staining was observed which was not associated with hemocyte infiltration. The high doses of bacterial cells used in this experimental infection and the lack of disease signs from Octopus vulgaris suggest that, under normal conditions, octopus is resistant to Photobacterium damsela subsp. piscicida.

Insights into the virulence-related genes of Edwardsiella tarda isolated from turbot in Europe: genetic homogeneity and evidence for vibrioferrin production

Edwardsiella tarda has long been known as a pathogen that causes severe economic losses in aquaculture industry. Insights gained on E. tarda pathogenesis may prove useful in the development of new methods for the treatment of infections as well as preventive measures against future outbreaks. In this report, we have established the correlation between the presence of virulence genes, related with three aspects typically involved in bacterial pathogenesis (chondroitinase activity, quorum sensing and siderophore-mediated ferric uptake systems), in the genome of E. tarda strains isolated from turbot in Europe and their phenotypic traits. A total of 8 genes were tested by PCR for their presence in 73 E. tarda isolates. High homogeneity was observed in the presence/absence pattern of all the strains. Positive results in the amplification of virulence-related genes were correlated with the detection of chondroitinase activity in agar plates, in vivo AHL production during fish infection and determination of type of siderophore produced by E. tarda. To the best of our knowledge, this is the first study carried out with European strains on potential virulence factors. Furthermore, we demonstrated for the first time that E. tarda produces the siderophore vibrioferrin.

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.

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.

Photobacteriosis: prevention and diagnosis

Photobacteriosis or fish pasteurellosis is a bacterial disease affecting wild and farm fish. Its etiological agent, the gram negative bacterium Photobacterium damselae subsp. piscicida, is responsible for important economic losses in cultured fish worldwide, in particular in Mediterranean countries and Japan. Efforts have been focused on gaining a better understanding of the biology of the pathogenic microorganism and its natural hosts with the aim of developing effective vaccination strategies and diagnostic tools to control the disease. Conventional vaccinology has thus far yielded unsatisfactory results, and recombinant technology has been applied to identify new antigen candidates for the development of subunit vaccines. Furthermore, molecular methods represent an improvement over classical microbiological techniques for the identification of P. damselae subsp. piscicida and the diagnosis of the disease. The complete sequencing, annotation, and analysis of the pathogen genome will provide insights into the pathogen laying the groundwork for the development of vaccines and diagnostic methods.

Iron bioavailability in larvae yellow snapper (Lutjanus argentiventris): cloning and expression analysis of ferritin-H

Ferritin is a major intracellular iron storage protein in higher vertebrates and plays an important role in iron metabolism. In this study, ferritin H subunit was cloned from the larvae of yellow snapper, Lutjanus argentiventris, by rapid amplification of cDNA ends (RACE) following in silico transcriptome analysis. The full-length cDNAs of the LaFeH was 1231 bp in length encoding 177 amino acids with a predicted molecular mass (MW) about 20.82 kDa and theoretical isoelectric point (pI) of 5.79. Amino acid alignment revealed that LaFeH shared high similarity with other known ferritins. It shared high degree identity to the ferritin H subunits of Lates calcarifer (99%), Takifugu rubripes (97%) and Dicentrarchus labrax (97%), and low identity to that of human (82%) and mouse (84%). By real-time PCR assays, the mRNA transcripts of LaFeH was found to be higher expressed in head-kidney, eye, heart and brain. Moreover, mRNA expression levels of LaFeH was measured by real-time PCR in larvae exposed with graded levels of iron (6.8 μg/ml and 13.6 μg/ml (Fe2x and Fe4x, respectively) and an iron chelation assay. Results showed that the expression of the LaFeH mRNA increased gradually with Fe2x in water. The LaFeH gene expression declined with increasing iron exposure levels at Fe4x. Finally, we can observe a high expression of LaFeH gene in larvae exposed to iron chelation therapy at 2 h; however this increase was gradually decreasing over time. In summary, the LaFeH gene expression for larvae yellow snapper showed a dose-depend increase following the iron treatment. These data indicated that iron bioavailability regulates LaFeH at transcriptional level in larvae yellow snapper. Further studies are necessary to ascertain their role in the immune response in teleost fish.

Enhanced efficacy of an attenuated Flavobacterium psychrophilum strain cultured under iron-limited conditions

An attenuated strain of Flavobacterium psychrophilum (CSF259-93B.17) has shown potential as a vaccine for prevention of bacterial coldwater disease (BCWD) in rainbow trout, Oncorhynchus mykiss (Walbaum). Because BCWD outbreaks can result in high mortality in other salmonid species, specifically coho salmon, Oncorhynchus kisutch (Walbaum), the live-attenuated strain was tested as a vaccine in this species. Additionally, we hypothesized that culture of the vaccine strain under iron-limited conditions would lead to improved protection against BCWD. To test this hypothesis, coho salmon were either injection or immersion immunized with CSF259-93B.17 cultured in iron-replete or iron-limited medium. Resultant antibody titers were low and not significantly different between the two treatments regardless of vaccine delivery method (P > 0.05). Following injection challenge with a virulent F. psychrophilum strain, mortality for injection vaccinated fish was significantly reduced compared to the control but did not differ by treatment (P > 0.05). Relative percent survival (RPS) was high in both treatments (90% in iron-replete, 98% in iron-limited medium). Fish immunized by immersion with CSF259-93B.17 grown in iron-replete medium exhibited lower mortality (29.3%; RPS 46%) when compared to mock immunized fish, but this was not significant. However, mortality was significantly lower in fish immunized with CSF259-93B.17 grown in iron-limited medium (14.7%; RPS 73%) when compared to mock immunized fish. The results demonstrate that the live-attenuated F. psychrophilum strain can confer protection to coho salmon and vaccine efficacy is enhanced by culturing the strain under iron-limited conditions.

High mortality of juvenile gilthead sea bream (Sparus aurata) from photobacteriosis is associated with alternative macrophage activation and anti-inflammatory response: results of gene expression profiling of early responses in the head kidney

The halophilic bacterium Photobacterium damselae subsp. piscicida (Phdp) represents a substantial health problem for several fish species in aquaculture. Bacteria that reside free and inside phagocytes cause acute and chronic forms of photobacteriosis. Infections of juveniles rapidly kill up to 90-100% fish. Factors underlying failure of the immune protection against bacteria remain largely unknown. The reported study used a transcriptomic approach to address this issue. Juvenile sea breams (0.5 g) were challenged by immersion in salt water containing 2.89 × 10(8) CFU of a virulent Phdp and the head kidney was sampled after 24- and 48-h. Analyses were performed using the second version of a 44 k oligonucleotide DNA microarray that represents 19,734 sea bream unique transcripts and covers diverse immune pathways. Expression changes of selected immune genes were validated with qPCR. Results suggested rapid recognition of the pathogen, as testified by up-regulation of lectins and antibacterial proteins (bactericidal permeability-increasing protein lectins, lysozyme, intracellular and extracellular proteases), chemokines and chemokine receptors. Increased expression of proteins involved in iron and heme metabolism also could be a response against bacteria that are dependent on iron. However, negative regulators of immune/inflammatory response were preponderant among the up-regulated genes. A remarkable finding was the increased expression of IL-10 in concert with up-regulation of arginase I and II and proteins of the polyamine biosynthesis pathway that diverts the arginine flux from the production of reactive nitrogen species. Such expression changes are characteristic for alternatively activated macrophages that do not develop acute inflammatory responses. Immune suppression can be induced by the host to reduce tissue damages or by the pathogen to evade host response.

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.

Microbiota of Atlantic cod (Gadus morhua L.) rearing systems at pre- and posthatch stages and the effect of different treatments

AIMS

To study the effect of ova disinfection, antibiotic and microbial treatments on the dominant cultivable cod rearing microbiota at pre- and posthatch stages, determining some virulence-related phenotypic traits among bacterial isolates and their relation to larval survival.

METHODS AND RESULTS

Sampling of rearing systems (rearing water, ova, larvae, feeds and supplement) for analysis of cultivable microbiota took place at early stages in 2004 and 2005. Cultivation, phenotypic and genotypic (16S rRNA gene) analyses were performed. The production of putative virulence factors (PVFs), including haemolysin, siderophores and quorum-sensing signals, by bacterial isolates was investigated and related to larval survival. The study was performed during two spawning seasons, evaluating current hatchery practices (ova disinfection and antibiotic treatment of unhealthy larvae) and specific putative probiotics applied to ova and larvae or rotifers. A diversified microbiota (75 operational taxonomic units, OTUs) was observed in cod rearing systems influenced by the feeds and treatments, with prevailing γ-Proteobacteria prior to hatching towards a multiphyla microbiota posthatch. Phenotypic tests demonstrated the heterogeneity within some OTUs. Multivariate analysis of survival data in larval silos and the corresponding larval microbiota was used to divide the genotypic groups into beneficial/harmless and detrimental/opportunistic clusters. PVFs were common among the proposed detrimental/opportunistic OTUs.

CONCLUSIONS

The results clearly demonstrate the influence of exogeneous feeding and treatments on larval gastrointestinal microbiota and the role of bacteria in larval survival.

SIGNIFICANCE AND IMPACT OF THE STUDY

Increased understanding of the microbiota in rearing systems may contribute to successful implementation of microbial management in cod aquaculture.

Invasion and survival of Photobacterium damselae subsp. piscicida in non-phagocytic cells of gilthead sea bream, Sparus aurata L

Fluorescence microscopy and gentamicin protection assays were used to investigate the ability of four Photobacterium damselae subsp. pisicida (Phdp) strains to adhere to and to invade the fish epithelial cell line, SAF-1, derived from Sparus aurata. All strains tested were detected intracellularly using both techniques, although internalization levels varied among strains. Treatment with cytochalasin D and experiments carried out at 4 degrees C demonstrated that a functional host cell cytoskeleton and active cell metabolism are necessary for bacterial internalization. Intracellular bacteria were detected for up to 7 days with a round morphology and were stained with DAPI, indicating that some bacterial cells may remain viable inside SAF-1 cells. Our in vitro findings indicate that Phdp are capable of adhering, entering and surviving within the non-phagocytic epithelial cell line SAF-1, which may be important for persistence and establishment of a carrier state in S. aurata.

In vivo morphological and antigenic characteristics of Photobacterium damselae subsp. piscicida

The present study was conducted to examine the morphology and antigenicity of Photobacterium damselae subsp. piscicida by culturing the bacterium in vivo in the peritoneal cavity of sea bass (Dicentrarchus labrax) within dialysis bags with either a low molecular weight (LMW) cut-off of 25 kDa or a high molecular weight (HMW) cut-off of 300 kDa. Differences were observed in the growth rate between the bacteria cultured in vivo or in vitro. Bacteria cultured in vivo were smaller and produced a capsular layer, which was more prominent in bacteria cultured in the HMW bag. Antigenicity was examined by Western blot analysis using sera from sea bass injected with live Ph. d. subsp. piscicida. The sera recognised bands at 45 and 20 kDa in bacteria cultured in vivo in the LMW bag. Bacteria cultured in vivo in the HMW bag did not express the 45 kDa band when whole cell extracts were examined, although the antigen was present in their extracellular products. In addition, these bacteria had a band at 18 kDa rather than 20 kDa. Differences in glycoprotein were also evident between bacteria cultured in vitro and in vivo. Bacteria cultured in vitro in LMW and HMW bags displayed a single 26 kDa band. Bacteria cultured in the LMW bag in vivo displayed bands at 26 and 27 kDa, while bacteria cultured in vivo in the HMW bag possessed only the 27 kDa band. These bands may represent sialic acid. The significance of the changes observed in the bacterium's structure and antigenicity when cultured in vivo is discussed.

Cloning and characterization of Photobacterium damselae ssp. piscicida phospholipase: an enzyme that shows haemolytic activity

A phospholipase gene of Photobacterium damselae ssp. piscicida (ppp) was cloned from a genomic library and its nucleotide sequence was determined. The open reading frame consisted of 1218 bp encoding a protein of 405 amino acids with a predicted molecular mass of 46 kDa. The PPP had identities (53-55%) with phospholipase and haemolysin of Vibrio spp., while it showed low identities (23-26%) with glycerophospholipid cholesterol acyltransferase of Aeromonas spp. A recombinant PPP (rPPP) with a His tag at the C-terminus expressed in Escherichia coli and purified showed phospholipase activity. The rPPP also showed lecithin-dependent haemolytic activity against mammalian erythrocytes and direct haemolytic activity against fish erythrocytes. The culture supernatant of wild-type P. damselae ssp. piscicida showed phospholipase activity, while that of a PPP gene knockout mutant did not.

Variation in the molecular weight of Photobacterium damselae subsp. piscicida antigens when cultured under different conditions in vitro

The antigenicity of Photobacterium damselae (Ph. d.) subsp. piscicida, cultured in four different growth media [tryptone soya broth (TSB), glucose-rich medium (GRM), iron-depleted TSB (TSB + IR^-), and iron-depleted GRM (GRM + IR^-)] was compared by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis using sera obtained from sea bass (Dicentrarchus labrax) raised against live or heat-killed Ph. d. subsp. piscicida. The antigenic expression of Ph. d. subsp. piscicida was found to differ depending on the culture medium used. A significantly higher antibody response was obtained with iron-depleted bacteria by ELISA compared with non-iron depleted bacteria obtained from the sera of sea bass raised against live Ph. d. subsp. piscicida. The sera from sea bass raised against live bacteria showed a band at 22 kDa in bacteria cultured in TSB + IR^- or GRM+ IR^- when bacteria that had been freshly isolated from fish were used for the screening, while bands at 24 and 47 kDa were observed with bacteria cultured in TSB or GRM. When bacteria were passaged several times on tryptic soya agar prior to culturing in the four different media, only bands at 24 and 47 kDa were recognized, regardless of the medium used to culture the bacteria. It would appear that the molecular weight of Ph. d. subsp. piscicida antigens change in the presence of iron restriction, and sera from sea bass infected with live bacteria are able to detect epitopes on the antigens after this shift in molecular weight.

A siderophore biosynthesis gene cluster from the fish pathogen Photobacterium damselae subsp. piscicida is structurally and functionally related to the Yersinia high-pathogenicity island

Photobacterium damselae subsp. piscicida, the causative agent of fish pasteurellosis, produces a siderophore which is distinct from that produced by P. damselae subsp. damselae. Using suppression subtractive hybridization, a subsp. piscicida-specific DNA region of 35 kb was identified in strain DI21, and 11 genes were defined: dahP, araC1, araC2, frpA, irp8, irp2, irp1, irp3, irp4, irp9 and irp5. The sequence of the predicted proteins encoded by these genes showed significant similarity with the proteins responsible for the synthesis and transport of the siderophore yersiniabactin, encoded within the Yersinia high-pathogenicity island (HPI). Southern hybridization demonstrated that this gene cluster is exclusive to some European subsp. piscicida isolates. Database searches revealed that a similar gene cluster is present in Photobacterium profundum SS9 and Vibrio cholerae RC385. An irp1 gene (encoding a putative non-ribosomal peptide synthetase) insertional mutant (CS31) was impaired for growth under iron-limiting conditions and unable to produce siderophores, and showed an approximately 100-fold decrease in degree of virulence for fish. The subsp. piscicida DI21 strain, but not CS31, promoted the growth of a Yersinia enterocolitica irp1 mutant. Furthermore, a yersiniabactin-producing Y. enterocolitica strain as well as purified yersiniabactin were able to cross-feed strains DI21 and CS31, suggesting that the subsp. piscicida siderophore might be functionally and structurally related to yersiniabactin. The differential occurrence among P. damselae strains, and the low sequence similarity to siderophore synthesis genes described in other members of the Vibrionaceae, suggest that this genetic system might have been acquired by horizontal transfer in P. damselae subsp. piscicida, and might have a common evolutionary origin with the Yersinia HPI.

Heme, an iron supply for vibrios pathogenic for fish

One of the main mechanisms present in gram-negative bacterial pathogens to obtain iron is the utilization of free heme or heme proteins from the host tissues. Vibrio anguillarum, the etiological agent of vibriosis in fish, and Photobacterium damselae subsp. piscicida, the causative agent of fish pasteurellosis, can acquire iron from free heme or heme-containing proteins present in the host tissues by a siderophore-independent mechanism. Similarly to other animal and human pathogens, the general mechanism for heme uptake in these two species consists in the presence of an outer membrane receptor that transport the heme molecule into the periplasm via a TonB-dependent process, and additional proteins that complete the transport of heme from the periplasm into the cell cytoplasm. Expression of heme uptake genes is iron-regulated at the transcriptional level by the repressor protein Fur. The heme uptake mechanisms are believed to contribute to virulence for fish. The existence of variability in the distribution of heme transport genes among strains suggests that gene inactivation and/or horizontal transfer might play a significant role in generating intraspecific genetic diversity.

Superoxide dismutase and catalase activities in Photobacterium damselae ssp. piscicida

The ability of a set of Photobacterium damselae ssp. piscicida strains isolated from different fish species to produce different superoxide dismutase (SOD) and catalase enzymes was determined. Unlike other bacterial pathogens, P. damselae ssp. piscicida is not able to produce different isoforms of SOD or catalase containing different metal cofactors when cultured under oxidative stress induced by hydrogen peroxide or methyl viologen, or under iron depleted conditions. However, iron content of the growth medium influenced the levels of SOD and catalase activity in cells, these levels decreasing with iron availability of the medium. Comparison of virulent and non-virulent strains of P. damselae ssp. piscicida showed similar contents of SOD, but higher levels of catalase were detected in cells of the virulent strain. Incubation of bacteria with sole, Solea senegalensis (Kaup), phagocytes has shown that survival rates range from 19% to 62%, these rates being higher for the virulent strain. The increased levels of catalase activity detected in the virulent strain indicates a possible role for this enzyme in bacterial survival.

Capture stress and the bactericidal competence of blood and plasma in five species of tropical birds

In wild birds, relatively little is known about intra- or interspecific variation in immunological capabilities, and even less is known about the effects of stress on immune function. Immunological assays adaptable to field settings and suitable for a wide variety of taxa will prove most useful for addressing these issues. We describe a novel application of an in vitro technique that measures the intrinsic bacteria-killing abilities of blood. We assessed the capacities of whole blood and plasma from free-living individuals of five tropical bird species to kill a nonpathogenic strain of E. coli before and after the birds experienced an acute stress. Killing invasive bacteria is a fundamental immune function, and the bacteria-killing assay measures constitutive, innate immunity integrated across circulating cell and protein components. Killing ability varied significantly across species, with common ground doves exhibiting the lowest levels and blue-crowned motmots exhibiting the highest levels. Across species, plasma killed bacteria as effectively as whole blood, and higher concentrations of plasma killed significantly better. One hour of acute stress reduced killing ability by up to 40%. This assay is expected to be useful in evolutionary and ecological studies dealing with physiological and immunological differences in birds.

Iron uptake mechanisms in the fish pathogen Tenacibaculum maritimum

We present here the first evidence of the presence of iron uptake mechanisms in the bacterial fish pathogen Tenacibaculum maritimum. Representative strains of this species, with different serotypes and origins, were examined. All of them were able to grow in the presence of the chelating agent ethylenediamine-di-(o-hydroxyphenyl acetic acid) (EDDHA) and also produced siderophores. Cross-feeding assays suggest that the siderophores produced are closely related. In addition, all T. maritimum strains utilized transferrin, hemin, hemoglobin, and ferric ammonic citrate as iron sources when added to iron-deficient media. Whole cells of all T. maritimum strains, grown under iron-supplemented or iron-restricted conditions, were able to bind hemin, indicating the existence of constitutive binding components located at the T. maritimum cell surface. This was confirmed by the observation that isolated total and outer membrane proteins from all of the strains, regardless of the iron levels of the media, were able to bind hemin, with the outer membranes showing the strongest binding. Proteinase K treatment of whole cells did not affect the hemin binding, indicating that, in addition to proteins, some protease-resistant components could also bind hemin. At least three outer membrane proteins were induced in iron-limiting conditions, and all strains, regardless of their serotype, showed a similar pattern of induced proteins. The results of the present study suggest that T. maritimum possesses at least two different systems of iron acquisition: one involving the synthesis of siderophores and another that allows the utilization of heme groups as iron sources by direct binding.

Subtractive hybridization reveals a high genetic diversity in the fish pathogen Photobacterium damselae subsp. piscicida: evidence of a SXT-like element

Photobacterium damselae subsp. piscicida is the causative agent of fish pasteurellosis, a severe disease affecting cultured marine fish worldwide. In this study, suppression subtractive hybridization was used to identify DNA fragments present in the virulent strain PC554.2, but absent in the avirulent strain EPOY 8803-II. Twenty-one genomic regions of this type (that included twenty-six distinct putative ORFs) were analysed by DNA sequencing. Twenty ORFs encoded proteins with homology to proteins in other bacteria, including four homologues involved in siderophore biosynthesis, and four homologues related to mobile elements; three of these were putative transposases and one was a putative conjugative transposon related to the Vibrio cholerae SXT element. This sequence was shown to be integrated into a prfC gene homologue. Six ORFs showed no significant homology to known bacterial proteins. Among the 21 DNA fragments specific to strain PC554.2, 5 DNA fragments (representing 7 ORFs) were also absent in the avirulent strain ATCC 29690. The analysis of these differential regions, as well as the screening of their presence in a collection of strains, demonstrated the high genetic heterogeneity of this pathogen.

Heme uptake genes in human and fish isolates of Photobacterium damselae: existence of hutA pseudogenes

The marine bacterium Photobacterium damselae includes strains classified into two distinct subspecies, namely subsp. damselae and subsp. piscicida, which have been reported to cause disease in a variety of marine animals and in humans. P. damselae strains utilize heme compounds as sole iron sources. In the present study, ten potential heme uptake and utilization genes are described in P. damselae subsp. damselae and subsp. piscicida. One gene cluster includes the genes coding for putative proteins HutZ, HutX and HutW; TonB, ExbB and ExbD, the three components of the TonB system; HutB, the putative periplasmic binding protein; HutC, the putative inner membrane permease; and HutD, the putative ABC-transporter ATP-ase. A gene coding for HutA, the outer membrane heme receptor, has also been identified, but it is not linked to the rest of the heme transport genes. RT-PCR analyses showed that heme uptake genes are arranged in three iron-regulated transcriptional units. A plasmid carrying the gene for the heme receptor HutA in combination with a plasmid carrying tonBexbBDhutBCD genes conferred to Escherichia coli 101ESD (ent) the ability to use heme and hemoglobin as iron sources. The hutA gene was present in strains isolated from humans and a variety of fish species, but it was shown to be interrupted in some subsp. piscicida strains, constituting a pseudogene. This is the first description of a heme-uptake system in a Photobacterium species, and shows some structural and functional similarities to heme-uptake systems reported in other gram-negative bacteria.

Identification of an iron-regulated hemin-binding outer membrane protein, HupO, in Vibrio fluvialis: effects on hemolytic activity and the oxidative stress response

In pathogenic bacteria, iron acquisition is important for colonization and proliferation in the host under iron-limited conditions. The ability of Vibrio spp. to acquire iron is often critical to their virulence, causing gastroenteritis or excessive watery diarrhea in humans. In the study described here, we cloned the 2,100-bp heme utilization protein gene hupO in Vibrio fluvialis. HupO had high homology to iron-regulated outer membrane receptor proteins in Vibrio sp. and contained motifs that are common to bacterial heme receptors, including a consensus TonB box, a FRAP domain, and an NPNL domain. To characterize the hemin-binding activity of HupO, we purified the recombinant HupO protein (rHupO) from Escherichia coli by using an overexpression system. HupO was found to bind to hemin but not to hemoglobin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting demonstrated that the 77-kDa outer membrane protein HupO of V. fluvialis was induced under iron-restricted conditions. We constructed a hupO mutant, HP1, to investigate the biochemical function of HupO in V. fluvialis. The hemolytic activity of HP1 was reduced compared to that of wild-type cells and, when exposed to hydrogen peroxide, significantly lower numbers of HP1 survived than was the case in the wild type. These results suggest that HupO is associated with virulence expression in V. fluvialis through stimulation of hemolysin production and resistance to oxidative stress. In experimentally infected mice, the 50% lethal dose value of the wild-type was lower than that of the mutant, HP1.

Molecular cloning and functional analysis of Photobacterium damselae subsp. piscicida haem receptor gene

A haem receptor gene from Photobacterium damselae subsp. piscicida (formerly known as Pasteurella piscicida) has been cloned, sequenced and analysed for its function. The gene, designated as pph, has an open reading frame consisting of 2154 bp, a predicted 718 amino acid residues and exists as a single copy. It is homologous with the haem receptors of Vibrio anguillarum hupA, V. cholerae hutA, V. mimicus mhuA and V. vulnificus hupA at 32.7, 32.7, 45.6 and 30.9%, respectively, and is highly conserved, consisting of a Phe-Arg-Ala-Pro sequence (FRAP), an iron transport related molecule (TonB) and a Asn-Pron-Asn-Leu sequence (NPNL), binding motifs associated with haem receptors. As a single gene knockout mutant P. damselae subsp. piscicida was able to bind haem in the absence of pph, suggesting that other receptors may be involved in its iron transport system. This study shows that the P. damselae subsp. piscicida pph belongs to the haem receptor family, is conserved and that its iron-binding system may involve more than one receptor.

Identification of Fur regulated genes in the bacterial fish pathogen Photobacterium damselae ssp. piscicida using the Fur titration assay

Bacteria have developed a series of iron-scavenging and transport systems. The expression of many of the iron utilization genes is tightly regulated by the Fe2+ loaded Fur repressor protein. In this study, the Fur titration assay (FURTA) was used to screen for DNA fragments from a genomic DNA library of Photobacterium damselae ssp. piscicida containing potential Fe2+ Fur binding sites or iron binding-proteins which withdraw iron from Fur. One of the clones encoded a tonB gene and adjacent a functionally related exbB gene. An additional and complete tonB exbB exbD gene cluster was identified and sequenced. A gene homologous to the ferritin gene was found whose FURTA-positive phenotype may be explained by its iron-binding ability. Genes encoding a putative complete iron-regulated outer membrane transport protein and a pseudogene of a transport protein were found. The FURTA assay also revealed iron regulation of the AraC type transcriptional regulation.

The effect of growth medium salinity of Photobacterium damselae subsp. piscicida on the immune response of hybrid bass (Morone saxatilis x M. chrysops)

Photobacterium damselae subsp. piscicida (P. damselae) was grown on various media and the effect of media salinity on certain immune responses of hybrid bass was studied. In Israel, pasteurellosis outbreaks have not been reported at water salinities below 1.38 per thousand. During vaccination experiments the salinity of the medium on which P. damselae is grown, was shown to affect stimulation of the immune system. No correlation was found between antibody response and protection. Bacterial envelopes separated by electrophoresis and subjected to western blot analysis revealed an antibody response against some protein bands. Band sequencing was performed to identify the protein stimulating the immune response. Sequence identity of 80% was seen in 10-amino-acid overlap of the 36-kDa band with a specific gene of alkalophilic Bacillus firmus. A preparation of P. damselae grown in a 2.5% NaCl medium at 25 degrees C is the most effective vaccine against pasteurellosis, providing hybrid bass with quite good protection.

Isolation of mutants of Vibrio anguillarum defective in haeme utilisation and cloning of huvA, a gene coding for an outer membrane protein involved in the use of haeme as iron source

The isolation of Vibrio anguillarum mutants lacking the ability to use haemin and haemoglobin as the only iron sources, as well as the identification of a gene involved in haeme utilisation are described. One of the isolated mutants defective in haeme utilisation lacked an iron-regulated outer membrane protein of 79-kDa. Although growth on haeme as iron source was completely abolished, the haemin and haemoglobin binding activities remained intact in the mutant, suggesting that the absent protein is not the only one involved in haeme binding. The wild-type phenotype in this mutant was restored by transformation with a cosmid clone (pML1) containing a 21-kb DNA fragment isolated from a gene library derived from the parental strain of V. anguillarum. Sequence analysis of pML1 subclones led to the finding of an ORF, huvA, that codes for a 79-kDa protein (HuvA) and whose sequence shows high identity with haeme receptors from Vibrio choleare (HutA) and Vibrio vulnificus (HupA). The sequence of huvA from the V. anguillarum haeme-utilisation mutant revealed a single mutation, leading to the synthesis of a truncated HuvA protein of 70 kDa. The parental strain and the cosmid-complemented mutant showed a higher degree of virulence for fish than the mutant strain in experimental infections in which fish were previously overloaded with haemin. This finding suggests that haeme uptake plays an important role in V. anguillarum multiplication in fish tissues when free haeme is available.

Investigation of media formulations promoting differential antigen expression by Photobacterium damsela ssp. piscicida and recognition by sea bass, Dicentrarchus labrax (L.), immune sera

Photobacterium damsela ssp. piscicida (Phdp) isolates were grown in various bacteriological media, in eukaryotic cell culture media and in the presence of fish cells (resembling some aspects of in vivo growth environments). Bacterial cells, extracellular products (ECPs) and crude capsular polysaccharide were isolated and analysed by electrophoresis and Western blot using sea bass sera. Growth in bacteriological media conserved the synthesis of cell and extracellular components when these were compared with those prepared under near-in vivo growth conditions. In fact, synthesis of a larger range of cell components was induced after growth in bacteriological media. Certain media based on yeast extract and peptones from various sources and a specific salt formulation induced the synthesis of novel cell components at approximately 21.3 and 14 kDa. These antigens were recognized by sea bass sera collected after natural pasteurellosis outbreaks and other sea bass sera raised against live or inactivated Phdp cells. The ECPs of the pathogen were not good immunogens in their soluble form despite various treatments prior to immunization. The results are discussed with respect to vaccine development.

Survival of Flavobacterium psychrophilum in rainbow trout (Oncorhynchus mykiss) serum in vitro

Virulent and non-virulent strains of Flavobacterium psychrophilum of different serotypes were examined for survival and growth in non-immune and immune rainbow trout serum, in vitro. A majority of the examined strains consumed complement of non-immune serum, but the complement cascade was not able to cause an immediate (after 3 h incubation) notable reduction in viability of the inoculated cells. After 24 h incubation a more pronounced reduction in the number of viable bacteria was observed in untreated serum as well as in serum heated at 45 degrees C. In serum heated at 56 degrees C this reduction in cell number was not observed, but an increase in cell number did not occur either. The serum survival of one of the examined strains was different from the others in showing cell multiplication after 24 h incubation in normal as well as heat-treated (45 and 56 degrees C) serum. In immune serum no immediate reduction in viability of inoculated cells, of all tested strains, was observed. The number of viable cells showed a slow decrease or remained almost unchanged for up to 72 h post-inoculation in untreated serum, at 5 degrees C as well as 15 degrees C. In heat-treated serum (45 degrees C) the number of viable cells decreased slowly at 5 degrees C and 15 degrees C for up to 72 h. The results suggest that the examined strains were unaffected by the alternative complement reaction present in fish serum as well as by antibodies against F. psychrophilum. However, some unknown component(s) in the fish sera, or lack of nutrients or essential growth factors, inhibited the growth of most of the examined strains in the tested fish sera.

Temperature dependent siderophore production in Vibrio salmonicida

Vibrio salmonicida is the causative agent of cold water vibriosis, a haemorrhagic septicaemia of Atlantic salmon (Salmo salar). The disease is observed only at low water temperatures and not normally above 10 degrees C. Siderophore production and iron regulated outer membrane protein expression was studied at various temperatures. Although in iron-limited media optimal cell growth was identified at 12 degrees C, significant quantities of a single hydroxamate siderophore were produced only at 10 degrees C or below. Dependent on inoculant size, good growth without significant siderophore production was also observed in iron-limited media at temperatures above and below 10 degrees C. It is therefore likely that V. salmonicida also possesses one or more non-siderophore based iron assimilation systems. Expression of high molecular weight iron-regulated outer membrane proteins appeared to be suppressed at 15 degrees C compared to expression at 6 and 10 degrees C. It is proposed that temperature sensitive iron sequestration may constitute a significant virulence factor in V. salmonicida and may have implications for vaccine manufacture.

The capacity of Porphyromonas gingivalis to multiply under iron-limiting conditions correlates with its pathogenicity in an animal model

Isolates of Porphyromonas gingivalis have various abilities to induce infections in an animal model. The hypothesis of this study was that pathogenic strains of P. gingivalis could multiply under iron-limiting conditions, while non-pathogenic strains could not. Three pathogenic strains (W50, W83, and ATCC 49417) grew to a final optical density (660 nm) > 2 in horse serum, while the growth of the 3 non-pathogenic strains (ATCC 33277, LB13D-2, and HW24D-1) was negligible. When an excess of hemin or ferric chloride was added to the serum, significant growth of the non-pathogenic strains occurred. Under iron-limiting conditions, the pathogenic strains of P. gingivalis had a much lower requirement for human iron-loaded transferrin and hemin than the non-pathogenic strains. Proteolytic degradation of transferrin, which may be associated with the release of iron, was not markedly different for pathogenic and non-pathogenic strains. In addition, no relationship could be established between the level of 55Fe uptake from 55Fe-transferrin and the pathogenicity of strains. Our study provided evidence that the ability of P. gingivalis to multiply in vitro under iron-limiting conditions may be correlated with its ability to induce infections in an animal model. Isolates of P. gingivalis possessing a low requirement for iron are likely to have a higher potential for initiating periodontal infections.

Invasion of fish epithelial cells by Photobacterium damselae subsp. piscicida: evidence for receptor specificity, and effect of capsule and serum

Photobacterium damselae subsp. piscicida is a fish pathogen which causes serious disease in commercial warmwater fish species. Because information on the initial stages of the infection is scarce, an investigation of the invasion ability of this pathogen was undertaken utilizing a fish epithelial cell line (epithelioma papillosum carpio, EPC), a virulent capsulated strain of P. damselae (MT1415), an avirulent non-capsulated strain of P. damselae (EPOY-8803-ii) and Escherichia coli HB101 as a non-invasive control. P. damselae was found to be able to adhere to and invade fish epithelial cells and remain inside them for 6-9 h. There were no significant differences in invasiveness between the capsulated and non-capsulated strains. A kinetics study demonstrated that P. damselae invasiveness was more efficient at low m.o.i., reaching saturation at higher m.o.i., suggesting internalization may be receptor-mediated. Invasion efficiency (IE) was significantly higher than in the control E. coli HB101. Engulfment of bacteria was possibly by an endocytic process and was unaffected by killing the bacteria with UV light. However, heat-killed bacteria had significantly reduced invasion capability. Ultrastructural studies showed that inside the epithelial cells, the bacteria remained within large vacuoles for a few hours and no evidence of intracellular replication was found, by either fluorescence or electron microscopic studies. Normal sea bass serum slightly reduced the invasion capability of the MT1415 strain, but heat-inactivated normal serum had no effect. On the other hand, heat-inactivated fish antiserum raised against the same strain reduced the percentage of invaded epithelial cells by 50%. As for other pathogens, an intracellular phase of P. damselae may be a mechanism to delay or avoid phagocytosis and host immune responses, favouring the spread of infection.

Pathogenicity of Vibrio alginolyticus for cultured gilt-head sea bream (Sparus aurata L.)

The in vivo and in vitro pathogenic activities of whole cells and extracellular products of Vibrio alginolyticus for cultured gilt-head sea bream were evaluated. The 50% lethal doses ranged from 5.4 x 10(4) to 1.0 x 10(6) CFU/g of body weight. The strains examined had the ability to adhere to skin, gill, and intestinal mucus of sea bream and to cultured cells of a chinook salmon embryo cell line. In addition, the in vitro ability of V. alginolyticus to adhere to mucus and skin cells of sea bream was demonstrated by scanning electron microscopy. The biological activities of extracellular products of V. alginolyticus were hydrolytic activities; the products were able to degrade sea bream mucus. V. alginolyticus was cytotoxic for fish cell lines and lethal for sea bream. Moreover, the extracellular products could degrade sea bream tissues. However, experiments performed with the bath immersion inoculation technique demonstrated that V. alginolyticus should be considered a pathogen for sea bream only when the mucus layer is removed and the skin is damaged.