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

Characterization of the genome and cell invasive phenotype of Vibrio diabolicus Cg5 isolated from mass mortality of Pacific oyster, Crassostrea gigas

Vibrio is an important group of aquatic animal pathogens, which has been identified as the main pathogenic factor causing mass summer mortality of Crassostrea gigas in northern China. This study aims to investigate the potential pathogenic mechanisms of Vibrio Cg5 isolate in C. gigas. We sequenced and annotated the genome of Vibrio Cg5 to analyze potential virulence factors. The gentamicin protection assays were performed with C. gigas primary cells to reveal the cell-invasive behavior of Cg5. The genome analysis showed that Cg5 was a strain of human disease-associated pathogen with multiple antibiotic resistance, and four virulence factors associated with intracellular survival were present in the genome. The gentamicin protection assays showed that Cg5 could potentially invade the cells of C. gigas, indicating that Cg5 could be a facultative intracellular pathogen of C. gigas. These results provide insights into the pathogenic mechanism of V. diabolicus, an emerging pathogenic Vibrio on aquatic animals, which would be valuable in preventing and controlling diseases in oysters.

Emerging Research Topics in the Vibrionaceae and the Squid-Vibrio Symbiosis

The Vibrionaceae encompasses a cosmopolitan group that is mostly aquatic and possesses tremendous metabolic and genetic diversity. Given the importance of this taxon, it deserves continued and deeper research in a multitude of areas. This review outlines emerging topics of interest within the Vibrionaceae. Moreover, previously understudied research areas are highlighted that merit further exploration, including affiliations with marine plants (seagrasses), microbial predators, intracellular niches, and resistance to heavy metal toxicity. Agarases, phototrophy, phage shock protein response, and microbial experimental evolution are also fields discussed. The squid-Vibrio symbiosis is a stellar model system, which can be a useful guiding light on deeper expeditions and voyages traversing these "seas of interest". Where appropriate, the squid-Vibrio mutualism is mentioned in how it has or could facilitate the illumination of these various subjects. Additional research is warranted on the topics specified herein, since they have critical relevance for biomedical science, pharmaceuticals, and health care. There are also practical applications in agriculture, zymology, food science, and culinary use. The tractability of microbial experimental evolution is explained. Examples are given of how microbial selection studies can be used to examine the roles of chance, contingency, and determinism (natural selection) in shaping Earth's natural history.

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.

Evidence for the facultative intracellular behaviour of the fish pathogen Vibrio ordalii

Vibrio ordalii is an extracellular, Gram-negative bacterium that produces vibriosis in salmonids. While pathogenesis is not fully understood, this bacterium has numerous likely genes for adhesion, colonization, invasion factors and, as recently suggested, intracellular behaviour. Therefore, this study aimed to clarify possible intracellular behaviour for V. ordalii Vo-LM-18 and ATCC 33509^T in the fish-cell lines SHK-1 and CHSE-214. Confocal microscopy revealed Vo-LM-18 and ATCC 33509^T inside cytoplasm in both fish-cell lines at 4 hr post-inoculation (hpi). At 8 and 16 hpi, the proportion of fish cells invaded by both strains increased. Moreover, intracellular V. ordalii were observed after 8 hpi inside mouse embryonic fibroblasts (MEF), demonstrating that entry was not due to a cellular phagocytosis process. Flow cytometry confirmed immunocytochemistry results, with both V. ordalii evidencing statistically significant differences in the number of infected cells between 8 and 16 hpi. Interestingly, V. ordalii infection did not significantly damage fish cells, as determined by LDH liberation. Viable counts at 8 hpi detected, on average for both lines, 176 ± 47 CFU/ml of culturable intracellular Vo-LM-18 and ATCC 33509^T cells. These in vitro findings support the facultative intracellular behaviour of V. ordalii and may be of importance for understanding pathogenicity and survival in aquatic environments.

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.

Genomic analysis of the marine fish pathogen Photobacterium damselae subsp. piscicida: Insertion sequences proliferation is associated with chromosomal reorganisations and rampant gene decay

Photobacterium damselae subsp. piscicida (Pdp) is an intracellular fish pathogen that causes photobacteriosis, a disease proven deadly in farmed fish worldwide. This work focuses on the analysis of genome sequences, chromosomes structure and gene contents of two strains from Sparus aurata (DI21) and Solea senegalensis (L091106-03H), isolated on the Spanish Atlantic coast. The comparative genomic analysis revealed that DI21 and L091106-03H share 98% of their genomes, including two virulence plasmids: pPHDP70 encoding siderophore piscibactin synthesis and pPHDP10 encoding the apoptotic toxin AIP56. Both genomes harbour a surprisingly large number of IS elements accounting for 12-17% of the total genome, representing an IS density of 0.15 elements per kb, one of the highest IS density values in a bacterial pathogen. This massive proliferation of ISs is responsible for the generation of a high number of pseudogenes that caused extensive loss of biological functions. Pseudogene formation is one of the main features of Pdp genome that explains most of the ecological and phenotypic differences with respect to its sibling subspecies P. damselae subsp. damselae and to other Vibrionaceae. Evidence was also found proving the existence of two chromosomal configurations depending on the origin of the strains: an European and an Asian/American types of genome organisation, reinforcing the idea of the existence of two geographically-linked clonal lineages in Pdp. In short, our study suggests that the host-dependent lifestyle of Pdp allowed massive IS proliferation and gene decay processes, which are major evolutionary forces in the shaping of the Pdp genome.

In vitro effects of Italian Lavandula multifida L. leaf extracts on gilthead seabream (Sparus aurata) leucocytes and SAF-1 cells

Lavandula multifida is very appreciated by pharmaceutical and cosmetic industries. In Italy is only found in Calabria and Sicily and, at present, urge its valorization due to its high extinction and genetic erosion risks. Possible applications of L. multifida extracts as immunostimulant in fish aquaculture were assayed by using gilthead seabream (Sparus aurata) as a marine fish model, due to its importance in fish aquaculture. The in vitro effects of both aqueous and ethanolic leaf extracts obtained from two Italian populations of L. multifida on head kidney leucocyte activities (viability, phagocytosis, respiratory burst and peroxidase content) were assessed. Furthermore, the possible cytotoxic effects of the extracts on SAF-1 cells and their bactericidal effects on three fish pathogenic bacteria (Vibrio harveyi, Vibrio anguillarum, Aeromonas salmonicida) were also evaluated. All the assays were performed in comparison with leaf extracts obtained from a widely-distributed species as L. angustifolia. Results showed that water and ethanolic leaf extracts obtained from L. multifida enhanced innate immune activities of S. aurata HK leucocytes. Furthermore, SAF-1 cell viability was not affected significantly after being incubated with the extracts. These extracts did not exert any bactericidal activity on the pathogenic bacterial strains tested in the present study. Results obtained in the present work suggested the possibility of use such extracts in in vivo studies in order to corroborate the possibility of their use in aquaculture. Their use could prevent to improve fish defense against pathogenic infections through enhancement of the fish immune status.

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.

Francisella noatunensis subsp. noatunensis invades, survives and replicates in Atlantic cod cells

Systemic infection caused by the facultative intracellular bacterium Francisella noatunensis subsp. noatunensis remains a disease threat to Atlantic cod Gadus morhua L. Future prophylactics could benefit from better knowledge on how the bacterium invades, survives and establishes infection in its host cells. Here, facilitated by the use of a gentamicin protection assay, this was studied in primary monocyte/macrophage cultures and an epithelial-like cell line derived from Atlantic cod larvae (ACL cells). The results showed that F. noatunensis subsp. noatunensis is able to invade primary monocyte/macrophages, and that the actin-polymerisation inhibitor cytochalasin D blocked internalisation, demonstrating that the invasion is mediated through phagocytosis. Interferon gamma (IFNγ) treatment of cod macrophages prior to infection enhanced bacterial invasion, potentially by stimulating macrophage activation in an early step in host defence against F. noatunensis subsp. noatunensis infections. We measured a rapid drop of the initial high levels of internalised bacteria in macrophages, indicating the presence and action of a cellular immune defence mechanism before intracellular bacterial replication took place. Low levels of bacterial internalisation and replication were detected in the epithelial-like ACL cells. The capacity of F. noatunensis subsp. noatunensis to enter, survive and even replicate within an epithelial cell line may play an important role in its ability to infect live fish and transverse epithelial barriers to reach the bacterium's main target cells-the macrophage.

Two routes of infection with Photobacterium damselae subsp. piscicida are effective in the modulation of the transcription of immune related genes in Solea senegalensis

The marine fish pathogen Photobacterium damselae subsp. piscicida (Phdp) is responsible for important disease outbreaks affecting cultured fish species including the flatfish Solea senegalensis. In the present work, transcription of iron metabolism related genes (TF, FERR-M, HP-1 and HAMP-1) as well as innate immune system components such as complement proteins (C3 and C7), lysozyme (LYS-G), TNF family (TNFα, TRAF-3), NCCRP-1 and heat shock protein encoding genes (HSP70, HSP90AA, HSP90AB and GP96) has been determined in the liver and kidney of S. senegalensis specimens after Phdp infection. Intraperitoneal injection (IP) and immersion (IM) routes have been used for infection. Fish developed specific antibodies in both cases, higher levels being detected in IP infected specimens. Both infection routes resulted in increased relative transcript levels of FERR-M, HP-1 and HAMP-1 genes and TF decreased relative transcription, conducting to lower iron availability for the pathogen. This response can be considered as a strategy to limit iron availability for Phdp, a pathogen capable to obtain iron from transferrin. Relative transcription of genes encoding lysozyme and complement factors C3 and C7 were also increased regardless the infection route; the liver was the main organ involved in the initial stages and the kidney in later stages of the infection. TNFα and TRAF-3 relative gene transcription increased 24h post-infection. TRAF-3 gene induction was detected 30 d post-infection, whilst no changes in TNFα were observed 72h or 30 d post-infection. NCCRP-1 changes were observed after IP infection in the liver and kidney; however, IM infection resulted only in slight changes in the kidney of infected fish. This different response observed maybe related to a lower number of invaded cells by the pathogen. Finally, changes in HSP90AB and GP96 have been detected after infection by both routes. Different late modulation has been observed in assayed genes depending on the route of infection. Thus, only LYS-G, TF, NCCRP-1, GP96 and HSP90AB gene transcription was modulated 30 d post-infection in the kidney of IM infected specimens; however, IP infected fish showed modulation in a higher number of genes both in liver and kidney tissues. The implications of these responses in resistance to infection by Phdp need to be elucidated.

Immunization of sea bream (Sparus aurata) juveniles against Photobacterium damselae subsp. piscicida by short bath: Effect on some pro-inflammatory molecules and the Mx gene expression

Cytokines are a family of proteins derived from macrophages, lymphocytes, granulocytes, mast cells and epithelial cells and can be divided into interferons (IFNs), Interleukins (ILs) and Tumor Necrosis factors (TNFs) among others. The presence of cytokines in a wide number of fish species has been proved and several molecules types have been already cloned and sequenced. In this work some proinflamatory molecules and Mx gene were detected in the liver of vaccinated sea bream juveniles with an average body weight of 5 g. The method of immunization was by short bath and three different bacterins against the marine pathogen Photobacterium damselae subsp. piscicida were designed and used to immunize fish. Five genes encoding for five different molecules were analyzed by real time PCR: IL-1β, IL Ir-2, Cox-2, Mx and TNFα. Gene expression was quantified along four days after fish immunization and results were compared among groups. Results show that the heat-inactivated vaccine stimulates the up-regulation of IL-1β, IL Ir-2, Cox-2 and TNFα genes whereas the UV-light inactivated vaccine was the unique vaccine which stimulates the expression of Mx gene. The present is a novel study that shows by the first time the effect of the inactivation process of vaccines on the expression levels of genes involved in the defense against Photobacterium damselae subsp piscicida.

New aspects in the biology of Photobacterium damselae subsp. piscicida: pili, motility and adherence to solid surfaces

We describe for the first time the presence of pilus-like structures on the surface of Photobacterium damselae subsp. piscicida (Phdp). The hint to this discovery was the ability of one strain to hemagglutinate human erythrocytes. Further analysis of several Phdp strains ultrastructure by electron microscopy revealed the presence of long, thin fibers, similar to pili of other Gram-negative bacteria. These appendages were also observed and photographed by scanning, transmission electron microscopy and immunofluorescence. Although this fish pathogen has been described as non-motile, all strains tested exhibit twitching motility, a flagella-independent type IV-dependent form of bacterial translocation over surfaces. As far as we are aware, the movement of Phdp bacteria on semi-solid or solid surfaces has not been described previously. Moreover, we speculate that Phdp twitching motility may be involved in biofilm formation. Microscopic examination of Phdp biofilms by microscopy revealed that Phdp biofilm architecture display extensive cellular chaining and also bacterial mortality during biofilm formation in vitro. Based on our results, standardized analyses of Phdp surface appendages, biofilms, motility and their impact on Phdp survival, ecology and pathobiology are now feasible.

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.

Experimental Lactococcus garvieae infection in zebrafish and first evidence of its ability to invade non-phagocytic cells

Zebrafish has been used for studying infections and host-pathogen interactions in different bacterial fish pathogens. In the present study we evaluated the ability of Lactococcus garvieae to infect zebrafish when inoculated intraperitoneally with 2 × 10(7)UFC of this pathogen. L. garvieae can colonize and invade zebrafish at multiple anatomical sites causing a lethal acute septicemic infection with clinical signs and lesions consistent with those observed in lactococcosis outbreaks. Immunohistochemical studies showed the presence of L. garvieae into macrophages as well as into non-phagocytic zebrafish cells of liver (hepatocytes). The internalization capacity showed by L. garvieae in zebrafish cells was confirmed in the rainbow trout cell line RTG-2. Our results provide the first evidence that L. garvieae is able to invade non-phagocytic host cells.

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.

In vitro study of adherence, invasion, and persistence of Streptococcus iniae in fibroblastic-like fish cell line SAF-1

Streptococcus iniae is a major fish pathogen producing invasive infections that result in economic losses in aquaculture. Gentamicin protection assays were used to investigate the ability of different S. iniae strains to invade and adhere to fibroblastic-like fish cell line SAF-1. All strains tested were detected intracellularly using both techniques, with variable internalization degrees between strains. The experiments carried out at 4°C demonstrated that active cell metabolism is necessary for bacterial internalization. Intracellular bacteria were detected for up to 3 d with a round morphology and were stained with 4',6-diamidino-2-phenylindole (DAPI), indicating that some bacterial cells may remain viable inside SAF-1 cells. Our in vitro findings indicate that S. iniae is capable of adhering, entering, and surviving within fibroblastic cells, which may be important for the persistence and establishment of a carrier state.

Infectivity study of Streptococcus phocae to seven fish and mammalian cell lines by confocal microscopy

Streptococcus phocae is a beta-haemolytic bacterium that causes systemic infections in Atlantic salmon, Salmo salar L., cultured in southern Chile and also in seals. In this study, the host-pathogen interaction between S. phocae and seven types of cell lines (fish and mammalian) was examined using an indirect fluorescent antibody and confocal microscopy (CM). Chinook salmon embryo (CHSE-214), epithelioma papulosum cyprini (EPC), salmon head kidney (SHK-1) and Atlantic salmon kidney were used as the fish cell lines, while human cervix epithelial adenocarcinoma (HeLa), African green monkey kidney fibroblast (Cos-7) and mouse leukaemic monocyte macrophage (Raw 264.7) were included as mammalian cell lines. Streptococcus phocae type strain ATCC 51973(T) and isolates LM-08-Sp and P23 were selected as representatives from the salmon and seal host, respectively. For the CM examination, monolayers seeded on round coverslips were studied at 2- and 20-h post-inoculation (pi). The results showed that there is no common infectivity pattern between the three S. phocae strains at 2-h pi and the cell lines tested, regardless of the source of isolation (seal or salmon). All S. phocae strains could internalize and were found inside the fish and mammalian cell cytoplasm after 20-h pi. Regardless of the cells studied (fish or mammal) and incubation (2 and 20 h), S. phocae was never observed inside the nuclei. Seal and salmon isolates showed the highest number of bacteria entering into the primate cell lines (HeLa and Cos-7) from 2-h pi, while ATCC 51973(T) was not found outside or inside the HeLa and Cos-7 cells.

Bacteria-induced phagocyte secondary necrosis as a pathogenicity mechanism

Triggering of phagocyte apoptosis is a major virulence mechanism used by some successful bacterial pathogens. A central issue in the apoptotic death context is that fully developed apoptosis results in necrotic cell autolysis (secondary necrosis) with release of harmful cell components. In multicellular animals, this occurs when apoptosing cells are not removed by scavengers, mainly macrophages. Secondary necrotic lysis of neutrophils and macrophages may occur in infection when extensive phagocyte apoptosis is induced by bacterial cytotoxins and removal of apoptosing phagocytes is defective because the apoptotic process exceeds the available scavenging capacity or targets macrophages directly. Induction of phagocyte secondary necrosis is an important pathogenic mechanism, as it combines the pathogen evasion from phagocyte antimicrobial activities and the release of highly cytotoxic molecules, particularly of neutrophil origin, such as neutrophil elastase. This pathogenicity mechanism therefore promotes the unrestricted multiplication of the pathogen and contributes directly to the pathology of several necrotizing infections, where extensive apoptosis and necrosis of macrophages and neutrophils are present. Here, examples of necrotizing infectious diseases, where phagocyte secondary necrosis is implicated, are reviewed.