Cooperation and cheating orchestrate Vibrio assemblages and polymicrobial synergy in oysters infected with OsHV-1 virus.

Polymicrobial infections threaten the health of humans and animals but remain understudied in natural systems. We recently described the Pacific Oyster Mortality Syndrome (POMS), a polymicrobial disease affecting oyster production worldwide. In the French Atlantic coast, the disease involves coinfection with ostreid herpesvirus 1 (OsHV-1) and virulent Vibrio. However, it is unknown whether consistent Vibrio populations are associated with POMS in different regions, how Vibrio contribute to POMS, and how they interact with OsHV-1 during pathogenesis. By connecting field-based approaches in a Mediterranean ecosystem, laboratory infection assays and functional genomics, we uncovered a web of interdependencies that shape the structure and function of the POMS pathobiota. We show that Vibrio harveyi and Vibrio rotiferianus are predominant in OsHV-1-diseased oysters and that OsHV-1 drives the partition of the Vibrio community observed in the field. However only V. harveyi synergizes with OsHV-1 by promoting mutual growth and accelerating oyster death. V. harveyi shows high-virulence potential and dampens oyster cellular defenses through a type 3 secretion system, making oysters a more favorable niche for microbe colonization. In addition, V. harveyi produces a key siderophore called vibrioferrin. This important resource promotes the growth of V. rotiferianus, which cooccurs with V. harveyi in diseased oysters, and behaves as a cheater by benefiting from V. harveyi metabolite sharing. Our data show that cooperative behaviors contribute to synergy between bacterial and viral coinfecting partners. Additional cheating behaviors further shape the polymicrobial consortium. Controlling cooperative behaviors or countering their effects opens avenues for mitigating polymicrobial diseases.

Emergence and clonal expansion of Vibrio aestuarianus lineages pathogenic for oysters in Europe.

Crassostrea gigas oysters represent a significant global food source with 4.7 million tons harvested per year. In 2001, the bacterium Vibrio aestuarianus subsp. francensis emerged as a pathogen that causes adult oyster mortality in France and Ireland. Its impact on oyster aquaculture has increased in Europe since its re-emergence in 2012. To better understand the evolutionary mechanisms leading to the emergence and persistence over time of this pathogen, we conducted a survey of mollusc diseases through national reference laboratories across Europe. We analysed 54 new genomes of Vibrio aestuarianus (Va) isolated from multiple environmental compartments since 2001, in areas with and without bivalve mortalities. We used a combination of comparative genomics and population genetics approaches and show that Va has a classical epidemic population structure from which the pathogenic Va francensis subspecies emerged and clonally expanded. Furthermore, we identified a specific cus-cop-containing island conferring copper resistance to Va francensis whose acquisition may have favoured the emergence of pathogenic lineages adapted and specialized to oysters.

Functional Diversification of Oyster Big Defensins Generates Antimicrobial Specificity and Synergy against Members of the Microbiota.

Big defensins are two-domain antimicrobial peptides (AMPs) that have highly diversified in mollusks. Cg-BigDefs are expressed by immune cells in the oyster Crassostrea gigas, and their expression is dampened during the Pacific Oyster Mortality Syndrome (POMS), which evolves toward fatal bacteremia. We evaluated whether Cg-BigDefs contribute to the control of oyster-associated microbial communities. Two Cg-BigDefs that are representative of molecular diversity within the peptide family, namely Cg-BigDef1 and Cg-BigDef5, were characterized by gene cloning and synthesized by solid-phase peptide synthesis and native chemical ligation. Synthetic peptides were tested for antibacterial activity against a collection of culturable bacteria belonging to the oyster microbiota, characterized by 16S sequencing and MALDI Biotyping. We first tested the potential of Cg-BigDefs to control the oyster microbiota by injecting synthetic Cg-BigDef1 into oyster tissues and analyzing microbiota dynamics over 24 h by 16S metabarcoding. Cg-BigDef1 induced a significant shift in oyster microbiota ß-diversity after 6 h and 24 h, prompting us to investigate antimicrobial activities in vitro against members of the oyster microbiota. Both Cg-BigDef1 and Cg-BigDef5 were active at a high salt concentration (400 mM NaCl) and showed broad spectra of activity against bacteria associated with C. gigas pathologies. Antimicrobial specificity was observed for both molecules at an intra- and inter-genera level. Remarkably, antimicrobial spectra of Cg-BigDef1 and Cg-BigDef5 were complementary, and peptides acted synergistically. Overall, we found that primary sequence diversification of Cg-BigDefs has generated specificity and synergy and extended the spectrum of activity of this peptide family.

Vibrio aestuarianus subsp. cardii subsp. nov., pathogenic to the edible cockles Cerastoderma edule in France, and establishment of Vibrio aestuarianus subsp. aestuarianus subsp. nov. and Vibrio aestuarianus subsp. francensis subsp. nov.

Cockle mortality events have been reported in northern France since 2012. In the present study, we describe and investigate the implication of a potential bacterial causative agent in cockle mortality. Bacteria isolated from five different cockle mortality events were characterized and studied. Using phenotypic analysis combined with DNA-DNA hybridization (DDH) and whole genome sequencing, the isolates were shown to belong to Vibrio aestuarianus, a species regularly detected in France during oyster mortality events. Comparison of the strains from cockles with strains from French oysters and the type strain showed that the strains from cockles were genetically different to those from oysters and also different to the V. aestuarianus type strain. Moreover, the cockle and oyster strains were classified into two different, but close, groups both separated from the type strain by: (1) analyses of the ldh gene sequences; (2) DDH assays between 12/122 3T3T (LMG 31436T=DSM 109723T), a representative cockle strain, 02/041T (CIP 109791T=LMG 24517T) representative oyster strain and V. aestuarianus type strain LMG 7909T; (3) average nucleotide identity values calculated on the genomes; and (4) phenotypic traits. Finally, results of MALDI-TOF analyses also revealed specific peaks discriminating the three representative strains. The toxicity of representative strains of these cockle isolates was demonstrated by experimental infection of hatchery-produced cockles. The data therefore allow us to propose two novel subspecies of Vibrio aestuarianus: Vibrio aestuarianus subsp. cardii subsp. nov. for the cockle strains and Vibrio aestuarianus subsp. francensis subsp. nov. for the Pacific oyster strains, in addition to an emended description of the species Vibrio aestuarianus.

Susceptibility variation to the main pathogens of Crassostrea gigas at the larval, spat and juvenile stages using unselected and selected oysters to OsHV-1 and/or V. aestuarianus.

French commercial hatcheries are massively producing Crassostrea gigas selected for their higher resistance to OsHV-1, and soon should also implement selection for increasing resistance to Vibrio aestuarianus. The first objective of this study was to optimize the breeding programs for dual resistance to OsHV-1 and V. aestuarianus to determine the earliest life stage for which oysters are able to develop disease resistance. Wild stocks and selected families were tested using experimental infections by both pathogens at the larval, spat and juvenile stages. Oyster families could be evaluated for OsHV-1 as soon as the larval stage by a bath method, but this only highlighted the most resistant families; those that showed the highest resistance to V. aestuarianus could be determined using the cohabitation method at the juvenile stage. The second objective of this study was to determine if selection to increase/decrease the resistance to OsHV-1 and V. aestuarianus could have an impact on other major pathogens currently detected in hatchery at the larval stage, and in nursery and field at the spat/juveniles stages (V. coralliilyticus, V. crassostreae, V. tasmaniensis, V. neptunius, V. europaeus, V. harveyi, V. chagasi). No relationship was found between mortality caused by V. aestuarianus/OsHV-1 and the mortality caused by the other virulent bacterial strains tested regardless the stages, except between OsHV-1 and V. tasmaniensis at the juvenile stage. Finally, miscellaneous findings were evidenced such as (1) bath for bacterial challenges was not adapted for spat, (2) the main pathogens at the larval stage were OsHV-1 and V. coralliilyticus using bath, while it was V. coralliilyticus, V. europaeus, and V. neptunius at the juvenile stage by injection, and (4) variation in mortality was observed among families/wild controls for all pathogens at larval and juvenile stages, except for V. harveyi for larvae.

Vibrio-bivalve interactions in health and disease.

In the marine environment, bivalve mollusks constitute habitats for bacteria of the Vibrionaceae family. Vibrios belong to the microbiota of healthy oysters and mussels, which have the ability to concentrate bacteria in their tissues and body fluids, including the hemolymph. Remarkably, these important aquaculture species respond differently to infectious diseases. While oysters are the subject of recurrent mass mortalities at different life stages, mussels appear rather resistant to infections. Thus, Vibrio species are associated with the main diseases affecting the worldwide oyster production. Here, we review the current knowledge on Vibrio-bivalve interaction in oysters (Crassostrea sp.) and mussels (Mytilus sp.). We discuss the transient versus stable associations of vibrios with their bivalve hosts as well as technical issues limiting the monitoring of these bacteria in bivalve health and disease. Based on the current knowledge of oyster/mussel immunity and their interactions with Vibrio species pathogenic for oyster, we discuss how differences in immune effectors could contribute to the higher resistance of mussels to infections. Finally, we review the multiple strategies evolved by pathogenic vibrios to circumvent the potent immune defences of bivalves and how key virulence mechanisms could have been positively or negatively selected in the marine environment through interactions with predators.

Complex Relationships between the Blue Pigment Marennine and Marine Bacteria of the Genus Vibrio.

Marennine, the water-soluble blue pigment produced by the marine diatom Haslea ostrearia, is known to display antibacterial activities. Previous studies have demonstrated a prophylactic effect of marennine on bivalve larvae challenged with a pathogenic Vibrio splendidus, suggesting that the blue Haslea is a good candidate for applications in aquaculture as a source of a natural antimicrobial agent. Indeed, the genus Vibrio is ubiquitous in aquaculture ecosystems, and regular events of pathogenic invasion cause some of the biggest losses worldwide. To better characterize the effects of marennine on Vibrios, a panel of 30 Vibrio strains belonging to 10 different species was tested, including bivalve pathogenic species (e.g., Vibrio crassostreae and Vibrio harveyi). Vibrio strains were first exposed to 10 and 25 µg mL-1 of Blue Water (BW), a concentrated culture supernatant of H. ostrearia containing marennine. This screening evidenced a great diversity in responses, from growth stimulation to a total inhibition, at both the interspecific or intraspecific level. In a second series of experiments, 10 Vibrio strains were exposed to BW at concentrations ranging from 5 to 80 µg mL-1. The highest concentrations of BW did not systematically result in the highest growth inhibition as hormetic responses-opposite effects regarding the concentration-were occasionally evidenced. The relationships between marennine and Vibrio strains appear more complex than expected and justify further study-in particular, on the mechanisms of action-before considering applications as a natural prophylactic or antibiotic agent in aquaculture.

Ecologically realistic model of infection for exploring the host damage caused by Vibrio aestuarianus.

Although vibrios are frequently associated with marine organisms mortality outbreaks, knowledge on their ecology and pathogenicity is sparse, thus limiting disease management and prophylactic strategies. Here, we investigated V. aestuarianus infection onset and progression in the wild, taking advantage of a 'claire' pond: a semi-closed system with limited seawater renewal, theoretically more adapted to disease transmission. We showed a positive association of the bacteria with oysters, which can constitute a reservoir for the bacteria in the winter. Moreover, passage through oysters was found to be necessary for experimental disease reproduction as vibrios shedding from diseased oysters have higher infectivity than from in vitro grown. We next developed an experimental 'ecologically realistic' infection model in a mesocosm, allowing infection by natural route. By means of this non-invasive protocol, we analysed the pathogenesis of the bacteria and demonstrated the importance of haemolymph for initial colonization and the septicaemic nature of this disease.

Genomic abnormalities affecting mussels (Mytilus edulis-galloprovincialis) in France are related to ongoing neoplastic processes, evidenced by dual flow cytometry and cell monolayer analyses.

In the context of the abnormal mass mortality of mussels in France since 2014, Flow CytoMetry (FCM) was used in 2015 and 2016 to study the DNA content and cell cycle characteristics of hemic circulating cells collected from 2000 mussels. The mussels were sampled from 12 wild and cultivated blue mussels stocks distributed along the French Atlantic coast from the south Brittany to Pertuis Charentais areas. During these surveys, various genetic abnormalities were frequently detected, and ploidy characteristics revealed contrasting profiles that corresponded to respective contrasting sanitary status, i.e. healthy mussels with high cytogenetic quality (HCQ) versus diseased mussels with low cytogenetic quality (LCQ). In the present work, FCM and hemocytology cell monolayer techniques were combined in order to determine the putative causes of the observed genetic abnormalities that were significantly associated with mortality levels. FCM and cell monolayer approaches permitted the definition of new threshold values delimiting HCQ mussels from LCQ ones. FCM histograms of mussels from the HCQ group showed one single or a largely dominant population of diploid (2n) nuclei and a large majority of normal hemocytes. Hemolymph cell-monolayer analyses showed predominantly acidophil granulocytes characterized by nuclei of normal size and a large cytoplasm with numerous granulations. In contrast, FCM histograms for the LCQ group showed, in addition to the normal diploid (2n) nuclei, populations of nuclei that displayed aneuploidy patterns in a broad ploidy range, including diploid-triploid (2-3n), tetraploid-pentaploid (4-5n) and heptaploid-octaploid levels (7-8n). The corresponding hemolymph cell-monolayer showed cellular features characteristic of disseminated neoplasia disease with frequent abnormal anaplastic cells that exhibited noticeable numbers of mitotic figures with both normal and aberrant chromosomes segregation patterns. These neoplastic cells were a rounded shape with a reduced, granulation-free cytoplasm and large (11-12 µm) to very large (up to 21 µm) round or ovoid nuclei that correspond to the 4-5n and 7-8n nuclei previously detected by FCM analyses. These characteristics suggest that the genetic abnormalities detected by means of FCM were related to an ongoing neoplastic process that is affecting blue mussels in France, at least since the onset in 2014 of the mortality that heavily impacted French blue mussels stocks.

Several strains, one disease: experimental investigation of Vibrio aestuarianus infection parameters in the Pacific oyster, Crassostrea gigas.

This study investigated oyster infection dynamics by different strains of Vibrio aestuarianus isolated before and after the apparent re-emergence of this pathogen observed in France in 2011. We conducted experiments to compare minimal infective dose, lethal dose 50 and bacterial shedding for six V. aestuarianus strains. Whatever the strain used, mortality was induced in juvenile oysters by intramuscular injection and reached 90-100% of mortality within 5 days. Moreover, bacterial shedding was comparable among strains and reached its maximum after 20 h (≈10 EXP5 bacteria/mL/animal). Similarly, our first estimations of lethal dose 50 were comparable among strains (minimal infective dose around 0.4 × 10EXP5 bacteria/mL and LD50 around 10EXP5 bacteria/mL) by using seawater containing freshly shed bacteria. These results indicate that, at least with these criteria, despite V. aestuarianus strains genetic diversity, the disease process is similar. The strains isolated after the apparent re-emergence of the bacteria in 2011, do not present a more acute virulence phenotype than the reference strains isolated between 2002 and 2007. Finally, our study provides original and noteworthy data indicating that infected oysters shed bacteria at a level above the threshold of LD50 a few days before they die, meaning that infection is expected to spread in a susceptible population.

Infection dynamics of a V. splendidus strain pathogenic to Mytilus edulis: In vivo and in vitro interactions with hemocytes.

The pathogenic strain V. splendidus 10/068 1T1 has previously been reported for its virulence to the blue mussel and for its capacity to alter immune responses. In this study, we expanded the knowledge on hemocyte-pathogen interactions by using in vitro and in vivo assays. V. splendidus 10/068 1T1 severely inhibited cell adhesion and acidic vacuole formation unlike the innocuous phylogenetically related V. splendidus 12/056 M24T1 which had no effect on these cell functions. Furthermore, the virulent bacteria decreased hemocyte viability (59% of viability after 24 h). Infection dynamics were explored by using a model based on water tank cohabitation with septic mussels infected by GFP-tagged V. splendidus 10/068 1T1. Experimental infections were successfully produced (16.6% and 45% mortalities in 3 days and 6 days). The amount of GFP Vibrio in seawater decreased during the experiment suggesting its horizontal transfer from diseased animals to healthy ones. At the same time periods, bacteria were detected in hemocytes and in various organs and caused necrosis especially in gills. Total hemocyte count and viability were affected. Taken together, our results indicate that the pathogen V. splendidus 10/068 1T1 colonizes its host both by bypassing external defense barriers and impairing hemocyte defense activities.

Genetic parameters of resistance to Vibrio aestuarianus, and OsHV-1 infections in the Pacific oyster, Crassostrea gigas, at three different life stages.

BACKGROUND: In France, two main diseases threaten Pacific oyster production. Since 2008, Crassostrea gigas spat have suffered massive losses due to the ostreid herpesvirus OsHV-1, and since 2012, significant mortalities in commercial-size adults have been related to infection by the bacterium Vibrio aestuarianus. The genetic basis for resistance to V. aestuarianus and OsHV-1 and the nature of the genetic correlation between these two traits were investigated by using 20 half-sib sire families, each containing two full-sib families. For each disease, controlled infectious challenges were conducted using naïve oysters that were 3 to 26 months old. In addition, siblings were tested under field, pond and raceway conditions to determine whether laboratory trials reflected mortality events that occur in the oyster industry. RESULTS: First, we estimated the genetic basis of resistance to V. aestuarianus in C. gigas. Susceptibility to the infection was low for oysters in spat stage but increased with later life stages. Second, we confirmed a strong genetic basis of resistance to OsHV-1 infection at early stages and demonstrated that it was also strong at later stages. Most families had increased resistance to OsHV-1 infection from the spat to adult stages, while others consistently showed low or high mortality rates related to OsHV-1 infection, regardless of the life stage. Our third main finding was the absence of genetic correlations between resistance to OsHV-1 infection and resistance to V. aestuarianus infection. CONCLUSIONS: Selective breeding to enhance resistance to OsHV-1 infection could be achieved through selective breeding at early stages and would not affect resistance to V. aestuarianus infection. However, our results suggest that the potential to select for improved resistance to V. aestuarianus is lower. Selection for dual resistance to OsHV-1 and V. aestuarianus infection in C. gigas might reduce the impact of these two major diseases by selecting families that have the highest breeding values for resistance to both diseases.

Single or dual experimental infections with Vibrio aestuarianus and OsHV-1 in diploid and triploid Crassostrea gigas at the spat, juvenile and adult stages.

French production of the Pacific cupped oyster, Crassostrea gigas, is currently threatened by two pathogens, OsHV-1 and V. aestuarianus. While oysters selected for their higher resistance to OsHV-1 are now available for the industry, the impact of V. aestuarianus on such oysters is unknown, especially for triploids. In addition, experimental infection has used the virus or the bacteria alone, but there have been no investigations of dual exposure to these pathogens. This study is the first report of single or dual exposure in spat (Spat1 and Spat2), juvenile and adult naïve oysters. For each of the two stocks evaluated, unselected oysters and oysters selected for their higher resistance to OsHV-1 infection were tested, as well as their triploid siblings of the selected oysters produced using cytochalasin B. We confirmed that resistance to OsHV-1 infection and susceptibility to V. aestuarianus increased with age and size, although selected oysters were not significantly impacted by OsHV-1 whatever their ploidy, size or age. We found different mortality patterns depending on the pathogen tested. The mortality pattern was similar for oysters exposed to OsHV-1 or to both pathogens in the Spat1 trial (4months old and 1.9g). The mortality pattern was similar for oysters exposed to V. aestuarianus or to both pathogens in the Adult trial (25months old and 63.1g). Surprisingly, mortality was much higher (ranging from 75.9% to 100%), in particular for the selected oysters, for the Spat2 (8months old/3.9g) and Juvenile trials (16months old/18.4g) given a dual exposure, regardless of the level of selection for OsHV-1 and the ploidy state. Our findings highlight an important threat for oyster farmers: oysters exposed to both pathogens could experience dramatic mortality rates, even in oysters selected for their higher resistance to OsHV-1. Finally, our study demonstrated for the first time that triploid oysters were more susceptible to experimental challenges with V. aestuarianus at the spat stage than their diploid siblings. However, the difference in mortality between the triploids and diploids remained limited and ranged from 22.9% to 6.6% for spat and adults, respectively with a relatively regularly decrease in the difference with increased age.

Antimicrobial Compounds from Eukaryotic Microalgae against Human Pathogens and Diseases in Aquaculture.

The search for novel compounds of marine origin has increased in the last decades for their application in various areas such as pharmaceutical, human or animal nutrition, cosmetics or bioenergy. In this context of blue technology development, microalgae are of particular interest due to their immense biodiversity and their relatively simple growth needs. In this review, we discuss about the promising use of microalgae and microalgal compounds as sources of natural antibiotics against human pathogens but also about their potential to limit microbial infections in aquaculture. An alternative to conventional antibiotics is needed as the microbial resistance to these drugs is increasing in humans and animals. Furthermore, using natural antibiotics for livestock could meet the consumer demand to avoid chemicals in food, would support a sustainable aquaculture and present the advantage of being environmentally friendly. Using natural and renewable microalgal compounds is still in its early days, but considering the important research development and rapid improvement in culture, extraction and purification processes, the valorization of microalgae will surely extend in the future.

A single regulatory gene is sufficient to alter Vibrio aestuarianus pathogenicity in oysters.

Oyster diseases caused by pathogenic vibrios pose a major challenge to the sustainability of oyster farming. In France, since 2012 a disease affecting specifically adult oysters has been associated with the presence of Vibrio aestuarianus. Here, by combining genome comparison, phylogenetic analyses and high-throughput infections of strains isolated before or during the recent outbreaks, we show that virulent strains cluster into two V. aestuarianus lineages independently of the sampling dates. The bacterial lethal dose was not different between strains isolated before or after 2012. Hence, the emergence of a new highly virulent clonal strain is unlikely. Each lineage comprises nearly identical strains, the majority of them being virulent, suggesting that within these phylogenetically coherent virulent lineages a few strains have lost their pathogenicity. Comparative genomics allowed the identification of a single frameshift in a non-virulent strain. This mutation affects the varS gene that codes for a signal transduction histidine-protein kinase. Genetic analyses confirmed that varS is necessary for infection of oysters and for a secreted metalloprotease expression. For the first time in a Vibrio species, we show here that VarS is a key factor of pathogenicity.

Factors other than metalloprotease are required for full virulence of French Vibrio tubiashii isolates in oyster larvae.

Vibrio tubiashii is a marine pathogen isolated from larval and juvenile bivalve molluscs that causes bacillary necrosis. Recent studies demonstrated the isolation of this species in a French experimental hatchery/nursery affecting Crassostrea gigas spat in 2007. Here, using larvae of C. gigas as an interaction model, we showed that the French V. tubiashii is virulent to larvae and can cause bacillary necrosis symptoms with an LD50 of about 2.3 × 10(3) c.f.u. ml(-1) after 24 h. Moreover, complete or gel permeation HPLC fractionated extracellular products (ECPs) of this strain appeared toxic to larvae. MS-MS analysis of the different ECP fractions revealed the existence of an extracellular metalloprotease and other suspected virulence factors. This observation is also supported by the expression level of some potential virulence factors. The overall results suggest that the pathology caused by the French V. tubiashii in C. gigas oysters is caused by a group of toxic factors and not only the metalloprotease.

Can selection for resistance to OsHV-1 infection modify susceptibility to Vibrio aestuarianus infection in Crassostrea gigas? First insights from experimental challenges using primary and successive exposures.

Since 2008, the emergent virus OsHV-1µvar has provoked massive mortality events in Crassostrea gigas spat and juveniles in France. Since 2012, mortality driven by the pathogenic bacteria Vibrio aestuarianus has stricken market-sized adults. A hypothesis to explain the sudden increase in mortality observed in France since 2012 is that selective pressure due to recurrent viral infections could have led to a higher susceptibility of adults to Vibrio infection. In our study, two OsHV-1-resistant lines (AS and BS) and their respective controls (AC and BC) were experimentally challenged in the laboratory to determine their level of susceptibility to V. aestuarianus infection. At the juvenile stage, the selected lines exhibited lower mortality (14 and 33%) than the control lines (71 and 80%), suggesting dual-resistance to OsHV-1 and V. aestuarianus in C. gigas. Interestingly, this pattern was not observed at the adult stage, where higher mortality was detected for AS (68%) and BC (62%) than AC (39%) and BS (49%). These results were confirmed by the analysis of the expression of 31 immune-related genes in unchallenged oysters. Differential gene expression discriminated oysters according to their susceptibility to infection at both the juvenile and adult stages, suggesting that resistance to V. aestuarianus infection resulted in complex interactions between the genotype, stage of development and immunity status. Finally, survivors of the V. aestuarianus challenge at the juvenile stage still exhibited significant mortality at the adult stage during a second and third V. aestuarianus challenge, indicating that these survivors were not genetically resistant.

Mass mortality in bivalves and the intricate case of the Pacific oyster, Crassostrea gigas.

Massive mortality outbreaks in cultured bivalves have been reported worldwide and they have been associated with infection by a range of viral and bacterial pathogens. Due to their economic and social impact, these episodes constitute a particularly sensitive issue in Pacific oyster (Crassostrea gigas) production. Since 2008, mortality outbreaks affecting C. gigas have increased in terms of intensity and geographic distribution. Epidemiologic surveys have lead to the incrimination of pathogens, specifically OsHV-1 and bacteria of the Vibrio genus, in particular Vibrio aestuarianus. Pathogen diversity may partially account for the variability in the outcome of infections. Host factors (age, reproductive status...) including their genetic background that has an impact on host susceptibility toward infection, also play a role herein. Finally, environmental factors have significant effects on the pathogens themselves, on the host and on the host-pathogen interaction. Further knowledge on pathogen diversity, classification, and spread, may contribute toward a better understanding of this issue and potential ways to mitigate the impact of these outbreaks.

Bacterial diseases in marine bivalves.

Bivalve aquaculture is seriously affected by many bacterial pathogens that cause high losses in hatcheries as well as in natural beds. A number of Vibrio species, but also members of the genera Nocardia and Roseovarius, are considered important pathogens in aquaculture. The present work provides an updated overview of main diseases and implicated bacterial species affecting bivalves. This review focuses on aetiological agents, their diversity and virulence factors, the diagnostic methods available as well as information on the dynamics of the host-parasite relationship.

Dual transcriptomics of virus-host interactions: comparing two Pacific oyster families presenting contrasted susceptibility to ostreid herpesvirus 1.

BACKGROUND: Massive mortality outbreaks affecting Pacific oyster (Crassostrea gigas) spat in various countries have been associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). However, few studies have been performed to understand and follow viral gene expression, as it has been done in vertebrate herpesviruses. In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted in order to test the susceptibility of several bi-parental oyster families to this virus and to analyze host-pathogen interactions using in vivo transcriptomic approaches. RESULTS: The divergent response of these oyster families in terms of mortality confirmed that susceptibility to OsHV-1 infection has a significant genetic component. Two families with contrasted survival rates were selected. A total of 39 viral genes and five host genes were monitored by real-time PCR. Initial results provided information on (i) the virus cycle of OsHV-1 based on the kinetics of viral DNA replication and transcription and (ii) host defense mechanisms against the virus. CONCLUSIONS: In the two selected families, the detected amounts of viral DNA and RNA were significantly different. This result suggests that Pacific oysters are genetically diverse in terms of their susceptibility to OsHV-1 infection. This contrasted susceptibility was associated with dissimilar host gene expression profiles. Moreover, the present study showed a positive correlation between viral DNA amounts and the level of expression of selected oyster genes.