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The emergence and dissemination of aquatic pathogens pose significant risks to farmed species. Francisella halioticida, initially reported in abalones and Yesso scallops, was recently isolated from mussels in France, with some isolates showing high virulence. This study aimed to characterize and compare several F. halioticida isolates from mussels using phenotypic and genotypic approaches. Phenotypic analysis was performed using growth curves, biochemical profiles (API strips), and morphology assessed by electron microscopy. Genetic analysis has been performed through whole‐genome comparison using classification methods and virulence markers seeking. Phenotypic analyses highlighted similarities among FR22 isolates and notable differences with FR21 and AG1. Notably, AG1 displayed distinct features. Antibiotic resistance profiling revealed the species' capacity to withstand multiple antimicrobial agents with various modes of action. Complete, circular genomes were assembled and compared using targeted and untargeted approaches. These analyses confirmed the affiliation of FR22 isolates with the F. halioticida species, while FR21 and AG1 taxonomy need to be further investigated. Virulence factor screening revealed the presence of secretion system components (types I, IV, and VI) in all isolates. A novel variant of the Francisella Pathogenicity Island (FPI) was described, shared by all virulent isolates. However, this FPI was absent in the low virulence isolate FR22b. In conclusion, this study discriminates against F. halioticida isolates and proposes new hypotheses on their virulence, contributing to improved detection tools and expanding our understanding of this emerging aquatic pathogen. Phenotypic and genetic characterization of French isolates of Francisella halioticida and comparison with Canadian and Japanese strains. The study revealed similarities between FR22s and previous isolates but distinct differences with FR21. A major finding is the absence of the Francisella pathogenicity island in the less virulent isolate, FR22b.

The ostreid herpesvirus (OsHV-1) is one of the major diseases that affect the Pacific oyster Crassostrea gigas. Selective breeding programs were recently shown to improve resistance easily to OsHV-1 infections in spat, juvenile and adult oysters. Nevertheless, this resistance has never been investigated in larvae, whereas this developmental stage has crucial importance for the production of commercial hatcheries, as well as explaining the abundance of spatfall. A first trial tested several viral suspensions at several concentrations using contaminated water with OsHV-1 in four-day-old and ten-day-old larvae that were produced from an unselected broodstock. In follow up on the results, one viral suspension at a final concentration of 10+6 OsHV-1 DNA copies per L was used to assess resistance to OsHV-1 infection in C. gigas larvae that were produced from selected and unselected broodstock. A second trial evaluated OsHV-1 resistance in larvae from both broodstocks in trials 2a, 2b and 2c with 4, 10 and 16-day-old larvae for 7 days, which corresponded to post D larvae, umbo larvae and eyed larvae, respectively. The mortality of unchallenged larvae for both stocks were low (<15%) at day 7 in trials 2a and 2b, whereas it ranged from 48 to 56% in trial 2c. More interestingly, selected larvae had significantly lower mortality than unselected larvae when exposed to OsHV-1 in all of the trials. Thus, the mortality was 11% and 49% for the selected larvae at day 7 post-exposure in trials 2a and 2c, respectively, in comparison with 84% and 97% for the unselected larvae. Although this difference in mortality was observed at day 5 in trial 2b, it was reduced at day 7, to 86% and 98% for the selected and unselected larvae, respectively. For the first time in the literature, the difference in mortality or the delayed onset of mortality between selected and unselected larvae have indicated a genetic resistance to OsHV-1 infection at the larval stage. Such finding should facilitate the selective breeding programs focusing on resistance to OsHV-1 infection by reducing the span of the genetic evaluation, and thus decreasing its cost

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.