A core of functional complementary bacteria infects oysters in Pacific Oyster Mortality Syndrome

Infection with Vibrio aestuarianus limits the utility of increasing resistance of Pacific oyster Crassostrea (Magallana) gigas against OsHV-1 µVar

Infection with the oyster herpesvirus type 1 microvariant (OsHV-1 μVar) has caused mass mortalities of Pacific oyster larvae and spat in multiple countries. Selective breeding to enhance resilience against that virus had been shown as a promising defence strategy. Mass spat mortalities associated with OsHV-1 μVar affected Pacific oyster farms in Ría de Arousa (Galicia, NW Spain), which led us to explore the potential utility of selective breeding to increase cultured oyster survival. Thus, adult oysters that had survived through culture in that area, heavily affected by OsHV-1 μVar, and oysters collected from a naturalised oyster bed that had never been affected, were used as broodstocks in hatchery facilities to produce spat families from each origin. Spat families derived from each stock were transferred into a culture raft in Ría de Arousa; survival and occurrence of OsHV-1 μVar were monitored through cultivation. Spat mortality associated with OsHV-1 μVar was higher in the families deriving from the naïve stock. Adult oyster mortality was detected close to the end of growing-out, which was not associated with OsHV-1 μVar but putatively caused by Vibrio aestuarianus infection. Adult mortality was higher in the families with the highest V. aestuarianus loads; notably, the oyster families with the lowest spat mortality showed the highest adult mortality. Therefore, a potential increase of spat survival in Ría de Arousa through selective breeding to enhance oyster resilience against OsHV-1 μVar could be counteracted by high adult mortality associated with V. aestuarianus infection.

The Isolation and Identification of Pseudoalteromonas sp. H27, a Bacterial Strain Pathogenic to Crassostrea gigas

Bacterial infection is frequently observed in disease outbreaks of aquatic animals, making it of significance to isolate and identify the bacterial pathogens. In this study, diseased individuals of Crassostrea gigas were sampled from the nearshore area in Zhanjiang, Guangdong in May 2023. Culturable bacteria were isolated from the diseased tissue and a pathogenic strain labeled as H27 was screened through a hemolysis test and bacterial challenge experiments. Morphological characterization, 16S rRNA gene sequence-based molecular identification and biochemical tests showed that strain H27 belonged to the genus of Pseudoalteromonas, a dominant genus in the diseased tissue of C. gigas revealed by bacterial community structure analysis. The clinical signs originally observed in naturally diseased C. gigas were reproduced in strain H27-challenged adults, both with the red mantle and adductor. Histopathological analysis was further performed on the diseased tissues of the latter, which showed a significantly increased accumulation of pigment granules in the cytoplasm of the diseased mantle as well as enlarged muscle fiber distances in the diseased adductor. In addition, strain H27 was re-isolated from tissues of the moribund C. gigas after bacterial challenge, indicating the fulfillment of Koch's postulate. Our results help to enrich the knowledge of C. gigas diseases, possibly contributing to disease prevention and control.

DNA methylation landscapes before and after Pacific Oyster Mortality Syndrome are different within and between resistant and susceptible Magallana gigas

Pacific oysters face recurring outbreaks of Pacific Oyster Mortality Syndrome (POMS), a polymicrobial multifactorial disease. Although this interaction is increasingly understood, the role of epigenetics (e.g., DNA methylation) appears to be of fundamental importance because of its ability to shape oyster resistance/susceptibility and respond to environmental triggers, including infections. In this context, we comprehensively characterized basal (no infection) and POMS-induced changes in the methylome of resistant and susceptible oysters, focusing on the gills and mantle. Our analysis identified differentially methylated regions (DMRs) that revealed distinct methylation patterns uniquely associated with the susceptible or resistant phenotypes in each tissue. Enrichment analysis of genes bearing DMRs highlighted that these epigenetic changes were specifically linked to immunity, signaling, metabolism, and transport. Notably, 31 genes with well-known immune functions were differentially methylated after POMS, with contrasting methylation patterns between the phenotypes. Based on the methylome differences between phenotypes, we identified a set of candidate epibiomarkers that could characterize whether an oyster is resistant or susceptible (1998 candidates) and whether a site has been exposed to POMS (164 candidates). Overall, the findings provide a deeper understanding of the molecular interactions between oysters and POMS infection, opening new questions about the broader implications of epigenetic mechanisms in host-pathogen dynamics and offering promising strategies for mitigating the impacts of this devastating disease. Beyond its biological aspects, this study provides insights into potential epigenetic biomarkers for POMS disease management and targets for enhancing oyster health and productivity.

Microbial education plays a crucial role in harnessing the beneficial properties of microbiota for infectious disease protection in Crassostrea gigas

The increase in marine diseases, particularly in economically important mollusks, is a growing concern. Among them, the Pacific oyster (Crassostrea gigas) production faces challenges from several diseases, such as the Pacific Oyster Mortality Syndrome (POMS) or vibriosis. The microbial education, which consists of exposing the host immune system to beneficial microorganisms during early life stages is a promising approach against diseases. This study explores the concept of microbial education using controlled and pathogen-free bacterial communities and assesses its protective effects against POMS and Vibrio aestuarianus infections, highlighting potential applications in oyster production. We demonstrate that it is possible to educate the oyster immune system by adding microorganisms during the larval stage. Adding culture based bacterial mixes to larvae protects only against the POMS disease while adding whole microbial communities from oyster donors protects against both POMS and vibriosis. The efficiency of immune protection depends both on oyster origin and on the composition of the bacterial mixes used for exposure. No preferential protection was observed when the oysters were stimulated with their sympatric strains. Furthermore, the added bacteria were not maintained into the oyster microbiota, but this bacterial addition induced long term changes in the microbiota composition and oyster immune gene expression. Our study reveals successful immune system education of oysters by introducing beneficial microorganisms during the larval stage. We improved the long-term resistance of oysters against critical diseases (POMS disease and Vibrio aestuarianus infections) highlighting the potential of microbial education in aquaculture.

Diseases of marine fish and shellfish in an age of rapid climate change

A recurring trend in evidence scrutinized over the past few decades is that disease outbreaks will become more frequent, intense, and widespread on land and in water, due to climate change. Pathogens and the diseases they inflict represent a major constraint on seafood production and yield, and by extension, food security. The risk(s) for fish and shellfish from disease is a function of pathogen characteristics, biological species identity, and the ambient environmental conditions. A changing climate can adversely influence the host and environment, while augmenting pathogen characteristics simultaneously, thereby favoring disease outbreaks. Herein, we use a series of case studies covering some of the world's most cultured aquatic species (e.g., salmonids, penaeid shrimp, and oysters), and the pathogens (viral, fungal, bacterial, and parasitic) that afflict them, to illustrate the magnitude of disease-related problems linked to climate change.

Cross-talk and mutual shaping between the immune system and the microbiota during an oyster's life

The Pacific oyster Crassostrea gigas lives in microbe-rich marine coastal systems subjected to rapid environmental changes. It harbours a diversified and fluctuating microbiota that cohabits with immune cells expressing a diversified immune gene repertoire. In the early stages of oyster development, just after fertilization, the microbiota plays a key role in educating the immune system. Exposure to a rich microbial environment at the larval stage leads to an increase in immune competence throughout the life of the oyster, conferring a better protection against pathogenic infections at later juvenile/adult stages. This beneficial effect, which is intergenerational, is associated with epigenetic remodelling. At juvenile stages, the educated immune system participates in the control of the homeostasis. In particular, the microbiota is fine-tuned by oyster antimicrobial peptides acting through specific and synergistic effects. However, this balance is fragile, as illustrated by the Pacific Oyster Mortality Syndrome, a disease causing mass mortalities in oysters worldwide. In this disease, the weakening of oyster immune defences by OsHV-1 µVar virus induces a dysbiosis leading to fatal sepsis. This review illustrates the continuous interaction between the highly diversified oyster immune system and its dynamic microbiota throughout its life, and the importance of this cross-talk for oyster health. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

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.