Deciphering host–pathogen dynamics in Mytilus galloprovincialis: immune responses to multiple infections with bacterial isolates from bivalve mortality outbreaks

Bivalve aquaculture is facing several challenges due to disease outbreaks, that are often multifactorial and polymicrobial. Increasing evidence shows that initial infection by primary pathogens, or exposure to environmental stressors, can disrupt the host-associated microbiota, creating a favorable environment for opportunistic pathogens. In adult bivalves, the outcome of bacterial infection depends on the capacity of the immune system to fight potential pathogens. Although immune responses to individual bacteria are well documented, data on polymicrobial infections remain scarce. Mytilus spp. is generally considered resistant to bacterial challenge; however, increasing reports of mortality events have emerged across Europe more recently. In this study, we examined the immune response of Mytilus galloprovincialis to infection with two bacterial strains isolated from an oyster mortality event in the Ebro Delta (Spain): Vibrio aestuarianus 106 (Va) and Malaciobacter marinus R1 (Mm). The effects of Va alone on early larval development and on in vitro responses of hemocytes from adult mussels were first evaluated. Both in vivo/in vitro and in vivo/in vivo cross-infections were then performed to determine whether prior exposure to either isolate could influence immune responses to subsequent challenge with the other. The results demonstrate that first infection with Va weakened mussel immune defences, thus favoring the subsequent opportunistic behaviour of Mm. These data indicate that the simple multiple infection approach used in Mytilus could be effectively applied to other, more susceptible bivalve species. These findings can help decipher the interaction dynamics between potential bacterial pathogens within bivalve hosts and predicting the risk of disease.