Hemolymph microbiome of Pacific oysters in response to temperature, temperature stress and infection

Abstract

Microbiota provide their hosts with a range of beneficial services, including defense from external pathogens. However, host-associated microbial communities themselves can act as a source of opportunistic pathogens depending on the environment. Marine poikilotherms and their microbiota are strongly influenced by temperature, but experimental studies exploring how temperature affects the interactions between both parties are rare. To assess the effects of temperature, temperature stress and infection on diversity, composition and dynamics of the hemolymph microbiota of Pacific oysters (Crassostrea gigas), we conducted an experiment in a fully-crossed, three-factorial design, in which the temperature acclimated oysters (8 or 22 1C) were exposed to temperature stress and to experimental challenge with a virulent Vibrio sp. strain. We monitored oyster survival and repeatedly collected hemolymph of dead and alive animals to determine the microbiome composition by 16s rRNA gene amplicon pyrosequencing. We found that the microbial dynamics and composition of communities in healthy animals (including infection survivors) were significantly affected by temperature and temperature stress, but not by infection. The response was mediated by changes in the incidence and abundance of operational taxonomic units (OTUs) and accompanied by little change at higher taxonomic levels, indicating dynamic stability of the hemolymph microbiome. Dead and moribund oysters, on the contrary, displayed signs of community structure disruption, characterized by very low diversity and proliferation of few OTUs. We can therefore link short-term responses of host-associated microbial communities to abiotic and biotic factors and assess the potential feedback between microbiota dynamics and host survival during disease.

Oysters and Vibrios as a Model for Disease Dynamics in Wild Animals

“…Oyster hemolymph also shows the highest Vibrio load in comparison to other tissues, but mortalities under field conditions were observed only when the Vibrio load was also high in other tissues [42], suggesting that the spillover of virulent Vibrio populations from the hemolymph to solid tissues causes systemic disease. Interestingly, in [ 2 3 _ T D $ D I F F ] hemolymph of experimentally infected moribund animals, bacterial communities disturbed by Vibrio infection were not dominated by the Vibrio strain used for infection, but by unrelated secondary opportunists (e.g., Arcobacter) that profited from the initial community disturbance induced by the infection [63]. This stresses the right timing and tissue choice for sampling since moribund animals may be dominated by such secondary opportunists, masking the causative shifts in Vibrio and other microbial populations during the onset of disease.…”

“…Interestingly, in [ 2 3 _ T D $ D I F F ] hemolymph of experimentally infected moribund animals, bacterial communities disturbed by Vibrio infection were not dominated by the Vibrio strain used for infection, but by unrelated secondary opportunists (e.g., Arcobacter) that profited from the initial community disturbance induced by the infection [63]. This stresses the right timing and tissue choice for sampling since moribund animals may be dominated by such secondary opportunists, masking the causative shifts in Vibrio and other microbial populations during the onset of disease.…”

“…Abiotic environmental factors also strongly influence the dynamics of hemolymph microbiota by determining turnover rates as well as inter-individual differences. Importantly, oysters that are associated with more stable communities are more resistant to invasion by pathogens [63].…”

“…These populations may harbor virulence genes (vibrios in red), hence serving as reservoirs of pathogens [21]. To successfully colonize the host, cells must be taken up through the gills and be able to evade the immune system (large gray cell: hemocyte; orange crosses: immune effectors such as antimicrobial peptides) as well as compete with resident bacteria (colored microcells: microbiota) [19,63]. Evidence also suggests that cooperation between virulent and nonvirulent populations may enhance pathogenicity, especially in the early stages of colonization [21,49].…”

The new insights into the oyster antimicrobial defense: Cellular, molecular and genetic view

“…As filter feeders, oysters are in permanent contact and exchanges with microorganisms. Thus, they harbor on their surfaces and inside their body cavities and hemolymph a dense microbiota which has been shown to be greatly dominated by Vibrio species [1,2]. Indeed, a large attention has been paid over the past years, to populations of vibrios as they are among the most abundant cultivable bacteria isolated from oyster tissues [3].…”