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

Abstract

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.

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

“…To persist in such a hostile environment, the resident microbiota of oysters may have evolved low immunogenic properties, as recently evidenced in a Vibrio population showing preferential association with oyster tissues. In this population, a modified O-antigen structure was shown to reduce strain recognition by oyster immune receptors [ 46 ], unlike in other Vibrio that behave as opportunistic pathogens and colonize oysters by actively altering their cellular defences, thereby favouring immune evasion [ 45 , 47 ]. In addition, the oyster mucus proteins may confer immune tolerance.…”

“…The potent respiratory burst produced by haemocytes triggers a mechanism of ETosis by which haemocytes release DNA Extracellular Traps that contain antimicrobial histones and entrap bacteria, preventing them from disseminating outside the haemolymph or sites of injury [ 52 ]. Most oyster bacterial pathogens have evolved the capacity to escape oyster cellular defences, either by evading phagocytosis [ 53 ] or by exerting cytotoxicity against haemocytes [ 45 , 47 ], thereby causing systemic infections ( figure 1 , right panel). Such mechanisms of cytotoxicity are rarely observed in non-pathogenic bacteria [ 54 ], which instead are contained in the circulation by haemocyte clumps [ 45 ].…”

“…The fight for iron is key in the control of infections by Vibrio , which produce their own siderophores, such as vibrioferrin, for the uptake of this essential nutrient [ 77 ]. Recent results have shown the key role of iron homeostasis in the structuring and assembly of pathological microbial communities in oysters [ 47 ].…”

“…For instance, heat stress decreases the stability of the haemolymph microbiota in C. gigas , resulting in increased mortality in response to infections [ 114 ]. Particularly, destabilization of the oyster haemolymph microbiota can facilitate infection by Vibrio [ 40 ], which are recruited from the oyster environment [ 47 ]. Dramatic events are observed when an abiotic stress co-occurs with an exposure to a pathogen.…”

“…Arcobacter , Vibrio , but also Amphritea , Marinobacterium , Marinomonas , Oceanospirillum and Pseudoalteromonas , together represented up to 40% of the bacterial gene expression at the onset of oyster mortality. Bacteria of the Vibrio genus associated to POMS ( V. crassostrea, V. tasmaniensis or V. harveyi according to environments) were shown to actively participate in the pathogenic process by dampening oyster cellular defences and by producing public goods such as siderophores, which benefit not only the producer but also other members of the population or local community to import iron, which is key for bacterial growth [ 45 , 47 ]. Such cooperative behaviours, which make oyster a more favourable environment for microbial proliferation, were key in structuring the Vibrio community associated with POMS [ 47 ].…”