Persistence, Seasonal Dynamics and Pathogenic Potential of Vibrio Communities from Pacific Oyster Hemolymph

Abstract

Bacteria of the genus Vibrio occur at a continuum from free-living to symbiotic life forms, including opportunists and pathogens, that can contribute to severe diseases, for instance summer mortality events of Pacific oysters Crassostrea gigas. While most studies focused on Vibrio isolated from moribund oysters during mortality outbreaks, investigations of the Vibrio community in healthy oysters are rare. Therefore, we characterized the persistence, diversity, seasonal dynamics, and pathogenicity of the Vibrio community isolated from healthy Pacific oysters. In a reciprocal transplant experiment we repeatedly sampled hemolymph from adult Pacific oysters to differentiate population from site-specific effects during six months of in situ incubation in the field. We characterized virulence phenotypes and genomic diversity based on multilocus sequence typing in a total of 70 Vibrio strains. Based on controlled infection experiments we could show that strains with the ability to colonize healthy adult oysters can also have the potential to induce high mortality rates on larvae. Diversity and abundance of Vibrio varied significantly over time with highest values during and after spawning season. Vibrio communities from transplanted and stationary oysters converged over time, indicating that communities were not population specific, but rather assemble from the surrounding environment forming communities, some of which can persist over longer periods.

Adaptation to enemy shifts: rapid resistance evolution to local Vibrio spp. in invasive Pacific oysters

“…While oyster associated microbiota can assemble according to host genotype [41], Vibrio spp. in oyster haemolymph are most probably taken up from the environment and vary seasonally with environmental temperature [24,42]. Vibrio spp.…”

“…Thus, Pacific oysters do not have a constant Vibrio community but rather take up strains from the local environment making it highly unlikely that the strains used here share a longer coevolutionary history exceeding the time since invasion into the Wadden Sea. Warm summer months also showed the highest chances of encountering highly virulent strains [24]. Therefore, our sampling in September should have captured a representative high diversity of Vibrio spp., including several virulent strains ( figure 1).…”

“…Indeed, Vibrio communities associated with benthic marine invertebrates including oyster populations used here were not host specific and rather reflected a random assemblage of Vibrio spp. from the environment [24,45]. For generalist pathogens, adaptation to a particular host species would be disadvantageous, as it will limit the host range [46,47].…”

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

“…This also applies to the hemolymph (Olafsen et al, 1993;Garnier et al, 2007, Wendling et al, 2014-the functional analog of blood in vertebrates (Bachere et al, 2004). The presence of viable bacteria in the hemolymph of healthy oysters can influence the outcome of pathogen infections either by stimulating immunity or by competitive exclusion (Schmitt et al, 2012); isolation of antimicrobial compounds of bacterial origin from oyster hemolymph has provided support for the latter hypothesis (Defer et al, 2013).…”

“…The presence of viable bacteria in the hemolymph of healthy oysters can influence the outcome of pathogen infections either by stimulating immunity or by competitive exclusion (Schmitt et al, 2012); isolation of antimicrobial compounds of bacterial origin from oyster hemolymph has provided support for the latter hypothesis (Defer et al, 2013). Yet oyster hemolymph microbiota have rarely been studied and so far only by means of culture-dependent methods (Olafsen et al, 1993;Garnier et al, 2007;Wendling et al, 2014). However, neither cultivation nor molecular fingerprinting methods provide realistic estimates of community diversity and composition (Pedros-Alio, 2006;Bent and Forney, 2008).…”