Abstract
Several Vibrio spp. cause acute and severe mortality events in hatcheries where larvae of bivalve mollusks are reared, potentially leading to subsequent shortage of bivalve seed for the grow-out industry. In particular, strains of Vibrio coralliilyticus have been identified as a major cause of disease in Pacific, Crassostrea gigas, and eastern, C. virginica, oyster hatcheries in the USA of America. Probiotic bacteria are an inexpensive, practical, and natural method of disease control. Previous research shows that pretreatment of larval oysters with probiotic bacteria Bacillus pumilus RI06–95 (RI) and Phaeobacter inhibens S4 (S4) significantly decreases mortality caused by experimental challenge with the bacterial pathogen V. coralliilyticus RE22 (RE22). This study aims to characterize the immune response of 6–10-day-old eastern oyster larvae to experimental challenge with pathogen V. coralliilyticus RE22 and probionts RI and S4. Treatments included (a) pathogen and probiont exposure at a concentration of 5 × 104 CFU per mL (~2500 bacterial cells per larva) for a duration of 6 h, (b) probiont exposure at the same concentration for a duration of 24 h, and (c) probiont RI daily treatment of larvae in the hatchery for 4, 11, and 15 days. Differential gene expression analysis compared pathogen or probiotic-treated transcriptomes to unexposed controls. Probiotic and pathogen treatment led to upregulation of transcripts coding for several immune pattern recognition receptors (PRRs) involved in environmental sensing and detection of microbes in oyster larvae. Larval oyster responses to pathogen RE22 suggested suppression of expression of genes in immune signaling pathways (myd88, tak1, nkap), failure in upregulation of immune effector genes, high metabolic demand, and oxidative stress that potentially contributed to mortality. On the other hand, the transcriptomic response to probiotic bacteria RI and S4 suggested activation of immune signaling pathways and expression of immune effectors (e.g., Cv-spi2, mucins and perforin-2). These key features of the host immune response to probiotic bacteria were shared despite the length of probiotic exposure, probiotic species, and the type of environment in which exposures were conducted. This study suggests that pre-exposure of eastern oyster larvae to probiotics for 6–24 h prior to pathogenic challenge leads to a robust and effective immune response that may contribute to protecting larvae from subsequent challenge with V. coralliilyticus RE22. This research provides new insights into host-microbe interactions in larval oysters that could be applied in the management of vibriosis in bivalve hatcheries.
Highlights
The Vibrionaceae constitutes a diverse bacterial family inhabiting a variety of ecological niches in aquatic environments
This research focused on characterizing the immune response of C. virginica to (a) bacterial pathogen V. coralliilyticus RE22 in order to understand mechanisms of pathogenesis in larval vibriosis and (b) bacterial probiotics B. pumilus RI06–95 and P. inhibens S4 in order to understand the role of immunomodulation in protection against RE22
This is in contrast with the responses of larval eastern oysters to probiotic treatment observed in our study, as well as results from previous studies in oysters and other bivalves showing a diversity of immune effectors, such as antimicrobial peptides, lysozymes, pore forming molecules like perforin, and serine protease inhibitors, produced in response to bacterial challenge
Summary
The Vibrionaceae constitutes a diverse bacterial family inhabiting a variety of ecological niches in aquatic environments. Several Vibrio spp. are well-known for their ability to cause disease in a broad range of marine, estuarine, and freshwater hosts, as well as in the terrestrial organisms that ingest vibrio-contaminated seafood or water [1,2,3]. These diseases, grouped under the generic name of vibriosis, have a significant economic and ecological impact, constraining the productivity of aquaculture and fisheries, affecting keystone species such as reef-building corals and bivalves, and placing a burden on public health, in coastal regions [4,5,6]. Recent studies suggest that bivalve larvae and juveniles are unable to mount an effective immune response against vibrio challenge, as seen in mussel larvae in response to V. coralliilyticus [18]
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