Impact of parasitism on levels of human-pathogenic Vibrio species in eastern oysters

A suite of ddPCR assays targeting microbial pathogens for improved management of shellfish aquaculture

The shellfish aquaculture industry is one of the fastest-growing sectors of global food production, but it is currently facing major challenges stemming from microbial pathogens. This study presents an optimized and validated suite of droplet digital PCR (ddPCR) assays using water samples proximal to oyster farms in North Carolina to quantify pathogens relevant to the aquaculture industry. Two of the molecular assays enable the quantification of the pathogens, Vibrio parahaemolyticus and Perkinsus marinus, that threaten human health and oyster performance, respectively. This work also introduces two ddPCR assays that enable the simultaneous quantification of at least nine ecologically relevant Vibrio spp. using only two sets of primers and probes targeting the glycosyl hydrolase family 18 (GH18) domain of the chiA gene in Vibrio bacteria. The entire suite of assays was applied to single assessments at 12 sites, revealing heterogeneity in microbial pathogen concentrations across the coastal landscape and variability of abundances within individual estuarine river systems. Additionally, a longitudinal study conducted at a demonstration lease elucidated unique temporal trends for all microbial targets. Notably, when concentrations of Vibrio spp. quantified using the two assays targeting the chiA gene reached their maximum, the daily probability of mortality increased, suggesting a role for other ecologically pertinent Vibrio spp. in the progression of mortality that would otherwise be missed. This study highlights the utility of ddPCR for the advancement of shellfish management by offering insights into the spaciotemporal dynamics of microbial pathogens.

IMPORTANCE

Climate change is drastically altering the environment and changing the abundance and geographical distribution of marine pathogens. These microbial species put additional pressure on the aquaculture industry by acting as sources of disease for animals important to the food industry as well as for humans upon consumption of contaminated food. To address growing concerns, high-resolution monitoring of pathogens can offer insights for effective management in a critical industry. Validated in the field, the suite of molecular droplet digital PCR assays presented here improves upon current methods, enabling the simultaneous quantification of several targets. This technology makes it possible to track pathogens as they move through the environment and reveals changes in abundance that may inform adjustments to farming practices aimed at mitigating negative outcomes. Additionally, this work presents a unique approach to molecular assay design that unveils potential drivers of ecological shifts and emerging etiologies of disease more efficiently.

Bivalve microbiomes are shaped by host species, size, parasite infection, and environment

Many factors affect an organism's microbiome including its environment, proximity to other organisms, and physiological condition. As filter feeders, bivalves have highly plastic microbiomes that are especially influenced by the surrounding seawater, yet they also maintain a unique core set of microbes. Using 16S ribosomal RNA sequencing, we characterized the bacterial microbiomes of four species of bivalves native to the Mid-Atlantic East Coast of North America: Crassostrea virginica, Macoma balthica, Ameritella mitchelli, and Ischadium recurvum and assessed the impact of their external environment, internal parasites, and size on their microbial communities. We found significant differences in bacterial amplicon sequence variants (ASVs) across species, with each species harboring a core ASV present across all individuals. We further found that some C. virginica co-cultured with I. recurvum had high abundances of the I. recurvum core ASV. We identified ASVs associated with infection by the parasites Perkinsus marinus and Zaops ostreum as well others associated with bivalve size. Several of these ASV are candidates for further investigation as potential probiotics, as they were found positively correlated with bivalve size and health. This research represents the first description of the microbiomes of A. mitchelli, I. recurvum, and M. balthica. We document that all four species have highly plastic microbiomes, while maintaining certain core bacteria, with important implications for growth, health, and adaptation to new environments.

Sources and contamination routes of seafood with human pathogenic Vibrio spp.: A Farm-to-Fork approach

Vibrio spp., known human foodborne pathogens, thrive in freshwater, estuaries, and marine settings, causing vibriosis upon ingestion. The rising global vibriosis cases due to climate change necessitate a deeper understanding of Vibrio epidemiology and human transmission. This review delves into Vibrio contamination in seafood, scrutinizing its sources and pathways. We comprehensively assess the contamination of human-pathogenic Vibrio in the seafood chain, covering raw materials to processed products. A "Farm-to-Fork" approach, aligned with the One Health concept, is essential for grasping the complex nature of Vibrio contamination. Vibrio's widespread presence in natural and farmed aquatic environments establishes them as potential entry points into the seafood chain. Environmental factors, including climate, human activities, and wildlife, influence contamination sources and routes, underscoring the need to understand the origin and transmission of pathogens in raw seafood. Once within the seafood chain, the formation of protective biofilms on various surfaces in production and processing poses significant food safety risks, necessitating proper cleaning and disinfection to prevent microbial residue. In addition, inadequate seafood handling, from inappropriate processing procedures to cross-contamination via pests or seafood handlers, significantly contributes to Vibrio food contamination, thus warranting attention to reduce risks. Information presented here support the imperative for proactive measures, robust research, and interdisciplinary collaboration in order to effectively mitigate the risks posed by human pathogenic Vibrio contamination, safeguarding public health and global food security. This review serves as a crucial resource for researchers, industrials, and policymakers, equipping them with the knowledge to develop biosecurity measures associated with Vibrio-contaminated seafood.

Environmental Reservoirs of Pathogenic Vibrio spp. and Their Role in Disease: The List Keeps Expanding

Vibrio species are natural inhabitants of aquatic environments and have complex interactions with the environment that drive the evolution of traits contributing to their survival. These traits may also contribute to their ability to invade or colonize animal and human hosts. In this review, we attempt to summarize the relationships of Vibrio spp. with other organisms in the aquatic environment and discuss how these interactions could potentially impact colonization of animal and human hosts.

Vibrio vulnificus and Vibrio parahaemolyticus in Oysters under Low Tidal Range Conditions: Is Seawater Analysis Useful for Risk Assessment?

Human-pathogenic Vibrio bacteria are acquired by oysters through filtering seawater, however, the relationships between levels of these bacteria in measured in oysters and overlying waters are inconsistent across regions. The reasons for these discrepancies are unclear hindering our ability to assess if -or when- seawater samples can be used as a proxy for oysters to assess risk. We investigated whether concentrations of total and human pathogenic Vibrio vulnificus (vvhA and pilF genes) and Vibrio parahaemolyticus (tlh, tdh and trh genes) measured in seawater reflect concentrations of these bacteria in oysters (Crassostrea virginica) cultured within the US lower Chesapeake Bay region. We measured Vibrio spp. concentrations using an MPN-qPCR approach and analyzed the data using structural equation modeling (SEM). We found seawater concentrations of these bacteria to predictably respond to temperature and salinity over chlorophyll a, pheophytin or turbidity. We also inferred from the SEM results that Vibrio concentrations in seawater strongly predict their respective concentrations in oysters. We hypothesize that such seawater-oyster coupling can be observed in regions of low tidal range. Due to the ease of sampling and processing of seawater samples compared to oyster samples, we suggest that under low tidal range conditions, seawater samples can foster increased spatial and temporal coverage and complement data associated with oyster samples.