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

Abstract

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.

Keywords: Crassostrea virginica; Vibrio parahaemolyticus; Vibrio vulnificus; aquaculture; ecology; model; seafood safety; tidal range.

Figure 1

Conceptual model generalizing the expected relationships between environmental factors (e.g., temperature and salinity) and biological processes (e.g., abundance of host organisms and carbon available for decomposition) that drive Vibrio spp. abundance both in the water column and in oysters. Arrows indicate which factors are influencing Vibrio levels in seawater and oysters in each model. Two competing models are represented; Model A (left) represents a null expectation that Vibrio spp. in seawater and oysters are driven only by local environmental conditions. Model B (right) builds on this expectation by including an additional path representing the case where Vibrio spp. concentrations in oysters respond to concentrations of Vibrio spp. in seawater.

Figure 2

Environmental parameters measured during the study. (A) water temperature and (B) salinity measured at each site during both studied years. (C) turbidity, (D) chlorophyll a and (E) pheophytin measured in Year 2 of the study.

Figure 3

Concentrations of (A) total (vvhA) and (B) pathogenic (pilF) V. vulnificus measured during the study in oyster (left panel) and water samples (right panel).

Figure 4

Concentrations of (A) total V. parahaemolyticus (tlh), (B) pathogenic V. parahaemolyticus tdh+ and (C) pathogenic V. parahaemolyticus trh+ measured in oysters (left panel) and water samples (right panel).

Figure 5

Pearson’s correlations between (A) concentrations of total (vvhA+) and pathogenic (pilF+) V. vulnificus, (B) concentrations of total (tlh+) and pathogenic V. parahaemolyticus tdh+, and (C) concentrations of total (tlh+) and pathogenic V. parahaemolyticus trh+ measured during both years of the study in oyster samples. The dashed lines illustrate a 1:1 identity line.

Figure 6

Results for the final selected models for each of the Vibrio vulnificus gene targets ((A): vvha: χ² = 0.23, df = 1, p = 0.63; pilF: χ² = 3.80, df = 1, p = 0.05) and V. parahaemolyticus gene targets ((B): tlh: χ² = 3.79, df = 2, p = 0.05; tdh: χ² = 3.25, df = 2, p = 0.20; trh: χ² = 3.63, df = 2, p = 0.16). Boxes represent observed variables and path coefficients are standardized values (standardized by the standard deviation of the variables) for each gene target. Proportional variance explained by the models for all dependent variables (Vibrio spp. concentrations in seawater and oysters) are shown with each of these variables. Note the linear relationship between salinity and seawater V. vulnificus concentrations (vvhA and pilF) but convex relationship (specifying the salinity² term) between salinity and seawater V. parahaemolyticus (tlh, tdh, and trh).