“Hot shellfish” host higher-than-expected levels of Vibrio spp. pathogens

Vibrio spp. are a group of heterotrophic and often pathogenic bacteria causing disease in corals, molluscs, fish, crustaceans, and humans. Of particular importance are V. parahaemolyticus and V. vulnificus, both food-borne human pathogens potentially fatal to raw or undercooked shellfish consumers. These estuarine species adsorb to particulate organic matter in the water column, and are drawn into bivalves’ digestive systems through the filter-feeding activity of these molluscs (Froelich, Ayrapetyan & Oliver, 2013; Kaneko & Colwell, 1975), where they proliferate to substantially higher abundance levels than those in sea water (FDA, 2005).
In terms of their pathogenicity to humans, the picture is made complex by the fact that pathogenic strains (meaning strains in nature, see Dijkshoorn, Ursing & Ursing, 2000) represent but a minor subset of their conspecifics isolated from environmental samples, e.g. only 0.18-3.2 % for oyster-associated V. parahaemolyticus at various coastal sites in North America (FDA, 2005). An official document assessing the risk of V. parahaemolyticus for public health in the US reported that the likelihood of contracting disease is positively correlated to the levels of V. parahaemolyticus consumed (FDA, 2005). Interestingly however, a new study published this year on FEMS Microbiology Ecology, found that the total abundance of V. parahaemolyticus is not correlated to the levels of pathogenic conspecific strains in shellfish and in the water column (Williams et al., 2017). The authors isolated V. parahaemolyticus and V. vulnificus from shellfish (American oyster, Crassostrea virginica, and hard clam, Mercenaria mercenaria) and water samples collected along the eastern US coast through selective medium-culturing in order to estimate bacterial abundance. Moreover, pathogenic isolates were identified using a DNA barcoding approach targeted at well-characterised virulence markers for both species. It was found that 64% of pathogenic V. parahaemolyticus isolates from all sources were not associated with non-pathogenic colonies. On the other hand, levels of pathogenic and total V. vulnificus were found to be positively correlated.
Perhaps most interestingly, the authors reported that a few oyster samples yielded exceptionally high numbers of V. vulnificus isolates. As noted by the authors, because the bacteria were isolated from pooled oyster samples (n = 5) from each site, it is not clear whether this resulted from one or more “hot oysters”. Nevertheless, considering that, as mentioned, a positive correlation was reported between total and pathogenic V. vulnificus abundance, the finding could have important implications for the development of future public health risk assessments.
Considering the frequent disagreement between culture-dependent bacterial abundance estimation methods and in situ bacterial counts (Munn, 2003), it would be interesting to repeat the study using techniques allowing to enumerate bacteria in situ. For instance, since the genomes of both V. parahaemolyticus and V. vulnificus are available, accurate quantification of bacterial levels could be performed by qPCR, a powerful technique used for bacterial disease diagnosis in a range of organisms. Flow-FISH (Fluorescence In Situ Hybridisation coupled with flow cytometry) would be an equally promising option.
In view of the study’s findings, current public health risk assessments are arguably outdated, as they heavily rely on total bacterial count as a predictor of the risk to human health (FDA, 2005). Furthermore, notably, the majority of V. parahaemolyticus pathogenic isolates was obtained in autumn/winter months, despite total Vibrio spp. abundance usually peaking in warmer periods, suggesting a more complex effect of temperature on Vibrio spp. life cycle than previously thought. The study clearly warrants further investigations on the ecology and physiology of V. parahaemolyticus and V. vulnificus, the symbiotic association with their bivalve hosts, and the ecological and physiological differences between pathogenic and non-pathogenic strains.

Reviewed article:
Williams, T. C., Froelich, B. A., Phippen, B., Fowler, P., Noble, R. T. & Oliver, J. D. (2017) 'Different abundance and correlational patterns exist between total and presumed pathogenic Vibrio vulnificus and V. parahaemolyticus in shellfish and waters along the North Carolina coast'. FEMS Microbiol Ecol, 93 (6).

References
Dijkshoorn, L., Ursing, B. M. & Ursing, J. B. (2000) 'Strain, clone and species: comments on three basic concepts of bacteriology'. J Med Microbiol, 49 (5), pp. 397-401.

FDA (2005) Quantitative Risk Assessment on the Public Health Impact of Pathogenic Vibrio parahaemolyticus In Raw Oysters. U.S. Department of Health and Human Services.

Froelich, B., Ayrapetyan, M. & Oliver, J. D. (2013) 'Integration of Vibrio vulnificus into Marine Aggregates and Its Subsequent Uptake by Crassostrea virginica Oysters'. Applied and Environmental Microbiology, 79 (5), pp. 1454-1458.

Kaneko, T. & Colwell, R. R. (1975) 'Adsorption of Vibrio parahaemolyticus onto chitin and copepods'. Appl Microbiol, 29 (2), pp. 269-274.

Munn, C. (2003) Marine Microbiology: Ecology & Applications. Taylor & Francis.