Seasonal and Geographical Differences in Total and Pathogenic Vibrio parahaemolyticus and Vibrio vulnificus Levels in Seawater and Oysters from the Delaware and Chesapeake Bays Determined Using Several Methods.

Oyster and seawater samples were collected from five sites in the Chesapeake Bay, MD, and three sites in the Delaware Bay, DE, from May to October 2016 and 2017. Abundances and detection frequencies for total and pathogenic and were compared using the standard most-probable-number-PCR (MPN-PCR) assay and a direct-plating (DP) method on CHROMagar Vibrio for total ( ) and pathogenic ( and ) genes and total () and pathogenic () genes. The colony overlay procedure for peptidases (COPP) assay was evaluated for total DP had high false-negative rates (14 to 77%) for most PCR targets and was deemed unsatisfactory. Logistic regression models of the COPP assay showed high concordances with MPN-PCR for and and in oysters (85.7 to 90.9%) and seawater (81.1 to 92.7%) when seawater temperature and salinity were factored into the model, suggesting that the COPP assay could potentially serve as a more rapid method to detect vibrios in oysters and seawater. Differences in total and pathogenic abundances between state sampling sites over different collection years were contrasted for oysters and seawater by MPN-PCR. Abundances of and were ∼8-fold higher in Delaware oysters than in Maryland oysters, whereas abundances of were nearly identical. For Delaware oysters, 93.5% were both and , compared to only 19.2% in Maryland. These results indicate that pathogenic was more prevalent in the Delaware Bay than in the Chesapeake Bay. While and cause shellfish-associated morbidity and mortality among shellfish consumers, current regulatory assays for vibrios are complex, time-consuming, labor-intensive, and relatively expensive. In this study, the rapid, simple, and inexpensive COPP assay was identified as a possible alternative to MPN-PCR for shellfish monitoring. This paper shows differences in total and pathogenic vibrios found in seawater and oysters from the commercially important Delaware and Chesapeake Bays. isolates from the Delaware Bay were more likely to contain commonly recognized pathogenicity genes than those from the Chesapeake Bay.