Characterization of clinical and environmental types of Vibrio vulnificus isolates from Louisiana oysters.

Vibrio vulnificus, a naturally occurring estuarine bacterium frequently transmitted to humans via raw oysters, is a leading cause of seafood-related deaths in the United States. Although unique virulence markers have not been identified to date, multiple biomarkers have been used previously to associate strains with clinical or environmental types of V. vulnificus. In this study, we evaluated the usefulness of these biomarkers in characterizing 349 V. vulnificus oyster isolates by the presence/absence of a viuB-associated fragment and genotypes of three biomarkers: the virulence-correlated gene (vcg), 16S rRNA, and the capsular polysaccharide (CPS) operon. Genotyping data indicated that environmental-type V. vulnificus strains accounted for the majority of oyster isolates, and the percentages ranged from 51.6% for 16S rRNA (type A) to 72.5% for CPS (allele 2 or none). There was also a small percentage (8%) of V. vulnificus isolates possessing both environmental (type A) and clinical (type B) genotypes of 16S rRNA. Additionally, the presence of the viuB fragment (41%) was significantly associated with clinical genotypes of V. vulnificus (p < 0.0001). An interesting seasonal pattern was observed, with clinical-type V. vulnificus isolates more frequently associated with warmer months. In conclusion, the majority of V. vulnificus isolates present in Louisiana raw oysters were of environmental type. There existed a seasonal variation in the V. vulnificus genotypes identified, which may help guide future control measures to focus more specifically on seasons that tend to accumulate more clinical-type V. vulnificus. The study also highlighted the need to identify unique virulence markers in this organism, which could facilitate future screening of virulent V. vulnificus strains from oysters.