Serogroup, virulence, and molecular traits of Vibrio parahaemolyticus isolated from clinical and cockle sources in northeastern Thailand

- The virulence and related genes of V. parahaemolyticus were more frequently detected in clinical than cockle isolates.
- Pandemic strains (O3:K6, or its probable serovariants) were more frequently detected in clinical than cockle isolates.
- There was likely to indicate a clonal relatedness between clinical and cockle strains.
- Most V. parahaemolyticus clinical and cockle isolates were resistant to ampicillin.
- Cockles in Thailand may serve as a source for virulent V. parahaemolyticus strains and have the potential to cause diarrhea.

Vibrio parahaemolyticus is responsible for seafood-borne gastroenteritis worldwide. Isolates of V. parahaemolyticus from clinical samples (n = 74) and cockles (Anadara granosa) (n = 74) in Thailand were analyzed by serotyping, determination of virulence and related marker genes present, response to antimicrobial agents, and genetic relatedness. Serological analysis revealed 31 different serotypes, 10 of which occurred among both clinical and cockle samples. The clinical isolates commonly included the pandemic serogroup O3:K6, while a few of the cockle isolates exhibited likely pandemic serovariants such as O3:KUT and O4:KUT, but not O3:K6. The pandemic (orf8 gene-positive) strains were more frequently found among clinical isolates (78.4%) than cockle isolates (28.4%) (p < 0.001). Likewise, the virulence and related marker genes were more commonly detected among clinical than cockle isolates; i.e., tdh gene (93.2% versus 29.7%), vcrD2 (97.3% versus 23.0%), vopB2 (89.2% versus 13.5%), vopT (98.6% versus 36.5%) (all p < 0.001) and trh (10.8% versus 1.4%) (p < 0.05). Pulsed-field gel electrophoresis of NotI-digested genomic DNA of 41 randomly selected V. parahaemolyticus isolates representing different serotypes produced 33 pulsotypes that formed 5 different clusters (clonal complexes) (A-E) in a dendrogram. Vibrio parahaemolyticus O3:K6 and likely related pandemic serotypes were especially common among the numerous clinical isolates in cluster C, suggesting a close clonal link among many of these isolates. Most clinical and cockle isolates were resistant to ampicillin. This study indicates that O3:K6 and its likely serovariants based on the PFGE clusters, are causative agents. Seafoods such as cockles potentially serve as a source of virulent V. parahaemolyticus, but further work is required to identify possible additional sources.

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