Vibrio parahaemolyticus was isolated from the internal organs of diseased sea bass (Dicentrarchus labrax) cultured in a marine cage farm and an inshore fish farm in the summer 2010. All strains isolated from diseased sea bass were tested for virulence by intraperitoneal injection. The isolates (n = 6) were pathogenic for sea bass. Only bacterial strains showing haemolytic activity were virulent. LD50 values ranged from 3.52 × 104 to 2.29 × 106 cfu fish−1. In this study, we report the characterization and the virulence of V. parahaemolyticus strains isolated from sea bass originating from two different fish farms (marine cage farm and inshore fish farm in Tunisia).

Vibrio parahaemolyticus and Vibrio alginolyticus are important pathogenic agents for both humans and aquatic animals. Twenty-five bacterial strains were isolated from infected sea bass (Dicentrarchus labrax) on thiosulfate citrate bile salts sucrose (TCBS) agar plates. For the species-specific detection of V. alginolyticus and V. parahaemolyticus, a multiplex PCR assay using two collagenase-targeted primer pairs allows the detection of four strains of V. parahaemolyticus and three strains of V. alginolyticus. The seven identified isolates were partitioned for capsule production, hydrophobicity, adherence, biofilm formation, invasion, and cytotoxicity against Hep-2 cells. Two V. parahaemolyticus (Spa2 and Spa3) and one V. alginolyticus (Va01) were capsule producers developing almost black colonies on CRA, they showed a strong hydrophobicity using bacterial adhesion to hydrocarbons test (BATH), and were able to produce high biofilm. Isolates were able to adhere and invade Hep-2 cells and exhibited dissimilar levels of cytotoxicity in epithelial cells. This study shows the strong relationship between adhesion, biofilm formation, invasion and the cytotoxicity of Vibrio strains. Thus, we found a strong and significant positive correlation between different virulence properties of these isolates. The present study shows that bacterial contact with the cells as well as adhesion and invasion are essential steps to induce cytotoxicity. However, the invasion is seen to be a post adherence event.