Vibriostatic bacteria isolated from rearing seawater of oyster brood stock: Potentiality as biocontrol agents for vibriosis in oyster larvae

To apply the biocontrol technique for vibriosis of larvae of the Pacific oyster, Crassostrea gigas, we isolated marine bacterial strains from the rearing seawater of oyster brood stock and screened the strains having suppressive activity for the growth of pathogenic vibrios. Twelve strains of the 51 isolated strains clearly showed inhibitory effects on growth of three vibrios (Vibrio alginolyticus, V. tubiashii and Vibrio sp.) by a smear method with agar plates. One strain (S21) demonstrated the highest vibriostatic activity among the 12 tested strains, while S21 did not have adverse effect on the survival of larval oysters. The addition of strain S21 suppressed the growth of V. alginolyticus in a value of 1.0 × 10⁵ CFU/mL, whereas mono-culture of V. alginolyticus augmented to 3.7 × 10⁸ CFU/mL in seawater. The larval survival rate at 24 h after challenge with V. alginolyticus was 78% in the group added with strain S21 at 10⁵ CFU/mL, while it was 8.4% in the control group. These results indicate that strain S21 protects larvae from V. alginolyticus and that strain S21 is a potential biocontrol agent for vibriosis in the larval oyster rearing system.