Isolation and molecular characterization of virulent and avirulent strains of Vibrio alginolyticus

Vibriosis caused by Vibrio alginolyticus has been recognised as serious disease problem. Although V. alginolyticus were reported to be pathogen but it also been used to formulate probiotics used in shrimp aquaculture. Therefore, indigenous marine strains of V. alginolyticus were isolated and selected to study their virulence in shrimp postlarvae. The bacteria were isolated from 20 water and 20 sediment samples collected from natural environment and shrimp ponds in Selangor and Sabah, Malaysia. From the total of 79 isolates which formed yellow colonies on TCBS agar plates, 15 were identified as V. alginolyticus using gyrB gene based PCR and showed 94 to 100% similarity to the partial sequence of V. alginolyticus 16S rRNA gene. From the 15 isolates, only six were chosen for pathogenicity study which consisted of one beta, three alpha and two gamma hemolysis type of strains. After 24 hours of incubation in TSB with shaking at 30 °C, these isolates showed different OD readings and the highest was 1.7554 ± 0.0031 with cell number of 4.72 X 109 ± 2.74 X 108 CFU.mL-1 indicating that V. alginolyticus M2 showed the best growth rate in TSB with 1.5% NaCl compared to other isolate In the pathogenicity test, three treatments consisting of different concentrations of bacteria (103, 105 and 107 CFU.mL-1) and one control group contained physiological saline were performed on shrimp postlarvae. There was a significant difference (p