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Antagonistic Activity Against Pathogenic Vibrio Isolates of Bioflocculant-Producing Bacteria Isolated from Shrimp Ponds.

Background and Objectives: Biofloc culture system has been used in aquaculture as an effective technology for water treatment due to many advantages of being biodegradable and environmentally friendly. This study aims to isolate bioflocculant-producing bacteria antagonistic to pathogenic Vibrio species from Pacific white shrimp ponds in Thua Thien Hue, Vietnam. Materials and Methods: Vibrio isolates were isolated by screening on medium with and without antibiotics. The resistance of Vibrio to antimicrobial agents was assessed by Minimum Inhibitory Concentration (MIC). Bioflocs formed in shrimp cultures were used to screen bioflocculant-producing bacteria. The identification of bacteria was performed by 16S rRNA sequencing. The flocculating activity was measured by a test with kaolin clay suspension. To evaluate the antagonistic activity against Vibrio isolates, an agar well diffusion assay was used. Results: The screening results have found that Vibrio isolates such as V. parahaemolyticus KS02 and V. alginolyticus KS08 from shrimp ponds can be resistant to many antibiotics with the highest resistance rate up to 66.49%. Four bioflocculant-producing isolates were obtained and identified as Bacillus species. Among them, Bacillus velezensis B9 when grown in YPG medium supplemented with 3% sucrose and 0.7% peptone had the highest bioflocculation with an activity of 49.2%. Two isolates of B. subtilis B2 and Bacillus sp. B6 had quite strong antagonistic activities against vibriosis shown in the zones of inhibition on the assay plates with diameters of about 20 mm. Conclusion: The present study has found some Bacillus isolates had bioflocculant-producing efficiency and inhibited pathogenic Vibrio bacteria. These Bacillus isolates will potentially be used as inoculum for bioflocculation to improve shrimp production.