Optimized production and isolation of antibacterial agent from marine Aspergillus flavipes against Vibrio harveyi .

Statistical methodologies, including Plackett-Burman design and Box-Behnken design, were employed to optimize the fermentation conditions for the production of active substances against aquatic pathogen Vibrio harveyi by marine-derived Aspergillus flavipes strain HN4-13. The optimal crucial fermentation values for maximum production of active substances against V . harveyi were obtained as follows: X 1 (peptone) = 0.3%, X 2 (KCl) = 0.25%, and X 3 (inoculum size) = 4.5%. The predicted diameter of inhibitory zone against V . harveyi was 23.39 mm, and the practical value reached 23.71 ± 0.98 mm with a 62.3% increase. Bioassay-guided fractionation resulted in the acquisition of two compounds whose structures were identified as questin ( 1 ) and emodin ( 2 ). Questin exhibited the same antibacterial activity against V . harveyi as streptomycin (MIC 31.25 µg/mL). This is the first time to report questin as a potential antibacterial agent against aquatic pathogen V . harveyi .