Screening, fermentation optimization and preliminary study on antibacterial activity of Bacillus siamensis LBY-7 producing poly-γ-glutamic acid

To explore the antibacterial activity of poly-γ-glutamic acid-producing strains against the main pathogenic Vibrio in aquaculture, a strain LBY-7 with high yield of poly-γ-glutamic acid and antimicrobial activity against the main pathogenic Vibrio in aquaculture was screened from the mud of mariculture pond bottom. It was identified as Bacillus siamensis. Results show that poly-γ-glutamic acid has no antibacterial activity but it can enhance the antibacterial activity of Bacillus siamensis LBY-7. When the fermentation conditions were 35 g/L sucrose, 6 g/L yeast powder, 35 g/L sodium L-glutamate, temperature 37 ℃ and pH 6.5, the yield of poly-γ-glutamic acid reached 23.15 g/L. The experiment showed that the fermentation broth of Bacillus siamensis LBY-7 had the best antibacterial effect and could reach 92% when the fermentation time was 24 h, which was equivalent to that of Bacillus siamensis LBY-7 seed liquid added with poly-γ-glutamic acid. The poly-γ-glutamic acid fermentation broth of Bacillus siamensis LBY-7 has an obvious bacteriostatic effect on Vibrio in aquaculture water, and the bacteriostatic effect is increased by 22.45% and 66.66% in aquaculture water with salinity of 10 and 30, respectively. In conclusion, Bacillus siamensis LBY-7 has the ability to produce poly-γ-glutamic acid, and the poly-γ-glutamic acid produced by fermentation enhance the antibacterial activity of the strain against Vibrio, which has a good application prospect in the prevention and control of Vibrio in aquaculture.