Surfactin–Bacillaene Copathway Engineering Strategy Boosts Fengycin Production and Antifungal Activity in Bacillus velezensis HN-Q-8

Abstract

Previous studies have demonstrated that Bacillus velezensis HN-Q-8 shows significant inhibitory effects against various plant pathogenic fungi causing potato diseases, primarily attributed to the production of fengycin. However, the low yield of fengycin in wild-type strains limits its practical application, and the influence of its biosynthesis pathway on volatile organic compound production remains unclear. In this study, to enhance fengycin production in Bacillus velezensis HN-Q-8, we applied metabolic engineering by targeting competitive pathways. Specifically, a double mutant (ΔsrfAAΔbaeBE) was constructed by knocking out the surfactin synthase gene srfAA and the bacillaene synthesis gene baeBE. The fengycin yield of the ΔsrfAAΔbaeBE mutant in the basal (sodium glutamate) fermentation medium reached 98.83 mg/L, representing a 2.39-fold increase over the wild-type strain. Subsequent medium optimization by supplementing peptone further boosted production to 155.61 mg/L, which was 3.77-fold higher than the wild-type level. The lipopeptide extract from the double mutant strain ΔsrfAAΔbaeBE demonstrated potentiated antifungal activity against four major potato fungal pathogens: Alternaria solani (early blight), Rhizoctonia solani (black scurf), Fusarium oxysporum (wilt), and Botrytis cinerea (gray mold). The active volatile compounds released by ΔsrfAAΔbaeBE, such as benzaldehyde and 2,5-dimethylpyrazine were significantly increased. The knockout of srfAA and baeBE also distinctly altered the physiology of the strain: the double mutant exhibited enhanced biofilm formation, an accelerated early growth rate followed by early decline, and a severely reduced sporulation capacity. These results confirmed the feasibility of molecularly modifying Bacillus velezensis HN-Q-8 to improve fengycin production and antifungal activity for further agricultural application.