Abstract
Antibiotics are essential for combating pathogens; however, their misuse has led to increased resistance, necessitating the search for effective, low-toxicity alternatives. Surfactin, due to its unique structure, exhibits significant antibacterial activity without easily inducing resistance, making it a focus of current research. Nonetheless, the effects of branched-chain amino acids (BCAAs) on surfactin's structure and activity are not well understood. This study examines the influence of BCAAs (L-valine, L-leucine, and L-isoleucine) on the lipopeptide (surfactin) produced by B. velezensis YA215. Process optimization revealed that adding 1 g/L of L-Leu and L-Ile, and 0.5 g/L of L-Val, maximizes surfactin production. Surfactin levels peaked with L-Val and L-Ile at 36 h, while L-Leu reached its maximum at 24 h. Notably, L-Val supplementation resulted in the highest relative surfactin content. Antimicrobial testing demonstrated that BCAAs significantly enhance the antibacterial effects of lipopeptides against Escherichia coli and Staphylococcus aureus, with Val showing the most pronounced effect. The addition of BCAAs notably altered the composition of surfactin fatty acid chains. Specifically, Val increased the proportions of iso C14 and iso C16 β-hydroxy fatty acids from 13.3% and 4.216–23.803% and 8.31%, respectively. Additionally, the amino acid composition at the 7th position of the peptide chain changed significantly, especially with Val addition, which increased the proportion of C14 [Val 7] surfactin by 3.29 times. These structural changes are likely associated with the enhanced antibacterial activity of surfactin. These findings provide valuable insights into the roles of BCAAs in microbial fermentation, underscoring their importance in metabolic engineering to enhance the production of bioactive compounds.