Antibacterial activity and mechanisms of Bacillus amyloliquefaciens HJ9 isolated from Polygonatum sibiricum against Escherichia coli

In this study, Bacillus amyloliquefaciens HJ9 ( B. amyloliquefaciens HJ9), an endophytic bacterium isolated from Polygonatum sibiricum ( P . sibiricum) , was investigated for its antibacterial activity and potential mechanisms against Escherichia coli ( E.coli ). The results showed that B. amyloliquefaciens HJ9 genome harbors 13 antimicrobial biosynthetic gene clusters. Crude lipopeptides were extracted from the fermentation supernatant by acid precipitation, and HPLC analysis confirmed the presence of surfactin-type lipopeptides. B. amyloliquefaciens HJ9 demonstrated broad-spectrum inhibitory effects against phytopathogenic fungi as well as both Gram-positive and Gram-negative bacteria, with a MIC of 1.48 mg/mL and a MBC of 5.92 mg/mL against E. coli . The CFS of B. amyloliquefaciens HJ9 appeared to compromise cell membrane integrity in a concentration-dependent manner. Specifically, nucleic acid leakage in the treatment groups at 8×MIC was elevated by 18.8-fold. Additionally, PI fluorescence intensity showed a significant increase at 8×MIC, and β-galactosidase activity demonstrated an increase 2.1-fold at 8×MIC. Furthermore, B. amyloliquefaciens HJ9 CFS could induce intracellular ROS accumulation. Concurrently, soluble protein concentration decreased by 14.09% at 8×MIC. Gel retardation assays further indicated DNA-binding capability of the CFS. Notably, B. amyloliquefaciens HJ9 CFS inhibited biofilm formation by up to 79.57% at 8×MIC. Gene expression analysis revealed downregulation of csgA , csgC , csgD , flhD , pfs , lsrG , and ibpA , alongside upregulation of fimA , fimC, ycfR , and luxS . These findings suggest that B. amyloliquefaciens HJ9 CFS may exert antibacterial effects against E. coli through multiple mechanisms, providing a preliminary theoretical basis for developing novel microbial-derived antibacterial agents.