Abstract: To screen for promising antimicrobial agents from natural sources, this study evaluated the antibacterial effect of nine polyphenols against Escherichia coli based on minimum inhibitory concentration (MIC) and fractional inhibitory concentration index (FICI). From these, ferulic acid and 2,4-dihydroxybenzoic acid were selected for their synergistic antibacterial effect against E. coli. Scanning electron microscopy (SEM) and fluorescence microscopy (FM) were used to observe micro-morphological changes in bacterial cells before and after treatment with ferulic acid and 2,4-dihydroxybenzoic acid. Fluorescent probes were applied to determine membrane potential, inner and outer membrane permeability, and reactive oxygen species (ROS) levels. The effect on biofilm formation was investigated using crystal violet staining. The results indicated that the two phenolic acids exerted synergistic antibacterial effect primarily by reducing transmembrane potential, increasing membrane permeability, disrupting membrane integrity as well as bacterial morphology, significantly lowering intracellular ROS levels to interfere with redox signaling, and effectively inhibiting biofilm formation. This study demonstrated great potential for the combined application of these phenolic acids as natural antibacterial agents.

Key words: ferulic acid; 2,4-dihydroxybenzoic acid; synergistic antibacterial mechanism; Escherichia coli