Abstract: The synergistic antibacterial effect and underlying mechanism of phenyllactic acid (PLA) combined with acetic acid (ACE) were studied against Escherichia coli. Based on the fractional inhibitory concentration index and the time-killing curve, PLA and ACE exhibited a synergistic antibacterial effect. The zeta potential analysis suggested that PLA and ACE could change the distribution of bacterial surface charge. The fluorescence spectral data obtained using fluorophore DiSC3(5) demonstrated that they synergistically dissipated the membrane potential of E. coli on the results of flow cytometry and fluorescence microscopy using the?fluorophores SYTO9 and propidium iodide (PI) showed that PI staining rates of the bacterial cells treated with PLA at 1/4 minimum inhibitory concentration (MIC), ACE at 1/2 MIC, and their combination were 8.8%, 1.6%, and 12.8%, respectively, indicating that the PLA and ACE could synergistically but weakly damage the integrity of the cell membrane. Scanning electron microscopy observation demonstrated that PLA and ACE could result in sunken deformation of E. coli cells. Finally, fluorescence spectroscopic analysis indicated that PLA and ACE, especially their combination, could lead to fluorescence quenching of genomic DNA, suggesting that PLA and ACE could synergistically damage genomic DNA. Conclusively, PLA and ACE could exert synergistic antibacterial effect against E. coli by changing the distribution of surface charge on the cells, dissipating the membrane potential, deforming the cells, and disrupting genomic DNA.

Key words: phenyllactic acid; acetic acid; antibacterial; synergistic effects