Antimicrobial mechanism of 4-hydroxyphenylacetic acid on Listeria monocytogenes membrane and virulence.

4-Hydroxyphenylacetic acid (4-HPCA) is the major intestinal metabolite of kaempferol and polymeric proanthocyanidins whereas the effect of 4-HPCA on Listeria monocytogenes remains unknown. In this study, we investigated the effect and mechanism of action of 4-HPCA on the highly lethal foodborne pathogen Listeria monocytogenes. Our results indicated that 4-HPCA inhibited L. monocytogenes growth and proliferation in a dose-dependent manner. In particular, L. monocytogenes displayed negligible growth or proliferation after 4-HPCA treatment (15.61 mM) for 24 h. The impact of 4-HPCA on cell membrane structure and function was investigated in terms of fluorometric cell membrane integrity, zeta potential and relative electrical conductivity. We observed an approximately 15 % fluorescence reduction in the cell membrane after MIC treatment. The zeta potential of the bacteria shifted significantly from -49.74 to -43.70 mV, -36.65 mV and -37.97 mV after treatment with 4-HPCA at the MIC for 0 h, 3 h and 12 h, respectively. The absolute value of the relative electrical conductivities increased significantly following 3 h, 6 h, 9 h and 15 h of 4-HPCA treatment at the MIC level. The results of scanning electron microscopy (SEM) showed that cells treated with 4-HPCA displayed a wrinkled morphology and irregular shapes. Moreover, 4-HPCA obviously decreased the expression of three virulence genes (hlyA, prfA, and inlA) in L. monocytogenes after 12 h of treatment. All these results verified that 4-HPCA, as an effective antibacterial compound against L. monocytogenes, could cause cell death through cell membrane damage and decrease the expression of three virulence factors.