The combined bactericidal effect of nisin and thymoquinone against Listeria monocytogenes in Tryptone Soy Broth and sterilized milk

In this study, the combined bactericidal effect of nisin and thymoquinone (TQ) on Listeria monocytogenes was investigated in Tryptone Soy Broth (TSB) and sterilized milk. The combined treatment resulted in a greater reduction in bacterial number than either nisin or TQ treatment alone. Nisin and TQ respectively caused depolarization and hyperpolarization of the cell membrane of L. monocytogenes , which indicated early injury to the bacteria. Confocal laser scanning microscopy and N -phenyl-l-naphthylamine uptake assay showed disruption of the cell membrane on treatment. Field emission scanning electron microscopy revealed depression and collapse of the cell surface following treatment with nisin and TQ. Growth curves showed that nisin and TQ acted on the bacteria in different growth phases. Fourier-transform infrared spectroscopy demonstrated a reversible hydrogen-bonding interaction between nisin and TQ. Our results suggest that nisin combined with TQ is a more effective antibacterial effect against L. monocytogenes in both TSB and in sterilized milk than either agent alone. Our study sheds light on the inhibitory mechanisms of nisin, TQ, and their combination, and provides a theoretical basis for the synergistic antibacterial application of nisin and TQ in the food industry.