Antibacterial combined effects of nisin and onion essential oil under different concentration of NaCl and pH against Listeria monocytogenes in vitro

In the present study, the anti-listeria activity of onion (Allium cepa L.) essential oil (EO), nisin as well as their combination at various pH values (4.8, 5.8 and 6.8) and different NaCl concentrations (0, 0.5, 2.5 and 4.5%) was investigated against Listeria monocytogenes by microtiter plates at 30°C. Minimum inhibitory concentrations (MICs) were assessed for the nisin, onion EO as well as their combination. Subsequently, fractional inhibitory concentration (FIC) was evaluated under different pH and NaCl concentrations. Both nisin and EQ showed significant antimicrobial effects against L.monocytogenes. Moreover, the nisin and onion EO exhibited MICs of 12.5 IU mL-1 and 125 μg mL-1, respectively. FICs of the nisin and onion EO alone and in combined form, along with various combinations of pH and NaCl concentration, showed clearly anti-listeria effect as synergistic, additional or indifference. Regardless of NaCl concentrations, the anti-listeria activity of both agents was strongly influenced by pH. Moreover, regardless of pH value, the growth of the bacterium was also affected by increasing of NaCl concentrations. It was concluded that, pH value of 4.8 and NaCl concentration of 4.5% had significant anti-listeria effect. In addition, it was found that, the effect of NaCl concentrations for each of the combination forms of treatments at various pH was different.Therefore, there are necessity to design and apply a comprehensive predictive model.