The Combined Effect of Mentha spicata Essential Oil and Nisin Against Listeria monocytogenes

Listeria monocytogenes is one of the major causes of infections in developing countries. The aim of the present study was to investigate chemical composition and the combined effect of Mentha spicata essential oil with nisin against L. monocytogenes at different temperatures (4, 9 and 14°C), pHs (5, 6 and 7) and NaCl concentrations (1, 2 and 4g/100ml) in in vitro condition. Chemical composition of the essential oil was evaluated by GC-MS analysis. Minimum inhibitory concentration (MIC) values of the essential oil and nisin were determined by using broth micro-dilution test. The Differences in Population (DP) assay and Fractional Inhibitory Concentration (FIC) index were applied to investigate their synergistic effects. The main components of the essential oil were carvone (78.76%) and limonene (11.50%). MIC values of the essential oil and nisin against L. monocytogenes were 320IU/ml and 160µl/ml, respectively. Concentration of 80μl/ml essential oil in combination with 160IU/ml nisin significantly (P<0.001) inhibited the bacterium at all pHs. Also, concentration of 40μl/ml and 80 essential oil in combination with 80 and 160IU/ml nisin significantly (P<0.001) inhibited the bacterium at all temperatures. 2g/100ml and 4g/100 ml NaCl concentration enhanced the sensitivity of L. monocytogenes toward four combinations. Reduction in the pH and incubation temperature and increasing of salt content led to enhance the anti-listerial effects of the essential oil and nisin. nisin and M. spicata essential oil could be considered as potential strong antimicrobials that can be used for the growth inhibition of L. monocytogenes in food products.