Inhibition ofListeria monocytogenesby a combination of chitosan and divergicin M35

The antimicrobial activities of the class IIa bacteriocin divergicin M35 and several types of chitosan against Listeria monocytogenes were quantified by agar diffusion, critical micro-dilution, and viable count and observed by electron microscopy. Antimicrobial activity of chitosan depended on its molecular mass (MM) and the pH. Three chitosans with MM values of 2, 20, and 100 kDa and 87.4% degree of deacetylation (DDA) were chosen for further study, based on high anti-listerial activity at pH 4.5. Electron microscopy suggested that the mechanism of anti-listerial activity also varied with the MM. Low-MM chitosan appeared to inhibit L. monocytogenes by affecting cell permeability and growth, whereas medium- and high-MM chitosan may form a barrier on the cell surface that prevents entry of nutrients. The minimum inhibitory concentrations (MICs) of 2, 20, and 100 kDa chitosan and divergicin M35 against a divergicin-resistant strain of L. monocytogenes (LSD 535) were 2.5, 2.5, 0.625, and 0.25 mg/mL, respectively. The combination of any of these 3 chitosans and divergicin M35 appeared to have an additive effect against L. monocytogenes, as determined by fractional inhibitory concentration (FIC) index. This study provides useful data for the development of chitosan films incorporating divergicin M35 for inhibiting L. monocytogenes in foods.