Effects of storage atmosphere on Listeria monocytogenes and competing microflora using a surface model system

A solid-surface model system was used to study the effects of gas atmospheres encountered in modified atmosphere packaging of vegetables on the survival and growth of Listeria monocytogenes and competing micro-organisms. The effects of increasing CO 2 levels (from 5% to 20%), 100% N 2 and 3% O 2 were determined. The model system allowed for estimation of the growth of L. monocytogenes alone or in the presence of competing microflora. CO 2 concentrations of 5-10% (with 5% O 2 in N 2 ) had no inhibitory effect, by comparison with air, on the growth and survival of pure cultures of L. monocytogenes. At 20% CO 2 population densities were reduced up to day 8, but the final population densities reached were not. An atmosphere of 100% N 2 allowed survival of pure cultures of L. monocytogenes, but populations did not significantly change (P > 0.05) during storage, whereas a low O 2 (3%, balance N2) atmosphere allowed significant growth (P < 0.05) of L. monocytogenes, Growth and inhibitory activities of Enterobacter cloacae and E. agglomerans were inversely related to the concentration of CO 2 . By contrast, the growth and anti-listerial activities of Leuconostoc citreum increased with elevated CO. concentrations. In the low O 2 atmosphere, L. monocytogenes grew considerably better in the presence of populations from the indigenous microflora of lettuce than when in pure culture. The results indicate that the gas atmospheres present within modified atmosphere packages of minimally processed vegetables may affect the interactions between the pathogen and the natural competitive microflora sufficiently to indirectly enhance L. monocytogenes growth.