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 atmospher es encountered in modified atmosphere packaging of vegetables on the surviv al and growth of Lister ia monocytogenes and competing micro-organisms. The effects of increasing CO2 levels (from 5% to 20%), 100% N-2 and 3% O-2 wer e determined. The model system allowed for estimation of the growth of L. m onocytogenes alone or in the presence of competing microflora. CO2 concentr ations of 5-10% (with 5% O-2 in N-2) had no inhibitory effect, by compariso n with air, on the growth and survival of pure cultures of L, monocytogenes . At 20% CO2 population densities were reduced up to day 8, but the final p opulation densities reached were not. An atmosphere of 100% N-2 allowed sur vival of pure cultures of L. monocytogenes, but populations did not signifi cantly change (P > 0.05) during storage, whereas a low O-2 (3%, balance N2) atmosphere allowed significant growth (P < 0.05) of L. monocytogenes. Grow th and inhibitory activities of Enterobacter cloacae and E. agglomerans wer e inversely related to the concentration of CO2. By contrast, the growth an d anti-listerial activities of Leuconostoc citreum increased with elevated CO2 concentrations. In the low O-2 atmosphere, L. monocytogenes grew consid erably better in the presence of populations from the indigenous microflora of lettuce than when in pure culture. The results indicate that the gas at mospheres present within modified atmosphere packages of minimally processe d vegetables may affect the interactions between the pathogen and the natur al competitive microflora sufficiently to indirectly enhance L. monocytogen es growth.