Inhibition of Listeria monocytogenes by a lactic acid bacterium isolated from Italian salami

Abstract

Listeria monocytogenes is an opportunistic psychrotroph foodborne pathogen that has been used as a model organism to study the efficacy of many different preservation methods. This work aimed to test the antilisterial activity of lactic acid bacteria isolated from Italian salami and study the development of resistance. Isolates were obtained from naturally fermented Italian salami and cultures that retained activity in the supernatants after pH neutralization and catalase treatment were further characterized. The isolate showing highest inhibitory activity (PD 6.9) was tested for sensibility to proteases, heat and pH. To evaluate if resistance developed, sensitive strains were transferred with sub-lethal doses of the partially purified inhibitory substance and then inoculated into media containing higher doses of the extract. Isolate PD 6.9 inhibited several L. monocytogenes strains obtained from different origins and retained its activity over a wide range of pH and temperature. When increasing concentrations (10–100 AU ml−1) of the partially purified inhibitory substance were added to culture media, growth of L. monocytogenes did not occur even after 12 h of incubation. Cultures of Listeria that were transferred with sub-lethal doses (10 AU ml−1) of the partially purified inhibitory substance could resist higher doses of the extract (50 AU ml−1), but were inhibited when the concentration was further increased (100 AU ml−1). These results indicate that isolate PD 6.9 could potentially be used as a bioprotective culture for salami fermentation.