Growth and anti-listerial activity of a nisin Z producer in a pork lean meat broth fermentation system

Listeria monocytogenes is widely distributed in nature and has been isolated from numerous sources such as meat and fermented meat products. This pathogenic microorganism can resist conditions of low pH, low water activity (Aw), high salt (NaCl) concentrations and the presence of sodium nitrite, being able to survive the commercial sausage manufacturing process. The aim of this work was to evaluate the antilisterial activity of a lactic acid bacterium (Lactococcus lactis subsp. lactis PD 6.9) isolated from Italian-style salami in conditions that simulate salami fermentation. L. lactis PD 6.9 produces nisin Z and grows well in pork lean meat broth, a feature that would be useful to compete with food-borne pathogens. The peak of nisin Z production by L. lactis PD 6.9 in pork lean meat broth occurred after 14 h of fermentation, but the inhibitory activity decreased if the producer organism was maintained in stationary phase. When L. lactis PD 6.9 (107CFU ml-1) and Listeria monocytogenes LMA 20 (106CFU ml-1) were co-inoculated in pork lean meat broth, growth of L. lactis PD 6.9 was unaffected. The decrease in viable cell number of Listeria coincided with an increase in bacteriocin activity produced by L. lactis PD 6.9 in pork lean meat broth. Co-culture experiments indicated that L. lactis PD 6.9 was able to control the growth of L. monocytogenes even if the Listeria population was 1000-fold greater than the L. lactis population. These results demonstrate the potential application of L. lactis PD 6.9 in controlling the growth of L. monocytogenes during salami fermentation and its usefulness as a starter culture for fermented sausages. Key words: Listeria monocytogenes, bacteriocins, lactic acid bacteria, co-cultivation, salami.