Abstract

The inhibitory effect of Lactococcus lactis WNC20 (Ll20), a nisin-producing lactic acid bacteria, on food-borne pathogens was investigated by adding Ll20 at ~7 log CFU mL^-1and or one of four different pathogenic bacteria at ~2 to 4 log CFU mL^-1 to the onset of yoghurt and Nham ferments. Enumeration and identification of the bacteria in the yoghurt and Nham were then monitored during the complete fermentation period. When initially seeded at either ~2- or ~3 log CFU mL^-1, Staphylococcus aureus and Listeria monocytogenes were not detected after 48 h when co-cultured with Ll20, and Escherichia coli and Salmonella typhimurium at the same initial seeding levels were not detectable after 72 h when co-cultured with Ll20. When S. aureus, L. monocytogenes and S. typhimurium were seeded at ~4 log CFU g^-1 in Nham fermentation, their growth inhibition by Ll20 was also demonstrated. In addition, nisin Z like antibacterial activity and the specific nis Z gene were always detected in yoghurt or Nham extracts during their fermentation when cocultured with Ll20 but not in its absence. Acid and nisin Z production by Ll20 are thus likely to be major factors in the inhibitory effect on pathogenic bacteria in these foods.

Key words: Lactococcus lactis WNC20, Nisin, foodborne pathogens, Nham, yoghurt.