Effect of Lactococcus lactis on the survival of Listeria monocytogenes and Staphylococcus aureus in yoghurt

This study evaluated Lactococcus lactis strains for their acidifying and antibacterial properties, with particular focus on their effect on artificially inoculated foodborne pathogens in milk and yoghurt matrices. Antibacterial activity was first assessed using agar spot and overlay methods against Listeria monocytogenes ATCC 7644 and Staphylococcus aureus ATCC 25923. Acidifying ability was monitored based on pH reduction at 6 and 24 hours. Strain LL16 demonstrated optimal fermentation activity, reaching a pH drop of 2.26 ± 0.01 with desirable aroma and gel-like consistency after 24 hours. LL29 showed the strongest inhibition against S. aureus (2.35 ± 0.19 cm zone), while LL16, LL29, and LL14C inhibited L. monocytogenes (2.15 ± 0.09 cm zones). For the challenge test, UAT cow’s milk was used to prepare yoghurt with a commercial starter and 1% LL29 and then artificially contaminated with 5 µl of L. monocytogenes and S. aureus. Samples were stored at 4 °C, and counts were monitored on days 1, 7, 14, and 21. This experiment showed complete inhibition of both pathogens throughout 21-day storage at 4°C. These findings support the application of LL16 and LL29 strains for safe dairy fermentation processes.