Strain-Specific Antimicrobial Mechanisms of Lactic Acid Bacteria Against Listeria monocytogenes Scott A Under Nutrient-Rich and Carbohydrate-Limited Conditions

Listeria monocytogenes remains a persistent food-safety challenge, sustaining interest in biopreservation with lactic acid bacteria (LAB). The mechanisms underlying LAB-mediated suppression, and their dependence on nutrient availability, remain incompletely resolved. Nine LAB isolates (four Lactiplantibacillus plantarum, two Lactiplantibacillus pentosus, three Lactococcus lactis subsp. lactis) were evaluated against L. monocytogenes Scott A using overlay agar-spot assays, co-culture kinetics at three effector-to-target ratios (1:1, 100:1, 10,000:1), co-culture under glucose limitation, and targeted chemical and enzymatic treatment of cell-free supernatants (CFSs). Competitive exclusion was dose-dependent: at equal inocula, most isolates permitted growth comparable to the monoculture control, whereas at 105 and 107 CFU/mL, pathogen growth was arrested within 6–8 h. Multivariate analysis of kinetic parameters at 105 CFU/mL resolved three efficacy clusters, and efficacy tracked the onset of acidification rather than LAB growth rate: strains that lowered pH within the first 6 h achieved 31–41% inhibition, while those with an 8–10 h delay achieved <20%. Lactiplantibacillus plantarum DSM 20205 reached the same acidification onset as the faster-growing Lc. lactis strains despite a lower μmax, indicating higher acidifying efficiency per unit biomass. Glucose removal reduced inhibition to <6% for all Lactiplantibacillus isolates, whereas Lc. lactis BL1 and BL8 retained approximately 16%. CFS assays indicated that organic acids provide the baseline activity in nutrient-rich medium, that catalase-sensitive activity appeared in co-culture but not monoculture supernatants, and that protease-sensitive activity was present in all nine isolates under carbohydrate limitation. These results support selecting bioprotective cultures on rapid acidification combined with acid-independent mechanisms rather than on acidification alone.