Abstract
Background: Live probiotics face safety and stability limitations, increasing interest in postbiotics (non-viable cell constituents and their metabolic products) as a more stable option for functional foods. Lactobacillus plantarum is a common source of antimicrobial agents, mainly bacteriocins and organic acids. However, its dose-dependent inhibitory effects against major foodborne pathogens are not well characterized, restricting assessment of its preservation potential.
Objective: This study assessed the antimicrobial and antioxidant activity of concentrated cell-free supernatant (CFS) postbiotics from L. plantarum against four foodborne pathogens: Listeria monocytogenes, Staphylococcus aureus, Escherichia coli, and Salmonella Typhimurium.
Methods: CFS postbiotics were centrifuged, filtered (0.22 µm), and freeze-dried to obtain lyophilized postbiotic (LPB). Six treatments were examined: MRS (negative control), enrofloxacin (positive control), non-lyophilized CFS postbiotics (NLPB), and LPB at 20%, 40%, and 60% (v/v). Antimicrobial efficacy was evaluated by agar well and disc diffusion, measuring inhibition zone diameter (mm) after 24 h. MIC was defined as the lowest concentration producing no visible turbidity, indicating complete growth inhibition. DPPH radical scavenging activity was measured.
Results: LPB60% displayed the strongest activity. Well diffusion produced inhibition zones of 32.33 ± 4.16 mm (L. monocytogenes), 33.00 ± 3.46 mm (S. aureus), 13.00 ± 2.00 mm (E. coli), and 20.33 ± 2.31 mm (S. Typhimurium). Disc diffusion produced smaller zones overall, consistent with the more limited diffusion inherent to the disc method. The MIC/MBC of LPB60% was the lowest concentration tested against all four pathogens. Gram-positive bacteria were significantly more susceptible than Gram-negative strains throughout. DPPH radical scavenging activity was measured to assess antioxidant capacity, and the scavenging rate reached 81.67% at LPB60%.
Conclusion: Concentrated L. plantarum postbiotics, particularly LPB60%, show concentration-dependent antimicrobial and antioxidant activity, supporting their potential as a natural preservative against Gram-positive pathogens. Weaker activity against Gram-negative bacteria and the purely in vitro nature of this study mean further validation in real food matrices is still required.