Effectiveness of Some Natural Compounds Against Antibiotic-Resistant Listeria spp. and Salmonella enterica Strains

The increasing prevalence of antibiotic resistance highlights the need to seek alternatives to currently used compounds. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of nine ethanolic extracts of propolis (EEPs) from northwestern Spain and five phenolic compounds (carvacrol, eugenol, thymol, resveratrol, and trans-cinnamaldehyde) against Listeria spp. and Salmonella enterica strains were determined. All EEPs showed a strong antimicrobial effect against Listeria spp., with MICs (ppm) ranging between 78.00 ± 0.00 and 1041.67 ± 360.84 and MBCs (ppm) ranging between 312.50 ± 0.00 and 3333.33 ± 1443.38. The concentration of caffeic acid phenethyl ester (CAPE), isoprenyl caffeate (IPC), or total polyphenols did not influence the antimicrobial activity of EEPs. However, EEPs had only slight antimicrobial activity against S. enterica, with MICs ≥ 10,000 ppm and MBCs exceeding 10,000 ppm. MIC values (ppm) for phenolic compounds with respect to Listeria spp. strains ranged from 125.00 ± 0.00 to 2500.00 ± 0.00, and those for MBCs (ppm) ranged from 250.00 ± 0.00 to 5000.00 ± 0.00. For S. enterica, MICs (ppm) ranged from 250.00 ± 0.00 to 2500.00 ± 0.00 and MBCs (ppm) ranged from 416.67 ± 144.34 to 5000.00 ± 0.00. The results suggest that EEPs and phenolic compounds may offer natural alternatives to other antimicrobials in common use, with phenolic compounds being especially promising because of their antimicrobial activity against both Gram-positive and Gram-negative species.