Synergistic Activity of Natural Antimicrobials Against Common Fish Spoilage and Pathogenic Bacteria

This study examined the antimicrobial activity (AA) of a suite of natural antimicrobials (NAM): low molecular weight chitosan (CS), essential oils (EOs; thyme, oregano, garlic, sage, laurel, marjoram and rosemary) and commercially available mixtures of weak organic acids (Articoat (ART) and Sulac (SUL)), tested individually and in combinations against common fish spoilage (Brochothrix thermosphacta (B. thermosphacta), Lactic acid bacteria (LAB) and Pseudomonas fluorescens (P. fluorescens)), pathogenic (Staphylococcus aureus (S. aureus), Listeria innocua (L. innocua) and Escherichia coli (E. coli)) bacteria and raw salmon microbiota. Individually, CS showed the strongest AA across all bacteria, with a minimum inhibitory concentration (MIC) of 0.125 mg/mL, outperforming all tested EOs, ART, and SUL. Among the 40 combinations tested (ART-EOs, CS-EOs, SUL-EOs and EO-EO), CS-garlic EO (GEO) had the strongest synergistic effect, significantly (p < 0.05) reducing CS and GEO MICs by 16- and 8-fold, respectively, against S. aureus, L. innocua, B. thermosphacta and LAB when compared to the individual MICs; a comparable synergistic effect was noticed for SUL combined with GEO or sage EO, significantly lowering (p < 0.05) SUL and EO MICs by 17- and 8-fold, respectively against the same bacteria. Oregano (OEO)-thyme (TEO), ART-OEO and ART-TEO demonstrated the strongest synergistic effect, reducing required concentrations (p < 0.05) by up to 8-fold, against E. coli, P. fluorescens and salmon microbiota compared to their individual MICs. Overall, synergistic NAM combinations enhanced AA while substantially reducing antimicrobial concentrations, highlighting their potential for application in foods and packaging to extend shelf-life, support clean-label preservation, reduce waste and enhance sustainability.