Antibacterial Activity of Different Essential Oils and Their Combinations Against Relevant Enteric Pathogenic Bacteria

Essential oils (EOs) and their components are being increasingly used in animal feeds for their antibacterial properties. However, in-feed concentrations are usually lower than the effective antibacterial concentrations reported by in vitro assays. Therefore, it is important to study the most efficient EOs and synergistic combinations to improve their effectiveness as feed additives. The objective of this study was to test the in vitro antibacterial activity of three EOs (clove, oregano and black pepper) and three isolated EOs components (cinnamaldehyde, thymol and carvacrol) both individually and in combinations against a collection of reference and field strains of E. coli, Salmonella enterica ssp. enterica and Cl. perfringens. The Minimum Inhibitory Concentrations (MIC) were studied per triplicate using a broth micro-dilution method at pH 6. Firstly, MICs of the 6 individual products were determined while in a second step, 11 combinations of the individual products were evaluated. The results indicated that all the individual products, except black pepper, showed antimicrobial activity against all gram positive and gram negative tested bacterial strains. MICs that inhibit 50 % of the isolates (MIC₅₀) ranged from 300 to 600 mg/L for E. coli, 300 to 600 mg/L for Salmonella and 300 to 1200 mg/L for Cl. perfringens. Carvacrol showed the best antibacterial activity, with a MIC₅₀ of 300 mg/L against all three bacteria, followed by cinnamaldehyde, with a MIC₅₀ of 600 mg/L against E. coli and Salmonella and 300 mg/L against Cl. perfringens. Most of the combinations showed an increased antibacterial activity compared to their individual components, with a MIC₅₀ range from 150 to 300 mg/L for all tested strains of the three bacterial species. The combination of cinnamaldehyde, thymol, carvacrol and clove EO showed the best antibacterial results, with a MIC₅₀ of 150 mg/L against all three bacteria, followed by the combination of cinnamaldehyde and carvacrol, with a MIC₅₀ of 150 mg/L for E. coli and Cl. perfringens and 300 mg/L for Salmonella. The results of this study confirm that the antibacterial activity of some EOs and their components can be increased by their use in combination. Further studies are needed to better understand these interactions as well as in vivo studies to show their effects on microbial populations.