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 generally lower than the effective antibacterial concentrations reported by in vitro assays. Therefore, it is important to study the most efficient EOs and combination synergies to improve their efficacy to be used as feed additives. The objective of this study (ASAS 2017) was to test the in vitro antibacterial activity alone and in combination of three EOs (clove, oregano and black pepper) and three isolated EOs components (cinnamaldehyde, thymol and carvacrol) with respect to strains of 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 separately and, subsequently, 11 combinations of the individual products were evaluated. The results indicated that all the individual products, with the exception of black pepper, showed antimicrobial activity against all gram positive and gram negative tested bacterial strains. Carvacrol had the highest antibacterial activity, with a MIC50 of 300 ppm against all three bacteria, followed by cinnamaldehyde, with a MIC50 of 600 ppm against E. coli and Salmonella and 300 ppm against Cl. perfringens.  Most of the combinations showed a remarkable increased antibacterial activity compared to their components alone for all tested strains of the three-bacterial species. The combination of cinnamaldehyde, thymol, carvacrol and clove EO had the best antibacterial results, with a MIC50 of 150 ppm against all three bacteria, followed by the combination of cinnamaldehyde and carvacrol. The results of this study confirm that the antibacterial activity of some EOs and their components can be increased by their combined use. Although, further studies are needed to better understand these interactions as well as their effects in vivo.