Mixture design of Origanum compactum, Origanum majorana and Thymus serpyllum essential oils: Optimization of their antibacterial effect

Several studies have revealed the efficiency of essential oils as natural antimicrobial agents in foods. However, limited reports describe the synergistic or antagonistic effects of their combinations. The aim of this study was to investigate the combined antibacterial effect of three Moroccan essential oils (Origanum compactum, Origanum majorana and Thymus serpyllum) and to predict the optimal combination using the mixture design approach coupled to microdilution assay, for the first time to our knowledge. The chemical composition of the oils under study was also explored. The experimental antibacterial activity exhibited by the essential oils mixtures depended on the proportion of each oil in the combination and on the target strain. Moreover, the response surface analysis showed significant synergistic effects in some binary and ternary mixtures. The optimal mixture predicted against Bacillus subtilis and Staphylococcus aureus corresponded to 28%, 30% and 42% of O. compactum, O. majorana and T. serpyllum, respectively. While the optimal mixture predicted against Escherichia coli was composed by O. compactum and O. majorana essential oils at 75% and 25%, respectively. Our findings demonstrated the usefulness of mixture design in the estimation of antibacterial interaction of essential oils. The synergistic effect showed for some combinations may contribute to their successful application as natural preservatives in foods. (C) 2016 Elsevier B.V. All rights reserved.