Optimizing the Antimicrobial Synergism of Melaleuca alternifolia (Tea Tree) Essential Oil Combinations for Application against Respiratory Related Pathogens

Abstract Antimicrobial research into the use of Melaleuca alternifolia essential oil has demonstrated broad-spectrum activity; however, much of the research published focuses on identifying the potential of this essential oil individually, rather than in combination for an enhanced antimicrobial effect. This study aimed to determine the antimicrobial activity of four essential oil combinations, all inclusive of M. alternifolia, against nine pathogens associated with the respiratory tract. The minimum inhibitory concentration assay was used to determine the antimicrobial activity of four essential oil combinations, M. alternifolia in combination with Cupressus sempervirens, Origanum majorana, Myrtus communis, and Origanum vulgare essential oils. The interactions between essential oil combinations were analyzed using isobolograms and SynergyFinder 2.0 software to visualize the synergistic potential at varied ratios. The antimicrobial activity of the different combinations of essential oils all demonstrated the ability to produce an enhanced antimicrobial effect compared to the essential oils when investigated independently. The findings of this study determined that isobolograms provide a more in-depth analysis of an essential oil combination interaction; however, the value of that interaction should be further quantified using computational modelling such as SynergyFinder. This study further supports the need for more studies where varied ratios of essential oils are investigated for antimicrobial potential.