Comprehensive Review of the Combined Antimicrobial Activity of Essential Oil Mixtures and Synergism with Conventional Antimicrobials

The combined effects of the essential oil (EO) mixtures can significantly enhance their antimicrobial activity by harnessing the synergistic interaction among their diverse bioactive compounds. Additionally, combining EOs with conventional antimicrobial agents can improve or restore the efficacy of these treatments, providing a promising strategy for combating pathogenic and resistant microorganisms. Various methods have been used to evaluate the EO-based combinations, resulting in different types of interactions. This review highlights that EOs from several medicinal and aromatic herbs exhibit synergistic effects when used in mixtures or in combination with antibiotic and antifungal drugs against pathogenic and resistant bacterial and fungal strains. Additive, indifferent and antagonistic effects have also been reported. Recorded synergistic effects suggest the potential therapeutic effects of EOs as natural alternatives or complements to conventional antimicrobial drugs to combat antimicrobial resistance. Despite the demonstrated synergistic effects of many EOs and conventional antimicrobials, there is a notable lack of studies elucidating the mechanisms underlying these synergistic interactions and their specific targets within microbial cells. In this review, the investigations of synergistic studies of EO with conventional antibiotics and the progress in antimicrobial effect of EO mixtures have been reported.