Synergistic Antimicrobial Effects of Melaleuca alternifolia Essential Oil and Kojic Acid Combinations.

Abstract

Melaleuca alternifolia (tea tree) essential oil (TTO) is widely recognized for its anti-inflammatory and antimicrobial properties, attributed primarily to its major constituent, terpinen-4-ol. Kojic acid, a well-established compound used mainly in cosmetic formulations, exhibits potent antioxidant and broad-spectrum antimicrobial activities. In this study, the antimicrobial potential of commercial Pharmacopoeia-grade TTO was evaluated with kojic acid and terpinen-4-ol combinations. The quality of TTO was confirmed through GC-FID and GC/MS analyses, where terpinen-4-ol was the major component (44.4%). In vitro antimicrobial assays were performed against human pathogens including Candida utilis, Candida albicans, Candida krusei, Candida tropicalis, Candida parapsilosis, Escherichia coli, and Salmonella Typhimurium. Notably, all tested samples demonstrated activity against S. Typhimurium. Kojic acid combined with TTO showed a synergistic effect, with a fractional inhibitory concentration index (FICI) of 0.266, while the combination of kojic acid and terpinen-4-ol showed an even stronger synergistic effect (FICI: 0.039). The combination of TTO and terpinen-4-ol produced an additive effect (FICI: 0.508). To the best of our knowledge, this study is the first to demonstrate the combined effectiveness of TTO and kojic acid against S. Typhimurium. The synergistic activity observed with the combination of terpinen-4-ol and kojic acid highlights its potential as an antimicrobial for a therapeutic strategy, after safety evaluations.