An agar dilution method for the determination of the minimum inhibitory concentration of essential oils

Methods for the measurement of the minimum inhibitory concentration (MIC) of antibacterial agents have been developed for water-soluble substances and require adaptation for use with water-insoluble essential oils. This paper reports a standardized agar dilution MIC method, using 0.5% v/v Tween 20 as a dispersant, which provides a reliable and reproducible technique. The method was tested using two Melaleuca alternifolia oil samples with two different levels of terpinen-4-ol (37% and 45% v/v). The MIC values of the tea tree oil samples against a wide selection of bacteria, moulds and yeast are reported.