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) o f 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 a s a dispersant, which provides a reliable and reproducible technique. The m ethod was tested using two Melaleuca alternifolia oil samples with two diff erent 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.