A NEW METHOD FOR DETERMINING THE MINIMUM INHIBITORY CONCENTRATION OF ESSENTIAL OILS

A new microdilution method has been developed for determining the mini mum inhibitory concentration (MIC) of oil-based compounds. The redox d ye resazurin was used to determine the MIC of a sample of the essentia l oil of Melaleuca alternifolia (tea tree) for a range of Gram-positiv e and -negative bacteria. Use of 0.15% (w/v) agar as a stabilizer over came the problem of adequate contact between the oil and the test bact eria and obviated the need to employ a chemical emulsifier. A rapid ve rsion of the assay was also developed for use as a screening method. A comparison of visual and photometric reading of the microtitre plates showed that results could be assessed without instrumentation; moreov er, if the rapid assay format was used, rigorous asepsis was not neces sary. Accuracy of the resazurin method was confirmed by plate counting from microwells and MIC values were compared with results obtained us ing an agar dilution assay. The MIC results obtained by the resazurin method were slightly lower than those obtained by agar dilution.