THE SEARCH FOR OPTIMALLY BIOACTIVE AUSTRALIAN TEA TREE OIL

Antimicrobial activity of tea tree oil was determined using the disc diffusion assay. Terpinen-4-ol, -terpineol and 1,8-cineole were found to have greater mobility through agar than hydrocarbon congeners in agar plate zone-of-inhibition measurements. Minimum inhibitory concentrations were measured using an agar plate dilution method where a range of oil concentrations in agar were tested against S. aureus, E. coli, P. aeruginosa, C. albicans and A. niger.

This method was used to show that concentrations of 1,8-cineole in excess of the upper limit specified in the Australian Standard are not inhibitory. At concentrations up to 30% cineole appears to be synergistic. Antimicrobial activity decreased only when terpinen-4-ol levels fell below 30%, the minimum limit specified in the standard. Preliminary results also indicated that (+) and (-)-terpinen-4-ol are equally bioactive.