Death kinetics of Staphylococcus aureus exposed to commercial tea tree oils s.l.

Staphylococcus aureus cells were exposed to increasing concentrations of Australian tea tree oil, cajuput oil, niaouli oil, Lema oil, kanuka oil and manuka oil as well as of a beta -triketone complex isolated from manuka oil. The death kinetics were determined by calculation of log(10) reduction factors after increasing exposure periods. Niaouli oil turned out to be highly active, followed by Lema (this is a registered trademark), tea tree and cajuput oils. Kill rate data indicated that 1.0% (v/v) were lethal to stationary phase cells in the assay conditions used. At 2.0% (v/v), niaouli oil and Lema oil yielded a complete 6.8 log(10) reduction of cell numbers in suspensions within 60 min, whereas cells treated with tea tree and cajuput oils were inactivated more slowly within 120 min and 240 min, respectively. Kanuka oil and manuka oil as well as the beta -triketone complex, the active principle of manuka oil, lacked any bactericidal properties. Their high effectiveness against Gram-positive bacteria can be explained by bacteriostatic effects. The results obtained with Lema oil, a blend of tea tree oil and a polar fraction of manuka oil (mainly beta -triketones), gave cause to discuss synergistic effects.