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Despite the many therapeutic claims made forthis essential oil, limited scientific evidence isavailable to substantiate these claims. Althoughrecent studies have evaluated the in vivo efficacy oftea tree oil products in the treatment of acne7 andonychomycosis,’ little is known concerning the invitro antimicrobial activity of tea tree oil againstthe pathogens involved in these and other conditions.This is due in part to difficulties arising from thenature of tea tree oil itself. Melaleuca oil is notsoluble in water and therefore cannot be tested bystandard antimicrobial susceptibility test methods,which are conducted in aqueous media. Inaddition, once solubilized, melaleuca oil forms aturbid suspension that precludes the visual determinationof MICs.There was wide variation in the susceptibility ofthe staphylococci to tea tree oil, with MBCs for thecoagulase-negative staphylococci (CNS) beinghigher than those for S. uuyeus. Lower concentrationsof tea tree oil were required for bactericidalactivity against the gram-negative organismsA. baumannii, Klebsiella pneumoniae, andS. munzescens. In previous studies we obtainedsimilar MIC,, and MBC,, results for E. coli9 anddemonstrated that methicillin-resistant S. aureushas an MIC,, of 0.25% and MBC,, of 0.5%.‘O Theresults suggest that S. aureus and most of thegram-negative bacteria tested are more susceptibleto tea tree oil than CNS and micrococci. Thismay afford tea tree oil a use in removing transientskin flora while suppressing but maintaining resi- AJICVolume 24, Number 3dent flora as a protective measure against colonizationby multiresistant pathogenic bacteria.The MIC and MBC values obtained in our studymay not be comparable to those of other studiesbecause of methodological differences. Previouslypublished MICs for S. uuretls, determined by agaror broth dilution methods, included 0.5%” and0.08%.12 MBCs were not determined. Altman’reported MICs in the range 0.5% to 1.0% for arange of bacteria, including E. coli, S. aUreM.s, andP. aemginosa. Despite possible methodologicaldifferences, these results are all similar to thoseobtained in our study, with the exception of theMIC of 0.08% for S. uz4reu.s” and Altman’s’ MIC of1% for P. aemginosa. Variation between studyresults also highlights the value of includingreference strains in any chosen method, both as aninternal control and for the purposes of externalcomparison.The hands of staff are one of the main modes oftransmission of hospital infection, and handwashingremains the principle method for reducingcross-infection in hospital wards. Consequently,the refinement of existing handwashing productsand the development of novel products continues.The natural product renaissance of the last decadehas seen a plethora of claims regarding theusefulness of many essential oils, including teatree oil, in numerous products. Given the antimicrobialactivity of tea tree oil seen in this study,particularly against transient bacterial pathogens,potential exists for tea tree oil to be used inhygienic hand disinfection. In addition, the lipophilicnature of tea tree oil means that it lends itselfreadily to incorporation in surfactant preparations.Tea tree oil is already incorporated into anumber of moisturizing products, and this propertymay constitute another benefit of this naturalproduct. The ability of tea tree oil to penetrate theouter layers of skin may enhance its antimicrobialactivity against transient flora by means of aresidual effect. Finally, anecdotal reports suggestthat constant handwashing with products containingtea tree oil does not lead to the dermatologicproblems associated with some hand care preparations.l3
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