Department of Life Sciences, University of East London, Romford Road, London E15 4LZ,UK
Sir,
The antimicrobial properties of essential oils of plant origin have been recognized forcenturies.
1 Carson & Riley
2 demonstrated that the antimicrobial activities of one such
compound, tea tree oil, which isobtained from Melaleuca alternifolia, are attributable to
its hydrocarbon and terpine constituents, including terpinen-4-ol, -terpineol and linalool.
With the increasing prevalence of methicillin-resistant strains of Staphylococcus aureus (MRSA) as pathogens in both hospitals and the community, the eradication of the carrier statehas become an important control measure. Mupirocin has been used widely for this purpose 3 but, recently, there have been reports of clinical isolates of MRSA exhibiting either high- or low-level resistance to it. 4,5 This has prompted a search for alternative agents and this search has extended to natural oils.The present study was undertaken in order to evaluate the in-vitro activities of tea tree oil and mupirocin against 100 recent clinical isolates of MRSA.
Tea tree oil (batch no. 3691) was obtained from Thursday Plantation Laboratories Ltd (Ballina, NSW, Australia); the batch complied with the International Standard (ISO 4730) in that the 1,8-cineole content was 15% and the terpinen-4-ol content was 30%. Mupirocin was provided by SmithKline Beecham Pharmaceuticals Ltd (Harlow, UK). The strains of MRSA were 100 non-replicate clinical isolates collected from laboratories throughout England and Wales. S. aureus NCTC 7447 was used as a control.
The susceptibilities of the strains to tea tree oil and mupirocin were determined by amicrobroth dilution method described previously by Carson et al., 6 except that the medium used was nutrient broth (Oxoid, Basingstoke, UK); the medium was supplemented with 0.1% Tween 80 (Sigma Aldrich Ltd, Poole, UK) when susceptibility to tea tree oil was determined. Doubling dilutions were performed in 96-well microtitre plates (Greiner Laboratories Ltd, Dursley, UK), giving tea tree oil concentrations ranging from 0.039% to2.5% (v/v) and mupirocin concentrations ranging from 0.25 mg/L to 2048 mg/L. An overnight culture of each isolate was adjusted and inoculated into the wells to give suspensions containingc. 2.5 x 10 9 cfu/L. The plates were incubated in air for 24 h at 30°C (chosen because of the volatility of the oils at higher temperatures). MICs were read with a programmable microtitre plate reader (Titertek Multiscan, Flow Laboratories, High Wycombe, UK) at 540 nm. MBCs were determined by withdrawing 5 µL aliquots from wells in which there was no visible growth and inoculating into 100 µL of nutrient broth supplemented with 0.1% Tween 80 in microtitre wells. The plates were incubated at 30°C for 24 h and scanned with the microtitre plate reader. This method of determining MICs and MBCs was chosen because it overcomes the inhibitory carryover effect of tea tree oil which can be a problem when methods involving determining viable counts areused.
The median MIC of tea tree oil for the MRSA isolates was 0.32% (range, 0.160.32%), while the median MBC was 0.64% (range, 0.321.25%); the median MIC fell within the range of previously published values. 6 The median MIC of mupirocin was 16 mg/L (range, 2>2048 mg/L) and the median MBC was 32 mg/L (range, 4>2048 mg/L). According to the definition of Poupard, 5 23% of the MRSA strains were categorized as susceptible to mupirocin, 45% as exhibiting low-level resistance and 32% high-level resistance. The isolates exhibited remarkably uniformsusceptibilities to tea tree oil, whereas the ranges of the MICs and MBCs of mupirocin were much broader. There was no difference between isolates that were susceptible or resistant tomupirocin in terms of their susceptibilities to tea tree oildata which are in accord with those of Carson et al. 6
As the proportion of MRSA isolates that are resistant to mupirocin increases, topical agents, such as tea tree oil, that might be used as alternatives to eradicate MRSA carriage, will assume greater importance. 4 However, as Nelson has already pointed out, 1 the widespread use of tea tree and other essential oils in sub-inhibitory concentrations in cosmetics and other topical formulations could undermine the potential efficacies of these compounds as antiseptic agents.
Notes
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References
1
.
Nelson, R. R. S. (1997). In-vitro activities
of five plant essential oils againstmethicillin-resistant Staphylococcus aureus and
vancomycin-resistant Enterococcus faecium. Journal of Antimicrobial
Chemotherapy 40, 3056.
2 . Carson, C. F. & Riley, T. V. (1995). Antimicrobial activity of the major components ofthe essential oil of Melaleuca alternifolia. Journal of Applied Bacteriology 78, 2649.[ISI][Medline]
3 . Eltringham, I. (1997). Mupirocin resistance and methicillin-resistant Staphylococcus aureus (MRSA). Journal of Hospital Infection 35, 18.[ISI][Medline]
4 . Miller, M. A., Dascal, A., Portnoy, J. & Mendelson, J. (1996). Development of mupirocinresistance among methicillin-resistant Staphylococcus aureus after widespread use of nasal mupirocin ointment. Infection Control and Hospital Epidemiology 17, 8113.[ISI][Medline]
5 . Poupard, J. A. (1995). Update on mupirocin resistance. Journal of Chemotherapy 7, Suppl. 3, 714.[ISI][Medline]
6 . Carson, C. F., Cookson, B. D., Farrelly, H. D. & Riley, T. V. (1995). Susceptibility ofmethicillin-resistant Staphylococcus aureus to the essential oil of Melaleuca alternifolia. Journal of Antimicrobial Chemotherapy 35, 4214.[Abstract]