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J Med Microbiol 55 (2006), 1375-1380; DOI: 10.1099/jmm.0.46558-0
© 2006 Society for General Microbiology
ISSN 1473-5644

In vitro activity of tea-tree oil against clinical skin isolates of meticillin-resistant and -sensitive Staphylococcus aureus and coagulase-negative staphylococci growing planktonically and as biofilms

Aaron Brady1, Ryan Loughlin1, Deirdre Gilpin1, Paddy Kearney2 and Michael Tunney1

1 Clinical and Practice Research Group, School of Pharmacy, Queen's University Belfast, Medical Biology Centre, 97 Lisburn Road, Belfast BT9 7BL, UK

2 Microbiology Department, United Hospitals Trust, Antrim, BT41, UK

Correspondence
Michael Tunney
m.tunney{at}qub.ac.uk

Received 8 February 2006
Accepted 19 June 2006

The susceptibility of Staphylococcus aureus [meticillin-resistant (MRSA) and meticillin-sensitive (MSSA)] and coagulase-negative staphylococci (CoNS), which respectively form part of the transient and commensal skin flora, to tea-tree oil (TTO) was compared using broth microdilution and quantitative in vitro time–kill test methods. MRSA and MSSA isolates were significantly less susceptible than CoNS isolates, as measured by both MIC and minimum bactericidal concentration. A significant decrease in the mean viable count of all isolates in comparison with the control was seen at each time interval in time–kill assays. However, the only significant difference in the overall mean log10 reduction in viable count between the groups of isolates was between CoNS and MSSA at 3 h, with CoNS isolates demonstrating a significantly lower mean reduction. To provide a better simulation of in vivo conditions on the skin, where bacteria are reported to grow as microcolonies encased in glycocalyx, the bactericidal activity of TTO against isolates grown as biofilms was also compared. Biofilms formed by MSSA and MRSA isolates were completely eradicated following exposure to 5 % TTO for 1 h. In contrast, of the biofilms formed by the nine CoNS isolates tested, only five were completely killed, although a reduction in viable count was apparent for the other four isolates. These results suggest that TTO exerts a greater bactericidal activity against biofilm-grown MRSA and MSSA isolates than against some biofilm-grown CoNS isolates.

Abbreviations: CoNS, coagulase-negative staphylococci; ISB, IsoSensitest broth; MBC, minimum bactericidal concentration; MRSA, meticillin-resistant Staphylococcus aureus; MSSA, meticillin-sensitive Staphylococcus aureus; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide; TTO, tea-tree oil.






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