Penfold and Grant (1925) first demonstrated the activity of tea tree oil in the RidealWalker test, a standard test of the period, which employed Bacillus typhosus (now known as Salmonella typhi) as the test organism. The Rideal-Walker coefficient was reported to be 11, indicating that tea tree oil is 11 times more effective than phenol. The results in Table 1 show that tea tree oil also compares very favourably with a number of other essential oils when tested by this method.
Very little further work was done until the 1970s when Low et al. (1974) reported MIC values of 1:16 against S. aureus and 1:32 against Salmonella typhi, and Beylier (1979)
Table 1 Antimicrobial acitivity of some essential oils (Rideal-Walker coefficient data adapted from Penfold and Grant (1925) and Schilcher (1985))
Source of oil
Melaleuca alternifolia Melaleuca linariifolia
liuccilyptus dives 'Type'
Eucalyptus citriodora BackJxusia citriodora Lavender
Clove Thyme included tea tree oil in a study of the antimicrobial activity of ten essential oils derived from Australian native plants. Tea tree oil was effective against the five test organisms, which included both Gram-positive and Gram-negative bacteria, a yeast and a mould. The MIC value against S. aureus (0.25-0.2%) was considerably lower than that reported by Low et al. (1974).
Both tea tree oil and formulations containing tea tree oil at concentrations of 5% have been reported to pass the Therapeutic Goods Act (TGA) test for antiseptics and disinfectants (Graham 1978), which includes type strains of the Gram-positive bacterium Staphylococcus aureus and the Gram-negative Escherichia coli, Proteus vulgaris and Pseudomonas aeruginosa as test organisms (Altman 1989). The oil has also been reported to pass both USP and BP preservative efficiency tests in semi-solid formulations at concentrations of approximately 0.5-1% (Altman 1991), although higher concentrations may be needed to satisfy the requirements of the test in relation to Aspergillus niger (this volume, Chapter
The very broad spectrum of activity of tea tree oil, a highly desirable characteristic in an antiseptic or disinfectant, has been confirmed by a number of studies published during the last decade. Table 2 identifies the organisms which have been tested by various authors and presents MIC values for a large number of organisms from a single study (Griffin et al. 1998). These results suggest the enormous potential of tea tree oil in a variety of applications, including the treatment of external conditions such as acne, tinea, thrush and staphylococcal and streptococcal infections, in oral hygiene products, in the disinfection of cooling towers contaminated with Legionella, and in agricultural uses. Not only has the sensitivity of many species of bacteria and fungi to tea tree oil been demonstrated, but some studies have examined the susceptibility of large numbers of recent clinical isolates, as well as type strains, of particular species (see Table 2 for references). Such data provides valuable information about the variability in sensitivity of organisms likely to be encountered in therapeutic use of the oil.
The majority of studies report MIC values and a comparison of results for commonly tested organisms from different studies is presented in Table 3. Although there is variation in MIC values against different strains within a species and between studies, the majority of values are less than 1%. Lack of consistency of results may be accounted for by differences in oil composition, in test organisms and methods of determining MIC values. For example,
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