- Email: [email protected]
Efficacy and safety of tea tree oil as a topical antimicrobial agent
M
Journalof HospitalInfection( 1998) 40: I 75-I 78
LEADING
ARTICLE
Efficacy and safety of tea tree oil as a topical antimicrobial agent C. F. Carson, T. V. Riley* and B. D. Cooksont Department of Microbiology, The University of Western Australia, Queen Elizabeth II Medical Centre, Nedlands, WA 6009, Australia; *Division of Microbiology and Infectious Diseases, Western Australian Centre for Pathology and Medical Research, Queen Elizabeth II Medical Centre, Nedlands, WA 6009, Australia; and +!.aboratory of Hospital Infection, PHLS Central Public Health Laboratory, 6 I Colindale Avenue, London NW9 5HT, UK
Keyzumls:
Tea tree oil; AIelaleuca
alternifolia;
topical antimicrobial
Introduction The Australian native plant Melaleuca alternifolia has been used medicinally by Aborigines for several millennia. European colonists soon recognized the therapeutic properties of the plant and furthered its applications by distilling oil from the leaves. The oil, known colloquially as tea tree oil, has gained considerable popularity as a topical antimicrobial agent in recent years, initially on the basis of anecdotal evidence.’ A recent survey on the use of alternative and complementary medicines in Australia reported tea tree oil as the most frequently used.’ Tea tree oil is reputed to have selreral medicinal properties including antibacterial, antifungal, antiviral, anti-inflammatory and analgesic properties. However, it is the antibacterial activity which has attracted the most attention and there are noxv considerable data available concerning the in-vitro activity.‘-’ The colonization of healthcare workers and Correspondence to: C. 1:. Carson, Department of Rlicrobiology, The University of lestern Australia, Queen Elizabeth II LIedial Centre, Nedlands, \%‘A 6009, Australia. Email: [email protected]. Tel: 61 8
9346 3288. Fax: 61 8 9382 8046.
0 195-670
I /98/ I IO I75 + 04 $ i2.00/0
agent
patients with methicillin-resistant Staphylococcus aweus (MRSA) poses significant challenges to infection control and antibiotic prescribing practices.” The emergence of ‘mupirocin-resistant MRSA” confounds the situation further. As MRSA, including strains resistant to mupirocin have been recently shown to be susceptible7 to tea tree oil, tea tree oil is being considered as a potential MRSA decolonization agent. In addition to the need for appropriate clinical data on the efficacy of tea data on safety are also retree oil products, quired. While some work has been conducted for industry purposes, little has been published. In addition, there is some confusion about the concentrations and amount of oil that may elicit an adverse reaction, either topically or systemically.
Production
and quality
assurance
Historically, tea tree oil was produced by manual harvesting of natural bush stands of M. alternifolia. Today, most tea tree oil is produced on large-scale, mechanized plantations. Quality assurance practices, which contribute to the
0 1998 The Hospital
Infection
Society
C. F. Carson
176
provision of a consistent product, have evolved also. Although a number of national and international standards for tea tree oil have been developed and implemented over the years, these have been largely superceded by the most recent international standard for tea tree oil, IS0 4730.8 This standard is the most comprehensive to date and defines the physical and chemical parameters which tea tree oil samples must fulfil. Tea tree oil contains approximately 100 components with less than 10 components, including terpinen-4-01, 1,8-cineole, a-terpineol, terpinolene and a- and y-terpinene, comprising up to 90% of the whole oil.” This standard provides ranges within which 14 components must fall, including several of the major antimicrobial components such as terpinen-401 and a-terpineol.“’
Antimicrobial
activity
Early work examining the antimicrobial activity of tea tree oil provided largely qualitative information about the broad spectrum of organisms susceptible to the oil, including Grampositive and Gram-negative bacteria, acid-fast bacilli, yeasts and fungi.’ More recently, and quantitative discriminating, with more methods, the antimicrobial activity has been characterized in detail. Minimum inhibitory concentrations (MICs) have been determined for many organisms including coagulase-negative staphylococci (0.06-3% v/v), StuphyZococcus aweus (including MRSA) (0.12-O*S%), Streptococcus spp. (0.03-0.12%), vancomycinresistant enterococci (0.5-l%), Acinetobacter baumannii (0.06-l %), Escherichia coli (0.120.25%), Klebsiella pneumoniae (0.12-0.50/o), Candida albicans (0.12-0.25%), other Candida spp. (0.12-0.5%) and Mulassezia furfur (0.12().250/0).3-“,W2 The wide range of organisms susceptible to tea tree oil suggests that this agent may be useful for skin antisepsis. Furthermore, many organisms that colonize skin transiently have been shown to be more susceptible to tea tree oil than commensal organisms.” This differential susceptibility may provide further impetus for the application of tea tree oil in
et al.
situations where eradication of commensal flora is not desirable, particularly where maintenance of the commensal flora supports the continuity of a barrier against colonization by transient flora. This would include settings in wound management, MRSA decolonization and hygienic hand disinfection.
Toxicity Much work has been performed in animals but the generalizability of these data is unclear.
Cutaneous
toxicity
The acute dermal LD,, in rabbits was recorded as in excess of 5.0 g/kg since this dose resulted in 2/10 deaths in rabbits.13 Furthermore, it was observed at necropsy that neat tea tree oil produced irritant effects and skin abnormalities in rabbits patch tested at this dose for 24 h with occlusion. Although some irritation was observed, undiluted tea tree oil did not produce phototoxic effects on the skin of hairless mice.13 Clinical reports of adverse cutaneous events involving tea tree oil and humans” have consisted mainly of individual case reports of cutaneous sensitivity to tea tree oi1.“-‘8 While these reports have served to alert users and the medical professional to the existence of tea tree oil sensitivity, they have not provided data on either incidence rates or the basis of sensitivity. Although some experimental work has been conducted, this has generally been with a small number of subjects. A 48-h occlusive patch test with 1% tea tree oil on 22 volunteers produced no irritation and a maximization test also produced no sensitization reactions.13 In 13 subjects patch-tested with 5% tea tree oil, no adverse reactions were elicited.‘” More recently, 50 participants were patch-tested with 1 and 5% tea tree oil and no adverse reactions were seen.” In another study, repeated challenge over 21 days with 25% tea tree,oil did not result in any sensitivity in 25 of 28 subjects. However, the remaining three subjects were removed from the trial ‘because of a severe allergic (as distinct from irritant) response to tea tree oi1’.2o The influence
177
Efficacy and safety of tea tree oil
of compromised skin on the incidence and severity of sensitivity reactions is unknown also. Tea tree oil is a complex mixture of terpenes and related alcohols.” Some components responsible for adverse reactions to tea tree oil have been identified, including 1 ,S-cineole,” Da-terpinene, aromadendrene, terlimonene, cx-phellandrene, p-cymene, cx-pipinen-4-01, nene, terpinolene22 and sl-terpinene.20 For many years, 1,8-cineole was regarded as an undesirable constituent in tea tree oil due to its reputation as a skin and mucous membrane irritant. However, the latter two studies2’s2’ suggest that this component is not responsible for a large proportion of sensitivity reactions. There are no data documenting the safety of tea tree oil for application to granulation tissue. This is an important area as many patients are colonized with YIRSA in wounds, varicose ulcers and pressure sores.23’2’
Systemic
toxicity
hlost of the work on the systemic toxicity of tea tree oil in animals has been performed for commercial purposes and has not been published in the public domain. The acute LD5,, in rats has been reported as 1.9 g/kg (1.4-2.7 g/ kg),‘j similar to the acute oral LD,,, of eucalyptus oil (2.5 g/kg).” Several cases of human tea tree oil poisoning have been reported, mostly involving the ingestion of modest volumes (N lo-25 mL) of oil. In two cases, ingestion of tea tree oil resulted in what appeared to be systemic contact dermatitis.2’.‘” Seawright” reported that a patient was comatose for 12 h and then semi-conscious for 36 h after ingestion of approximately half a cup of tea tree oil. In separate cases, two children who ingested less than 10 mL tea tree oil became ataxic and drowsy or disorientated.28x2” Both were treated supportively and recovered fully without further complications.
Discussion If the broad range of properties attributed to tea tree oil is corroborated, in particular the antimicrobial activity in ciao, it seems likely that
the oil will find application in a diverse range of conditions. Already, tea tree oil has been ‘grandfathered’ into several markets largely on the basis of its ‘long history of use’. To be considered for therapeutic use in humans, further information regarding the safety of its application must be sought. Since many of the potential applications of tea tree oil involve mucous membranes, such as nasal, oral or vaginal mucosa, this work must include mucosal as well as skin toxicity. In the first instance, the incidence of sensitivity to tea tree oil must be determined. Hopefully, the components or fractions responsible for sensitivity will be identified also and this may assist in the development of oils with a lower propensity to cause sensitivity. Elimination of allergenic components may be achieved by manipulating the distillation process, rectification or by the genetic selection of plants which contain lower quantities of these components. Due to its systemic toxicity, tea tree oil should only be used as a topical agent. Most product manufacturers, distributors and retailers support this qualification, although a few continue to advocate systemic use. In the absence of data supporting its safety in this context and in the presence of clinical experience clearly indicating its toxicity, any recommendations of systemic use appear injudicious. Furthermore, while current interest seems centred around the safety of acute exposure, whether topical or systemic, the question of chronic exposure remains neglected. Tea tree oil contains several compounds known to penetrate skin and to enhance the penetration of other compounds.“” While this may augment the action of its medicinal properties, it may also potentiate the opportunities for toxicity. Tea tree oil has exhibited some promising antimicrobial properties in z.itro and it remains for these properties to be evaluated suitably in ciao. Before it can be introduced widely for controlled, pretherapeutic use, randomized, ferably double-blind clinical trials must be performed and comprehensive safety data obtained.
References 1. Carson CF, Riley TV. Antimicrobial activity of the essential oil of Melaleuca altevnifolia. Lett Appl hficrobiol 1993; 16: 49-55.
C. F. Carson et al.
178
2. Kristoffersen SS, Atkin PA, Shenfield GM. Uptake of alternative medicine. Lancet 1996; 347: 972. 3. Carson CF, Hammer KA, Riley TV. Broth micro-dilution method for determining the susceptibility of Escherichia coli and Staphylococcus aweus to the essential oil of Melaleuca alternifolia (tea tree oil). Microbios 1995; 82: 181-185. 4. Carson CF, Hammer KA, Riley TV In-vitro activity of the essential oil of Melaleuca alternifolia against Streptococcus spp. J Antimicrob Chemother 1996; 37: 1177-1178. 5. Hammer KA, Carson CF, Riley TV. Susceptibility of transient and commensal skin flora to the essential oil of Melaleuca alternifolia (tea tree oil). Am 7 Infect Control 1996; 24: 186-l 89. 6. Cookson BD. The emergence of mupirocin resistance: a challenge to infection control and antibiotic prescribing practice. J Antimicrob Chemother 1998; 41: 1 l-l 8. 7. Carson CF, Cookson BD, Farrelly HD, Riley of methicillin-resistant TV. Susceptibility Staphylococcus aweus to the essential oil of Melaleuca alternifolia. J Antimicrob Chemother 1995; 35: 421-424. 8. International Standard IS0 4730 Oil of Melaleuca, terpinen-l-01 type (Tea tree oil). 1996; International Organisation for Standardisation, Geneva, Switzerland. 9. Brophy JJ, Davies NW, Southwell IA, Stiff IA, Williams LR. Gas chromatographic quality control for oil of Melaleuca terpinen-4-01 type (Australian tea tree). J Agricult Food Chem 1989; 37: 1330-1335. 10. Carson CF, Riley TV. Antimicrobial activity of the major components of the essential oil of Melaleuca alternifolia. J Appl Bacterial 1995; 78: 264-269. 11. Hammer KA, Carson CF, Riley TV. In-vitro susceptibility of Malassezia furfur to the essential oil of Melaleuca alternifolia. J Med Vet Mycol 1997; 35: 375-377. 12. Nelson RRS. In-vitro activities of five plant oils essential against methicillin-resistant Staphylococcus aweus and vancomycin-resistant Enterococcus faecium. J Antimicrob Chemother 1997; 40: 305-306. 13. Ford RA. Fragrance raw materials monographs (tea tree oil). Food Chem Toxic01 1988; 26: 407. 14. Carson CF, Riley TV. Toxicity of the essential oil of Melaleuca alternifolia or tea tree oil. r Toxicol-Clin Toxic01 1995; 33: 193-l 94. 15. Selvaag E, Holm J-O, Thune P. Allergic contact
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26. 27. 28. 29.
30.
dermatitis in an aromatherapist with multiple sensitizations to essential oils. Contact Dermatitis 1995;33: 354-355. de Groot AC. Airborne allergic contact dermatitis from tea tree oil. Contact Dermatitis 1996; 35: 304-30s. Bhushan M, Beck MH. Allergic contact dermatitis from tea tree oil in a wart paint. Contact Dermatitis 1997; 36: 117-l 18. Hackzell-Bradley M, Bradley T, Fischer T. Kontaktallergi av “tea tree-oil”. Liikartidningen 1997;94:4359-4361. Brenan JA, Dennerstein GJ, Sfameni SF, Drinkwater P, Marin G, Scurry JP. Evaluation of patch testing in patients with chronic vulvar symptoms. Aus 7 Dermatol 1996; 37: 40-43. Southwell IA, Freeman S, Rubel D. Skin irritancy of tea tree oil. J Essential Oil Res 1997; 9: 47-52. de Groot AC, Weyland JW’. Systemic contact dermatitis from tea tree oil. Contact Dermatitis 1992; 27: 279-280. Knight TE, Hausen BM. Melaleuca oil (tea tree oil) dermatitis. J Am Acad Dermatol 1994; 30: 423-427. Boyce ST, Warden GD, Holder IA. Cytotoxicity testing of topical antimicrobial agents on human keratinocytes and fibroblasts for cultured skin grafts. J Burn Care Rehab 1995; 16: 97-103. Coello R, Glynn JR, Gaspar C, Picazo JJ, Fereres J. Risk factors for developing clinical infection with methicillin-resistant Staphy(MRSA) amongst hospital lococcus aweus patients initially only colonized with MRSA. r Hosp Infect 1997; 37: 39-46. Jenner PM, Hagan EC, Taylor JM, Cook EL, Fitzhugh OG. Food flavourings and compounds of related structure. I. Food Cosmetic Toxic01 1964; 2: 327. Elliott C. Tea tree oil poisoning. Med 3 Aust 1993; 159: 830-831. Seawright A. Comment: Tea tree oil poisoning. MedJAust 1993; 159: 831. Jacobs MR, Hornfeldt CS. Melaleuca oil poisoning. J Toxicol-Clin Toxic01 1994; 32: 461-464. Del Beccaro MA. Melaleuca oil poisoning in a 17-month-old. Vet Hum Toxic01 1995; 37: 557558. Obata Y, Takayama K, Machida Y, Nagai T. Combined effect of cyclic monoterpenes and ethanol on percutaneous absorption of diclofenac sodium. Drug Design Delivery 1991; 8: 137-144.
Journalof HospitalInfection( 1998) 40: I 75-I 78
LEADING
ARTICLE
Efficacy and safety of tea tree oil as a topical antimicrobial agent C. F. Carson, T. V. Riley* and B. D. Cooksont Department of Microbiology, The University of Western Australia, Queen Elizabeth II Medical Centre, Nedlands, WA 6009, Australia; *Division of Microbiology and Infectious Diseases, Western Australian Centre for Pathology and Medical Research, Queen Elizabeth II Medical Centre, Nedlands, WA 6009, Australia; and +!.aboratory of Hospital Infection, PHLS Central Public Health Laboratory, 6 I Colindale Avenue, London NW9 5HT, UK
Keyzumls:
Tea tree oil; AIelaleuca
alternifolia;
topical antimicrobial
Introduction The Australian native plant Melaleuca alternifolia has been used medicinally by Aborigines for several millennia. European colonists soon recognized the therapeutic properties of the plant and furthered its applications by distilling oil from the leaves. The oil, known colloquially as tea tree oil, has gained considerable popularity as a topical antimicrobial agent in recent years, initially on the basis of anecdotal evidence.’ A recent survey on the use of alternative and complementary medicines in Australia reported tea tree oil as the most frequently used.’ Tea tree oil is reputed to have selreral medicinal properties including antibacterial, antifungal, antiviral, anti-inflammatory and analgesic properties. However, it is the antibacterial activity which has attracted the most attention and there are noxv considerable data available concerning the in-vitro activity.‘-’ The colonization of healthcare workers and Correspondence to: C. 1:. Carson, Department of Rlicrobiology, The University of lestern Australia, Queen Elizabeth II LIedial Centre, Nedlands, \%‘A 6009, Australia. Email: [email protected]. Tel: 61 8
9346 3288. Fax: 61 8 9382 8046.
0 195-670
I /98/ I IO I75 + 04 $ i2.00/0
agent
patients with methicillin-resistant Staphylococcus aweus (MRSA) poses significant challenges to infection control and antibiotic prescribing practices.” The emergence of ‘mupirocin-resistant MRSA” confounds the situation further. As MRSA, including strains resistant to mupirocin have been recently shown to be susceptible7 to tea tree oil, tea tree oil is being considered as a potential MRSA decolonization agent. In addition to the need for appropriate clinical data on the efficacy of tea data on safety are also retree oil products, quired. While some work has been conducted for industry purposes, little has been published. In addition, there is some confusion about the concentrations and amount of oil that may elicit an adverse reaction, either topically or systemically.
Production
and quality
assurance
Historically, tea tree oil was produced by manual harvesting of natural bush stands of M. alternifolia. Today, most tea tree oil is produced on large-scale, mechanized plantations. Quality assurance practices, which contribute to the
0 1998 The Hospital
Infection
Society
C. F. Carson
176
provision of a consistent product, have evolved also. Although a number of national and international standards for tea tree oil have been developed and implemented over the years, these have been largely superceded by the most recent international standard for tea tree oil, IS0 4730.8 This standard is the most comprehensive to date and defines the physical and chemical parameters which tea tree oil samples must fulfil. Tea tree oil contains approximately 100 components with less than 10 components, including terpinen-4-01, 1,8-cineole, a-terpineol, terpinolene and a- and y-terpinene, comprising up to 90% of the whole oil.” This standard provides ranges within which 14 components must fall, including several of the major antimicrobial components such as terpinen-401 and a-terpineol.“’
Antimicrobial
activity
Early work examining the antimicrobial activity of tea tree oil provided largely qualitative information about the broad spectrum of organisms susceptible to the oil, including Grampositive and Gram-negative bacteria, acid-fast bacilli, yeasts and fungi.’ More recently, and quantitative discriminating, with more methods, the antimicrobial activity has been characterized in detail. Minimum inhibitory concentrations (MICs) have been determined for many organisms including coagulase-negative staphylococci (0.06-3% v/v), StuphyZococcus aweus (including MRSA) (0.12-O*S%), Streptococcus spp. (0.03-0.12%), vancomycinresistant enterococci (0.5-l%), Acinetobacter baumannii (0.06-l %), Escherichia coli (0.120.25%), Klebsiella pneumoniae (0.12-0.50/o), Candida albicans (0.12-0.25%), other Candida spp. (0.12-0.5%) and Mulassezia furfur (0.12().250/0).3-“,W2 The wide range of organisms susceptible to tea tree oil suggests that this agent may be useful for skin antisepsis. Furthermore, many organisms that colonize skin transiently have been shown to be more susceptible to tea tree oil than commensal organisms.” This differential susceptibility may provide further impetus for the application of tea tree oil in
et al.
situations where eradication of commensal flora is not desirable, particularly where maintenance of the commensal flora supports the continuity of a barrier against colonization by transient flora. This would include settings in wound management, MRSA decolonization and hygienic hand disinfection.
Toxicity Much work has been performed in animals but the generalizability of these data is unclear.
Cutaneous
toxicity
The acute dermal LD,, in rabbits was recorded as in excess of 5.0 g/kg since this dose resulted in 2/10 deaths in rabbits.13 Furthermore, it was observed at necropsy that neat tea tree oil produced irritant effects and skin abnormalities in rabbits patch tested at this dose for 24 h with occlusion. Although some irritation was observed, undiluted tea tree oil did not produce phototoxic effects on the skin of hairless mice.13 Clinical reports of adverse cutaneous events involving tea tree oil and humans” have consisted mainly of individual case reports of cutaneous sensitivity to tea tree oi1.“-‘8 While these reports have served to alert users and the medical professional to the existence of tea tree oil sensitivity, they have not provided data on either incidence rates or the basis of sensitivity. Although some experimental work has been conducted, this has generally been with a small number of subjects. A 48-h occlusive patch test with 1% tea tree oil on 22 volunteers produced no irritation and a maximization test also produced no sensitization reactions.13 In 13 subjects patch-tested with 5% tea tree oil, no adverse reactions were elicited.‘” More recently, 50 participants were patch-tested with 1 and 5% tea tree oil and no adverse reactions were seen.” In another study, repeated challenge over 21 days with 25% tea tree,oil did not result in any sensitivity in 25 of 28 subjects. However, the remaining three subjects were removed from the trial ‘because of a severe allergic (as distinct from irritant) response to tea tree oi1’.2o The influence
177
Efficacy and safety of tea tree oil
of compromised skin on the incidence and severity of sensitivity reactions is unknown also. Tea tree oil is a complex mixture of terpenes and related alcohols.” Some components responsible for adverse reactions to tea tree oil have been identified, including 1 ,S-cineole,” Da-terpinene, aromadendrene, terlimonene, cx-phellandrene, p-cymene, cx-pipinen-4-01, nene, terpinolene22 and sl-terpinene.20 For many years, 1,8-cineole was regarded as an undesirable constituent in tea tree oil due to its reputation as a skin and mucous membrane irritant. However, the latter two studies2’s2’ suggest that this component is not responsible for a large proportion of sensitivity reactions. There are no data documenting the safety of tea tree oil for application to granulation tissue. This is an important area as many patients are colonized with YIRSA in wounds, varicose ulcers and pressure sores.23’2’
Systemic
toxicity
hlost of the work on the systemic toxicity of tea tree oil in animals has been performed for commercial purposes and has not been published in the public domain. The acute LD5,, in rats has been reported as 1.9 g/kg (1.4-2.7 g/ kg),‘j similar to the acute oral LD,,, of eucalyptus oil (2.5 g/kg).” Several cases of human tea tree oil poisoning have been reported, mostly involving the ingestion of modest volumes (N lo-25 mL) of oil. In two cases, ingestion of tea tree oil resulted in what appeared to be systemic contact dermatitis.2’.‘” Seawright” reported that a patient was comatose for 12 h and then semi-conscious for 36 h after ingestion of approximately half a cup of tea tree oil. In separate cases, two children who ingested less than 10 mL tea tree oil became ataxic and drowsy or disorientated.28x2” Both were treated supportively and recovered fully without further complications.
Discussion If the broad range of properties attributed to tea tree oil is corroborated, in particular the antimicrobial activity in ciao, it seems likely that
the oil will find application in a diverse range of conditions. Already, tea tree oil has been ‘grandfathered’ into several markets largely on the basis of its ‘long history of use’. To be considered for therapeutic use in humans, further information regarding the safety of its application must be sought. Since many of the potential applications of tea tree oil involve mucous membranes, such as nasal, oral or vaginal mucosa, this work must include mucosal as well as skin toxicity. In the first instance, the incidence of sensitivity to tea tree oil must be determined. Hopefully, the components or fractions responsible for sensitivity will be identified also and this may assist in the development of oils with a lower propensity to cause sensitivity. Elimination of allergenic components may be achieved by manipulating the distillation process, rectification or by the genetic selection of plants which contain lower quantities of these components. Due to its systemic toxicity, tea tree oil should only be used as a topical agent. Most product manufacturers, distributors and retailers support this qualification, although a few continue to advocate systemic use. In the absence of data supporting its safety in this context and in the presence of clinical experience clearly indicating its toxicity, any recommendations of systemic use appear injudicious. Furthermore, while current interest seems centred around the safety of acute exposure, whether topical or systemic, the question of chronic exposure remains neglected. Tea tree oil contains several compounds known to penetrate skin and to enhance the penetration of other compounds.“” While this may augment the action of its medicinal properties, it may also potentiate the opportunities for toxicity. Tea tree oil has exhibited some promising antimicrobial properties in z.itro and it remains for these properties to be evaluated suitably in ciao. Before it can be introduced widely for controlled, pretherapeutic use, randomized, ferably double-blind clinical trials must be performed and comprehensive safety data obtained.
References 1. Carson CF, Riley TV. Antimicrobial activity of the essential oil of Melaleuca altevnifolia. Lett Appl hficrobiol 1993; 16: 49-55.
C. F. Carson et al.
178
2. Kristoffersen SS, Atkin PA, Shenfield GM. Uptake of alternative medicine. Lancet 1996; 347: 972. 3. Carson CF, Hammer KA, Riley TV. Broth micro-dilution method for determining the susceptibility of Escherichia coli and Staphylococcus aweus to the essential oil of Melaleuca alternifolia (tea tree oil). Microbios 1995; 82: 181-185. 4. Carson CF, Hammer KA, Riley TV In-vitro activity of the essential oil of Melaleuca alternifolia against Streptococcus spp. J Antimicrob Chemother 1996; 37: 1177-1178. 5. Hammer KA, Carson CF, Riley TV. Susceptibility of transient and commensal skin flora to the essential oil of Melaleuca alternifolia (tea tree oil). Am 7 Infect Control 1996; 24: 186-l 89. 6. Cookson BD. The emergence of mupirocin resistance: a challenge to infection control and antibiotic prescribing practice. J Antimicrob Chemother 1998; 41: 1 l-l 8. 7. Carson CF, Cookson BD, Farrelly HD, Riley of methicillin-resistant TV. Susceptibility Staphylococcus aweus to the essential oil of Melaleuca alternifolia. J Antimicrob Chemother 1995; 35: 421-424. 8. International Standard IS0 4730 Oil of Melaleuca, terpinen-l-01 type (Tea tree oil). 1996; International Organisation for Standardisation, Geneva, Switzerland. 9. Brophy JJ, Davies NW, Southwell IA, Stiff IA, Williams LR. Gas chromatographic quality control for oil of Melaleuca terpinen-4-01 type (Australian tea tree). J Agricult Food Chem 1989; 37: 1330-1335. 10. Carson CF, Riley TV. Antimicrobial activity of the major components of the essential oil of Melaleuca alternifolia. J Appl Bacterial 1995; 78: 264-269. 11. Hammer KA, Carson CF, Riley TV. In-vitro susceptibility of Malassezia furfur to the essential oil of Melaleuca alternifolia. J Med Vet Mycol 1997; 35: 375-377. 12. Nelson RRS. In-vitro activities of five plant oils essential against methicillin-resistant Staphylococcus aweus and vancomycin-resistant Enterococcus faecium. J Antimicrob Chemother 1997; 40: 305-306. 13. Ford RA. Fragrance raw materials monographs (tea tree oil). Food Chem Toxic01 1988; 26: 407. 14. Carson CF, Riley TV. Toxicity of the essential oil of Melaleuca alternifolia or tea tree oil. r Toxicol-Clin Toxic01 1995; 33: 193-l 94. 15. Selvaag E, Holm J-O, Thune P. Allergic contact
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26. 27. 28. 29.
30.
dermatitis in an aromatherapist with multiple sensitizations to essential oils. Contact Dermatitis 1995;33: 354-355. de Groot AC. Airborne allergic contact dermatitis from tea tree oil. Contact Dermatitis 1996; 35: 304-30s. Bhushan M, Beck MH. Allergic contact dermatitis from tea tree oil in a wart paint. Contact Dermatitis 1997; 36: 117-l 18. Hackzell-Bradley M, Bradley T, Fischer T. Kontaktallergi av “tea tree-oil”. Liikartidningen 1997;94:4359-4361. Brenan JA, Dennerstein GJ, Sfameni SF, Drinkwater P, Marin G, Scurry JP. Evaluation of patch testing in patients with chronic vulvar symptoms. Aus 7 Dermatol 1996; 37: 40-43. Southwell IA, Freeman S, Rubel D. Skin irritancy of tea tree oil. J Essential Oil Res 1997; 9: 47-52. de Groot AC, Weyland JW’. Systemic contact dermatitis from tea tree oil. Contact Dermatitis 1992; 27: 279-280. Knight TE, Hausen BM. Melaleuca oil (tea tree oil) dermatitis. J Am Acad Dermatol 1994; 30: 423-427. Boyce ST, Warden GD, Holder IA. Cytotoxicity testing of topical antimicrobial agents on human keratinocytes and fibroblasts for cultured skin grafts. J Burn Care Rehab 1995; 16: 97-103. Coello R, Glynn JR, Gaspar C, Picazo JJ, Fereres J. Risk factors for developing clinical infection with methicillin-resistant Staphy(MRSA) amongst hospital lococcus aweus patients initially only colonized with MRSA. r Hosp Infect 1997; 37: 39-46. Jenner PM, Hagan EC, Taylor JM, Cook EL, Fitzhugh OG. Food flavourings and compounds of related structure. I. Food Cosmetic Toxic01 1964; 2: 327. Elliott C. Tea tree oil poisoning. Med 3 Aust 1993; 159: 830-831. Seawright A. Comment: Tea tree oil poisoning. MedJAust 1993; 159: 831. Jacobs MR, Hornfeldt CS. Melaleuca oil poisoning. J Toxicol-Clin Toxic01 1994; 32: 461-464. Del Beccaro MA. Melaleuca oil poisoning in a 17-month-old. Vet Hum Toxic01 1995; 37: 557558. Obata Y, Takayama K, Machida Y, Nagai T. Combined effect of cyclic monoterpenes and ethanol on percutaneous absorption of diclofenac sodium. Drug Design Delivery 1991; 8: 137-144.