Christine F. Carson

Antimicrobial activity of essential oils and other plant extracts

The antimicrobial activity of plant oils and extracts has been recognized for many years. However, few investigations have compared large numbers of oils and extracts using methods that are directly comparable. In the present study, 52 plant oils and extracts were investigated for activity against Acinetobacter baumanii, Aeromonas veronii biogroup sobria, Candida albicans, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella enterica subsp. enterica serotype typhimurium, Serratia marcescens and Staphylococcus aureus, using an agar dilution method. Lemongrass, oregano and bay inhibited all organisms at concentrations of ≤2·0% (v/v). Six oils did not inhibit any organisms at the highest concentration, which was 2·0% (v/v) oil for apricot kernel, evening primrose, macadamia, pumpkin, sage and sweet almond. Variable activity was recorded for the remaining oils. Twenty of the plant oils and extracts were investigated, using a broth microdilution method, for activity against C. albicans, Staph. aureus and E. coli. The lowest minimum inhibitory concentrations were 0·03% (v/v) thyme oil against C. albicans and E. coli and 0·008% (v/v) vetiver oil against Staph. aureus. These results support the notion that plant essential oils and extracts may have a role as pharmaceuticals and preservatives.

Mechanism of Action of Melaleuca alternifolia (Tea Tree) Oil on Staphylococcus aureus Determined by Time-Kill, Lysis, Leakage, and Salt Tolerance Assays and Electron Microscopy

ABSTRACT The essential oil of Melaleuca alternifolia (tea tree) has broad-spectrum antimicrobial activity. The mechanisms of action of tea tree oil and three of its components, 1,8-cineole, terpinen-4-ol, and α-terpineol, against Staphylococcus aureus ATCC 9144 were investigated. Treatment with these agents at their MICs and two times their MICs, particularly treatment with terpinen-4-ol and α-terpineol, reduced the viability of S. aureus. None of the agents caused lysis, as determined by measurement of the optical density at 620 nm, although cells became disproportionately sensitive to subsequent autolysis. Loss of 260-nm-absorbing material occurred after treatment with concentrations equivalent to the MIC, particularly after treatment with 1,8-cineole and α-terpineol. S. aureus organisms treated with tea tree oil or its components at the MIC or two times the MIC showed a significant loss of tolerance to NaCl. When the agents were tested at one-half the MIC, only 1,8-cineole significantly reduced the tolerance of S. aureus to NaCl. Electron microscopy of terpinen-4-ol-treated cells showed the formation of mesosomes and the loss of cytoplasmic contents. The predisposition to lysis, the loss of 260-nm-absorbing material, the loss of tolerance to NaCl, and the altered morphology seen by electron microscopy all suggest that tea tree oil and its components compromise the cytoplasmic membrane.

Melaleuca alternifolia (Tea Tree) Oil: a Review of Antimicrobial and Other Medicinal Properties

SUMMARY Complementary and alternative medicines such as tea tree (melaleuca) oil have become increasingly popular in recent decades. This essential oil has been used for almost 100 years in Australia but is now available worldwide both as neat oil and as an active component in an array of products. The primary uses of tea tree oil have historically capitalized on the antiseptic and anti-inflammatory actions of the oil. This review summarizes recent developments in our understanding of the antimicrobial and anti-inflammatory activities of the oil and its components, as well as clinical efficacy. Specific mechanisms of antimicrobial and anti-inflammatory action are reviewed, and the toxicity of the oil is briefly discussed.

Antifungal activity of the components of Melaleuca alternifolia (tea tree) oil

Aims: To investigate the in vitro antifungal activity of the components of Melaleuca alternifolia (tea tree) oil.

Terpinen-4-ol, the main component of the essential oil of Melaleuca alternifolia (tea tree oil), suppresses inflammatory mediator production by activated human monocytes

Abstract:Objective and Design: To evaluate potential anti-inflammatory properties of tea tree oil, the essential oil steam distilled from the Australian native plant, Melaleuca alternifolia.¶Material and Methods: The ability of tea tree oil to reduce the production in vitro of tumour necrosis factor-α (TNFα), interleukin (IL)-1β, IL-8, IL-10 and prostaglandin E2 (PGE2) by lipopolysaccharide (LPS)-activated human peripheral blood monocytes was examined.¶Results: Tea tree oil emulsified by sonication in a glass tube into culture medium containing 10% fetal calf serum (FCS) was toxic for monocytes at a concentration of 0.016% v/v. However, the water soluble components of tea tree oil at concentrations equivalent to 0.125% significantly suppressed LPS-induced production of TNFα, IL-1β and IL-10 (by approximately 50%) and PGE2 (by approximately 30%) after 40 h. Gas chromatography/ mass spectrometry identified terpinen-4-ol (42%), α-terpineol (3%) and 1,8-cineole (2%, respectively, of tea tree oil) as the water soluble components of tea tree oil. When these components were examined individually, only terpinen-4-ol suppressed the production after 40 h of TNFα, IL-1β, IL-8, IL-10 and PGE2 by LPS-activated monocytes. Conclusion: The water-soluble components of tea tree oil can suppress pro-inflammatory mediator production by activated human monocytes.

Antimicrobial activity of the essential oil of Melaleuca alternifolia

Melaleuca alternifolia has been used for medical purposes since Australia was colonized in 1788. Melaleuca alternifolia is commonly called tea tree, although this vernacular name is also given to many other species in the Leptospermum and Melaleuca genera. A small tree, it grows up to 5 m in height, has papery bark and narrow, tapered leaves up to 20 mm in length and flowers in summer. Melaleuca alternifolia is unique to Australia and its natural habitat is a relatively small area around the Clarence and Richmond rivers in the north‐east coastal area of New South Wales where the terrain is generally low lying and swampy. The essential oil of M. alternifolia, or tea tree oil. has enjoyed increased medicinal use in recent years. It is a pale yellow viscous liquid with a distinctive pungent odour and is composed of a complex mixture of monoterpenes, 1‐terpinen‐4‐ol, cineole and other hydrocarbons (Peña 1962).

Susceptibility of transient and commensal skin flora to the essential oil of Melaleuca alternifolia (tea tree oil).

OBJECTIVES The purpose of this study was to determine the susceptibility of a range of transient and commensal skin flora to the essential oil of Melaleuca alternifolia, or tea tree. METHODS A modified broth microdilution method was used. Polyoxyethylene sorbitan mono-oleate detergent was added to the test medium to enhance solubility of the tea tree oil. RESULTS Serratia marcescens had the lowest minimum inhibitory concentration (MIC90) of 0.25%. The highest MIC90 was 3% for Pseudomonas aeruginosa. The lowest minimum bactericidal concentration (MBC90) was 0.25% for S. marcescens and Klebsiella pneumoniae, whereas the highest was 8% for Staphylococcus capitis. CONCLUSIONS S. aureus and most of the gram-negative bacteria tested were more susceptible to tea tree oil than the coagulase-negative staphylococci and micrococci. These results suggest that tea tree oil may be useful in removing transient skin flora while suppressing but maintaining resident flora.

The water-soluble components of the essential oil of Melaleuca alternifolia (tea tree oil) suppress the production of superoxide by human monocytes, but not neutrophils, activated in vitro

Abstract:Objective: To evaluate the regulatory properties of the essential oil of Melaleuca alternifolia (tea tree oil) on the production of oxygen derived reactive species by human peripheral blood leukocytes activated in vitro.¶Materials and methods: The ability of tea tree oil to reduce superoxide production by neutrophils and monocytes stimulated with N-formyl-methionyl-leucyl-phenylalanine (fMLP), lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) was examined.¶Results: The water-soluble fraction of tea tree oil had no significant effect on agonist-stimulated superoxide production by neutrophils, but significantly and dose-dependently suppressed agonist-stimulated superoxide production by monocytes. This suppression was not due to cell death. Chemical analysis identified the water-soluble components to be terpinen-4-ol, α-terpineol and 1,8-cineole. When examined individually, terpinen-4-ol significantly suppressed fMLP- and LPS- but not PMA-stimulated superoxide production; α-terpineol significantly suppressed fMLP-, LPS- and PMA-stimulated superoxide production; 1,8-cineole was without effect.¶Conclusion: Tea tree oil components suppress the production of superoxide by monocytes, but not neutrophils, suggesting the potential for selective regulation of cell types by these components during inflammation.¶

Influence of organic matter, cations and surfactants on the antimicrobial activity of Melaleuca alternifolia (tea tree) oil in vitro

The effect of some potentially interfering substances and conditions on the antimicrobial activity of Melaleuca alternifolia (tea tree) oil was investigated. Agar and broth dilution methods were used to determine minimum inhibitory and cidal concentrations of tea tree oil in the presence and absence of each potentially interfering substance. Activity was determined against Gram‐positive and ‐negative bacteria, and Candida albicans. Minimum inhibitory or cidal concentrations differed from controls by two or more dilutions, for one or more organisms, where Tween‐20, Tween‐80, skim‐milk powder and bovine serum albumin were assessed. These differences were not seen when assays were performed in anaerobic conditions, or in the presence of calcium and magnesium ions. The effect of organic matter on the antimicrobial activity of tea tree oil was also investigated by an organic soil neutralization test. Organisms were exposed to lethal concentrations of tea tree oil ranging from 1–10% (v/v), in the presence of 1–30% (w/v) dry bakers’ yeast. After 10 min contact time, viability was determined. At ≥ 1%, organic matter compromised the activity of each concentration of tea tree oil against Staphylococcus aureus and C. albicans. At 10% or more, organic matter compromised the activity of each tea tree oil concentration against Pseudomonas aeruginosa. Organic matter affected 1 and 2% tea tree oil, but not 4 and 8%, against Escherichia coli. In conclusion, organic matter and surfactants compromise the antimicrobial activity of tea tree oil, although these effects vary between organisms.

Safety, efficacy and provenance of tea tree (Melaleuca alternifolia) oil

The identity, sources and composition of tea tree (Melaleuca alternifolia) oil are discussed, and earlier errors in the literature indicated. Reports of both therapeutic and allergenic effects are reviewed.