Julie Lmarkham

Centre for Biostructural and Biomolecular Research, University of Western Sydney, Hawkesbury, Richmond, NSW, Australia

INTRODUCTION

Captain Cook is reported to have used the leaves of the tea tree to brew a spicy tea and Sir Joseph Banks, the botanist with Cook's expedition, included samples of the plant in his collection. Exactly which plants were used is not known as the name 'tea tree' has been used for a number of similar plants in the genera Melaleuca and Leptospermum. Whilst essential oils from several species possess antimicrobial activity, the oil which is widely used today for its antiseptic properties is the oil of Melaleuca alternifolia.

The natural habitat of Melaleuca alternifolia is the Bungawalbyn region of north-eastern New South Wales, Australia and Drury (1991) reports that the Bundjalung Aborigines, who lived in this area, treated skin infections by crushing leaves of the tea tree over the injury and covering it with a warm mudpack. It is possible that early settlers also used this remedy.

Scientific studies on tea tree oil began in the 1920s at the Museum of Technology and Applied Sciences in Sydney. Penfold and Grant (1925) reported that the oil was a non-toxic and non-irritating antiseptic, which was more effective than phenol, the standard for comparison. Following this demonstration of the effectiveness of tea tree oil in laboratory tests, evidence of its therapeutic use began to accumulate. Humphrey (1930) tested a water-miscible preparation of tea tree oil and reported its value at concentrations of 2.5-10% for the treatment of a variety of conditions, such as wounds, peronychia (infection of the nails), coryza and sore throats, and suggested it would have potential applications in the treatment of vaginal infections and burns. In addition to its efficacy as an antiseptic, several other features of the oil were cited. These included the ability of the oil to dissolve pus in infected wounds, to cleanse dirty wounds and to resolve chronic infections, particularly of the nail, without any apparent damage to healthy tissue.

During the 1930's the reputation of tea tree oil as 'a medicine chest in a bottle' continued to develop. The major producers of the oil, Australian Essential Oils Ltd, published a report in 1936 on the medical and dental applications of Ti-trol, the name used for the neat oil and of Melasol, a water-miscible preparation (cited in Lassak and McCarthy 1983). The list of applications in external conditions continued to grow, and all reports stressed the non-toxic and non-irritating properties of the oil. The ability of the pure oil to penetrate unbroken skin was also reported (Drury 1991).

A reputation for tea tree oil as a safe, effective antiseptic had been established by the time of the outbreak of the Second World War, and its record impressive enough that cutters and producers were exempted from military service. In addition, the oil was incorporated in army and navy first-aid kits for use in tropical regions (Drury 1991) and added to machine

cutting oils to reduce infection following injury to the hands by metal filings and turnings (Lassak and McCarthy 1983).

A number of factors led to a decline in the industry following the Second World War. As well as competition in the market-place from newly-developed synthetic agents, the product was inadequately promoted and the supply was inconsistent, both in quality and quantity. The natural stands of trees, located in swampy areas, were difficult to harvest and different chemotypes, with oils of variable composition, were difficult to distinguish in a simple manner.

During the 1970's the interest in 'natural' products was renewed, and over the last twenty years the production of oil has continued to increase and so have the applications. Tea tree oil is incorporated as the active antimicrobial or as a natural preservative in a wide range of pharmaceutical, cosmetic and 'cosmeceutical' products including antiseptic creams, face washes, pimple gels, vaginal creams, veterinary skin care products, tinea preparations, foot powders, shampoos and conditioners. It is claimed to provide rapid soothing relief when added to burns blankets and creams. Alongside this, there has been a recognition by the industry of the need to support the early anecdotal claims about the efficacy of the oil with scientific data which will enable registration of the oil as a pharmaceutical product. This chapter will review the published studies which have examined in vitro and in vivo activity of the oil.

IN VITRO TESTING OF ANTIMICROBIAL ACTIVITY

The major therapeutic claims for tea tree oil involve its antimicrobial activity and its effectiveness against a wide range of bacteria and fungi has been demonstrated. The methods used by various authors vary both in principle and detail and it is relevant here to review the methods and evaluate their usefulness in the testing of essential oils.

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