Australian tea tree oil is derived from the leaves of Melaleuca alternifolia, a tree that grows in the subtropical coastal regions of New South Wales and Queensland, Australia. New Zealand tea tree oil, better known as manuka oil, is made from the leaves and young branches of the manuka plant (Leptospermum scoparium). Both oils are known to have antimicrobial properties and are used in a variety of products applied to the skin. Australian tea tree oil is widely used in skin creams, body lotions, shampoos, and oral hygiene products. It is also used medicinally to treat skin ulcers and infections. Manuka oil is used in a skin cream to treat sores on the feet. The purpose of this article was to review the antibacterial, antifungal, and antiviral effects of these oils for human and veterinary use.

The essential oils from the Australian tea tree and New Zealand manuka tree contain different compounds and display different antimicrobial activities. Tea tree oil contains primarily monoterpenes, with terpinen-4-ol as the main component. Tea tree oil contains only small amounts of sesquiterpenes. In contrast, the dominant components of manuka oil are sesquiterpenes. Manuka oil contains only traces of terpinen-4-ol.

Tea tree oil shows activity against a broad spectrum of microorganisms. These include the human pathogens Escherichia coli and various Streptococcus and Staphylococcus species. Tea tree oil inhibits the growth of the fungus Candida albicans, the culprit in common yeast infections, and other fungi that cause athlete's foot infections. It also inhibits the growth of Propionibacterium acnes, a bacterium implicated in acne. A clinical study found that a gel containing 5% tea tree oil was as effective as a 5% benzoyl peroxide lotion, the established treatment of acne vulgaris. Tea tree oil may be useful against skin and ear infections in dogs and cats, because it inhibits the growth of the common fungus Malassezia pachydermatis (some caution is warranted when treating animals with tea tree and other essential oils as cases of toxicosis have been repeatedly reported by veterinarians when the oil was applied topically to dogs, cats, and birds. In most cases, the oil was used to treat dermatological conditions at inappropriate high doses. The typical symptoms of toxicosis observed were depression weakness, un-coordination, and muscle tremors. In the majority of cases, treatment of clinical signs and supportive care have been sufficient to achieve recovery free from pathological conditions within several days.1

Manuka oil has antimicrobial activity against a more limited range of organisms than tea tree oil. It is largely ineffectively against Gram-negative bacteria. However, the investigated Gram-positive bacteria were susceptible to manuka oil at lower concentrations than tea tree oil. A â-triketones-enriched manuka oil inhibited the growth of Helicobacter pylori, a bacterium implicated in stomach ulcers, more effectively than tea tree oil.

The widespread use of antibiotics has contributed to the development of bacteria that are resistant to antibiotic treatment. Bacteria that are resistant to penicillins, vancomycin, and other antibiotics can cause life-threatening wound infections, primarily in hospitalized patients. There is a critical need to develop new antibiotics with new mechanisms of action. Tea tree oil and manuka oil are natural substances that appear to be good candidates for the development of new antimicrobial products.

This article provides a thorough review of the evidence for a role of these oils in inhibiting the growth of pathogenic bacteria, fungi, and viruses. However, the experimental conditions and the mode of administration of the oils are not always described. It is not clear how much of the evidence is based on test tube studies and how much is based on topical application of the oils in animal or human subjects.