As the source of the world famous manuka honey, the manuka or Leptospermum scoparium tree is frequently considered to be the "tea tree" of Aotearoa New Zealand. A small, aromatic evergreen shrub native to all three islands that make up New Zealand, Leptospermum scoparium has been used medicinally by the country's indigenous Maori people for over 800 years. Its antiseptic and antibacterial properties are well known to both folk medicine and science, although no single chemical constituent is said to be responsible for all of its properties. The compound methylglyoxal (MGO) has been identified as the chemical responsible for the majority of the tree's antimicrobial activity, leading to the development of a rating system for products based on their percentage of MGO content. Since a significant portion of research into the plant's antibacterial properties have focused on its essential oil, which contains no MGO, it is likely that a number of other compounds contribute strongly to its antibacterial action.
The essential oil contains a wide variety of potentially active ingredients, including α-pinene, β-pinene, α-farnesine, myrcene, ρ-cymene, 1,8-cineole, linalol, methylcinnamate, isoleptospermone and leptospermone as well as a number of different sesquiterpenes and triketones. This entire plant also contains a number of pharmacologically active flavonoids and triterpenoids, which may also moderate the action of Leptospermum scoparium's other constitutents. These also may be responsible for the spasmolytic properties of the plant that make it inadvisable for use by pregnant or breastfeeding mothers. Such muscle spasm reducing properties may explain its traditional use in easing the cramping and discomfort of diarrhea. Although its muscle relaxing properties are considered to be quite mild, the plant should not be used with benzodiazepine anxiety medications as the two can potentiate the activity of each other.
While the antibacterial activity of the extracted essential oil from Leptospermum scoparium is quite potent, it is effective only on a limited range of bacterial types — mostly Gram-positive organisms. It has proved to be particularly effective against infections of Staphylococcus aureus, however, generating significant interest in its use against multi-drug resistant Staphylococcus aureus (MRSA). As of 2011 limited studies have followed its efficacy as an antibiotic treatment for this condition, suggesting that further research is needed to conclusively demonstrate that Leptospermum scoparium essential oil is indeed effective as a treatment for MRSA.
Although not effective against Gram-negative bacteria, there is some evidence Leptospermum scoparium essential oil may be effective as a treatment for fungal infections. Most of this is extrapolation from traditional medical uses and other research into the activity of other aromatic species within the same family on fungal infections. When used in combination with other topical antifungal drugs, the diluted essential oil of the plant may increase their efficacy.