Manuka Essential Oil Monograph.
(Leptospermum scoparium J.R. & G. Forster).
Botany & Natural Habitat.
Manuka (Leptospermum scoparium J.R. & G. Forster) (Allan 1961) is the most abundant shrub/small tree found in New Zealand, & the only endemic Leptospermum spp. native to New Zealand (Thompson 1989, Porter 2004) out of some seventy-nine known Leptospermum spp. It is an invasive, bushy, usually conical-shaped shrub or tree, which typically grows to 4m., but can reach 6-8m., with stems measuring from 10 -12cm. in diameter (Ward 2000). The branches of the shrub are covered in string bark, which, on breaking, reveals a hard reddish-coloured, or sometimes whitish, wood. The shrub is covered all year in small lanceolate shaped spiky-ended leaves, and flowers periodically especially May-June, having individual white or sometimes pink hermaphroditic (therefore insect-pollinated) flowers, about 10-12mm. across. The plant shows considerable morphological variation in from, habitat, leaf size & shape, flower & leaf colouration, foliage density etc. (Porter 2004). The branches and leaves are covered in silky white hairs, which release essential oil when rubbed. The shrub is prone to attack by the scale insect Eriococcus orariensis, eradicating it in some areas (Anon 1956).
The manuka shrub is found in scrub-forest all over New Zealand, including the Stewart & Chatham Islands, & in Tasmania, as well as in Australia, growing at elevations ranging from sea level to 1000m. It has also been reported as an alien species on several islands in Hawaii. Manuka is capable of growing in a variety of acidic and low nutrient soils, from sand dunes to mountainous areas. Ward (2000) points out that manuka shrubs are often confused with the larger & faster growing kanuka plants [Kunzea ericoides (A. Rich) J. Thompson, and the author further provides a number of morphological indicators to distinguish the two species.
The word ‘manuka’ comes from the Maori term meaning nervousness or anxiety, and is famously associated with Captain Cook, who’s men who made a refreshing tea from manuka leaves, which has to be brewed for a longer period to release the flavour than for conventional tea from Camellia sinensis (although many consider it superior). The plant parts are used in traditional Maori remedies (Brooker et al. 1987; Riley 1994). The leaves exude a sweet manna which is composed of d-mannitol (Cambie & Seelye 1959) – there is a debate as to the cause of this exudation, whether it be natural or as a result of insect damage (Booker et al. 1991).
Essential oil of Manuka.
As with many species of the Myrtaceae, the essential oil of manuka occurs in schizogenous cavities (oil sacs) on the (underside) leaf surfaces and the seed capsules, and is obtained in practice by the steam distillation of the wild harvested terminal leaves and branches. Perry et al. (1997) reports the yield of essential oil as ranging from 0.14% to 0.80% dry weight of vegetation. The volatile oil is extremely variable in composition according to vegetation source (see chemotypes listed below), and variation of certain components has been reported from to maturity, and from natural variability within plants sourced at a single location. The ‘normal’ oil presented commercially has been described as an amber coloured liquid; the odour is fresh but rather unpleasant-bitter, clove-terpene like/bitter-herbaceous, resinous, with a hint of fruitiness. The dry-out on a perfumers strip is ambery, slightly scented, and soapy (Burfield 2000). Joulain (1996) previously commented that “the intense characteristic odour of this type of product (referring to tea tree oil from Melaleuca alternifolia) is often a handicap for wider uses, such as bodycare products. The problem also exists, albeit to a lesser extent, for the essential oil of Leptospermum scoparium (manuka) …” However the buying public has become familiar & accepting of the earthy aromatic odour of tea tree oil over the years, and these remarks may not now apply.
Earlier studies on Manuka oil chemistry.
Although analytical work on Manuka oil has been carried out for nearly 100 years initially with the identification of leptospermol (Penfold 1921, Gardner 1924; Gardner 1924a) – later to be renamed leptospermone (Short 1926), only in the last few years has the chemistry associated with the high variability of the oil started to become clear.. Flynn et al. (1979) identified several mono-& sequiterpenoids in manuka oil by GC-MS & IR spectroscopy. Häberlein & Tschiersch (1994) identified several isoflavones and triterpenoids in a dichloromethane extract of manuka vegetation. The b-triketones exhibit keto-enol tautomerism: one of the possible -enol forms of leptospermone is illustrated below:
Chemotypes.
Douglas et al. (2001) investigated oils from the foliage of 132 samples from 44 collecting sites on the North Island of New Zealand, and distinguished 5 chemotypes: mono-terpene-rich, sesquiterpene-rich, triketone-enriched, mono-sesquiterpene type and methyl cinnamate types. Previously an unpublished survey carried out by the New Zealand Institute for Crop & Food Research Limited, studied prepared essential oils from manuka leaves gathered from various locations on the S. Island, revealing the presence of four separate chemotypes: monoterpene rich; sesquiterpene rich; enhanced triketones in sesquiterpene rich oils and mixed oils with a balance of monoterpenes and sesquiterpenes (Ward 2000). Later Douglas et al. (2004) conducted a survey analyzing oils from 261 manuka plants across 87 sites in New Zealand and identified 11 chemotypes: a-pinene, sesquiterpene-rich with high myrcene, sesquiterpene-rich with elevated (β-)-caryophyllene and (α-)-humulene; sesquiterpene-rich with an unidentified sesquiterpene hydrocarbon; high geranyl acetate; sesquiterpene-rich with high a-ylangene + a-copaene and elevated triketones; sesquiterpene-rich with no distinctive components; sesquiterpene-rich with high trans-methyl cinnamate; high linalol; and sesquiterpene-rich with elevated elemene and selinene.
Monterpenes
Monoterpenes are generally below 3% in Manuka oils, although a high a-pinene chemotypes were identified in the North of the N. Island by Douglas et al. (2004). Other monoterpenes hydrocarbons such as myrcene, and oxygenated monoterpenes such as 1,8-cineole & linalool are also common. The presence of a cluster of a high geranyl acetate.(to 48.6%) chemoptype towards the South of the N. Island, was also identified by Douglas et al. (2004
Esters
Low levels of esters are found in manuka oils, but the ocuurence of a trans-methyl cinnamate chemotype (at up to 30% methyl cinnamate) was reported by Douglas et al. (2004) in several S. Island samples.
Sesquiterpenes.
The sesquiterpenes found in mauka oils include those components with cubebene/copaene, elemene, gurjunene/aromadendrene, farnescene/caryophyllene, selinene, calamenene & cadinene types of skeletons (Porter & Wilkins 1998)
Melching et al. (1997) succeeding in isolating & identifying the labile sesquiterpene (-)-(!R,7S,10R)-cadina-3,5-diene, zonarene & (+)-d-amorphene which constitute 5-10% of Manex oil (the trade name of Manuka oil from Te Araroa, East Cape as marketed by Tairawhiti Pharmaceuticals Ltd.). .
Beta-triketones.
Of the N. Island oils, the triketone-enriched East Cape chemotype is rich in the b-triketones flavesone, leptospermone & iso-leptospermone, & has a much lower discernable odour, especially if the oil is fractionated to enhance the concentration of these components. Analytically, the presence of the b-triketones distinguishes manuka oil from Kanuka oil from Kunzea ericoides.
The presence of 3 further minor ketonic compounds in Manuka oil illustrated below was established by Melching (1997) and was confirmed by Porter & Wilkins (1998):
One of these compounds, 2-(1-oxobutyl)-4,4,6,6-tetramethylcyclohexan-1,3,5-trione has previously been named grandiflorone after it was found as a substituent of the Australian essential oil of L. flavescens (Brooker et al. 1963; Hellyer 1968; Brophy et al. 1996).
The East Cape Chemotype of Manuka Oil.
Essential oils prepared from manuka vegetation in the N. Island were found to contain from 0.1 to 33.3% (average 5.8%) of the triketones flavescone, isoleptospermone, and leptosepermone (Douglas et al 2001). Later, Douglas et al. (2004) identified b-triketone levels of >20% with only a slight seasonal variation, from surveying 36 plants in the East Cape area, although triketone levels of up to 20% were also found in the Marlborough Sounds area of the S. Island. High triketone plants with triketone levels of >20% only have a limited distribution within the East Cape area, and commercial exploitation of this chemotype is dependent on maximizing foliage production.& regrowth (Douglas et al. 2004)..
Porter (Porter 2004) further comments that under agricultural pressure, wild stands of Manuka are being cleared and that therapeutic lines of the East Cape variety may be lost, although trial plantations have been established. High levels of these compounds are aided by companies involved in East Cape oil production (e.g. Tairawhiti Pharmaceuticals which distills foliage from Te Araroa, East Cape) by prolonging distillation times (4-6 hours), and/or by high-vacuum fractionation of the oil, making oil production a more expensive exercise than, for say, tea-tree oil from Melaleuca alternifolia. Careful analytical monitoring of production batches has to be maintained to ensure product consistency due to the variability of the essential oil from the East Cape vegetation sources. A high β-triketone containing fraction of East Cape manuka oil is commercially available containing over 96% β-triketone ocontent.:
|
Substituent |
%-age |
|
Leptospermone |
57.7% to 67.0% |
|
Isoleptospermone |
13.0% to 23.0% |
|
Flavesone |
13.0% to 23.0% |
Table 1. Constituents of high β-triketone fraction of manuka oil.
The biosynthetic pathway for the formation of these b-triketones is unknown at present, and Brophy et al. (1999) did not find any b-triketones in Australian L. scoparium samples. Further, Perry et al. (1997) proposes that New Zealand oils from L. scoparium are a different chemotype to the corresponding Australian oils, and that the New Zealand plants are morphologically different from Tasmanian L. scoparium specimens. Further, Porter & Wilkins (1998) advise that kanuka oil is characterized by high levels of a-pinene (>50%) whereas monoterpenes are typically present at low levels (<3%) in many manuka oils The presence of higher levels of b-triketones has been advised as offering a high level of anti-microbial activity against Gm-positive organisms such as Staphylococcus, Enterococcus & Streptococcus spp., and certain dermatophytic fungi (see below).
Flavonoids.
The flavonoids in a petroleum extract of the aerial parts of manuka, separated on silica gel were characterized by Mayer (1990) who confirmed the identity of seven compounds, four of which were already noted in the literature, and found that a triterpene diol previously identified as betulinol was in fact a mixture of uvaol & betulinol. The new flavonoids were 5-methoxy-7-hydroxy-6,8-dimethylflavone, 5-hydroxy-6-methyl-7-methoxyflavone & 5,7-dimethoxy-6-methylflavone, Further investigations were conducted by Tscheirsch et al. (1992) and Haberlein & Tschiersch (1993) who discovered a further flavanoid, 5,7-dimethoxy-6-methylflavone.
Tannins.
Tannins in Leptospermum scoparium were investigated by Cain (1963).
Triterpene Acids.
Triterpene acids in Leptospermum scoparium were investigated by Corbett & McDowell (1958).
Anti-microbial properties of Manuka oil.
General Remarks.
The manuka oil chemotype, the manuka oil composition and the microbiological testing method employed are some of the major factors with respect to reported anti-microbial activity of manuka oil. Intimate contact between essential oil molecules and micro-organisms, is notoriously difficult to achieve in aqueous media because of the hydrophobicity of essential oils. Various microbiological techniques employed to assess the anti-microbiological activity of manuka oils have included the inhibitory zone technique (Perry et al. 1997), the agar well technique (Lis-Balchin et al. 1996), the broth dilution method (Christolph et al. 2000; Harkenthal et al. 1999) and the broth susceptibility method (Carson & Riley 1994) amongst others. However, various considerations point to test method dependency. For example the effect of any surfactant employed may have a direct bearing on the results. Thus van Zyl et al. (2000) on testing 20 nature identical essential oil constituents remark that in their findings “the relative inactivity of citronellal, (+)-αβ-thujone, p-cymene and 1,8-cineol has been associated with low water solubility & hydrogen bonding capacity, thus limiting their entry into Gm-ive organisms that possess sufficient hydrophobic pathways in the outer membrane (quoting Griffin et al. 1999). Elsewhere Burt (2004) remarks that Gm –ive bacteria are less susceptible to the action of essential oils due to the presence of an lipopolysaccharide coving to the outer membrane to their cell wall which restricts the diffusion of lipophilic compounds.
The anti-microbial properties of mixtures of a high β-triketone fraction of manuka oil with other essential oils have also been investigated e.g. with niaouli or Australian tea tree oil by Christolph et al. (2001). In the latter study good activity was noted against Staphylococcus aureus and Moraxella catarrhalis, with total kill times determined at 240 mins. for both types of admixture, which was superior to that for myrtol, the proprietary product for the treatment of acute and chronic bronchitis and sinusitis. Combinations of manuka and tea tree, calendula and tea extracts & essential oils were tested for potential use as an oral mouthwash against the periodontal pathogens Actinobacillus actinomycetemcomitans, Tanerella forsythensis (Lauten et al. 2005) but the results did not reach statistical significance.
Combinations of the β-trietone fraction of manuka oil and antibiotics have also been investigated against a number of pathogenic organisms (Kim 1999).
Comparative anti-microbiological activity testing.
Christolph et al. (2000) found that Lema oil® came`second in kill time performance in a series of oils tested against Staphylococcus aureus (Australian tea-tree oil, cajuput oil, niaouli oil, Lema oil, kanuka oil, manuka & the beta-triketone isolate of manuka oil), where a 2% concentration of oil a complete 6.8 log10 reduction of cell numbers in suspensions within 60 min.
Harkenthal et al. (1999) found that manuka oil had a higher kill activity against Gm +ive bacteria than tea tree oil, with a MIC value of around 0.12%. The authours also found that both manuka and tea tree had a good activity against anti-biotic resistant strains of Staphylococcus aureus, but only a poor activity against Pseudomonas aeruginosa.
Takarada et al (2004) investigated a number of essential oils including manuka oil, tea tree oil, eucalyptus oil, lavandula oil, and rosmarinus oil against a number of oral pathogens, Porphyromonasgingivalis, Actinobacillus actinomycetemcomitans, Fusobacterium nucleatum, Streptococcus mutans, and Streptococcus sobrinus, finding that that, among the essential oils tested, manuka oil and tea tree oil in particular had strong antibacterial activity against periodontopathic and cariogenic bacteria.
Filoche et al. (2005) tried a number of essential oils including manuka oil, Listerine Coolmint, and menthol & thymol, alone and in combination with chlorhexidine gluconate, against biofilm & planktonic cultures of Streptococcus mutans & Lactobacillus plantarum. Manuka oil showed some activity but less than cinnamon oil.
Virucidal Properties.
Reichking et al.(2005) established the virucial activity of beta-triketone rich manuka oil fractions against the Herpes Simplex organisms HSV-1 & HSV-2 in vitro on RC-37 cells (monkey kidney cells) using a plaque reduction assay.Pretreatment of the viruses with manuka oil for 1 h prior to cell infection showed that significant inhibition could be achieved for both HSV-1 & HSV-2 strains.
Dermatophytic organisms.
Action of manuka oil against the dermatophyte Trichophyton mentagrophytes was investigated by Lis-Balchin et al (1996). Tea tree oil was found to have no action but the manuka oil was effective against this organism in this study.
Ethnic Uses of Manuka.
Manuka was renowned for its use as a tea substitute by sailors visiting Aotearoa, hence the name “tea-tree” was born, although manuka is of course, quite different from tea-tree.
The bark/leaves/sap/seed capsules of manuka have been used for beverages or medicinal preparations (Best 1905; Brooker et al. 1981). Decoctions of leaves used for aromatic teas for treating fevers & for treating colds, as an emetic, purgative & diuretic; oil infusion of leaves used against chronic sores (Porter 2004). Carr (Carr 2004) conveniently presents the ethnic uses of manuka plant parts in tabulated form, based on the above noted published information by Brooker et al. (1981).
Dyeing.
A yellow-green dye is obtained from manuka flowers & a greenish-black dye from the flowers, branches & leaves (Grae 1974). Daniels (1997) throws some light on the use of tannin-rich manuka vegetation which is boiled with the leaves of Phormiun tenax and plunged into mud to make a traditional black dye for bark-cloth & baskets by Maori weavers.
Other properties & applications of Manuka oil.
Spasmolytic effect.
Lis-Balchin & Heart (1998) & Lis-Balchin et. al. (2000) studying the effects of tea tree, manuka & kanuka oils on guinea-pig ileum, skeletal muscle (chick biventer muscle and the rat phrenic nerve diaphragm) and also rat uterus in vitro, noted a spasmolytic effect in smooth muscle for manuka oil, but it is unclear which precise chemotype of manuka oil was tested. Lis- Balchin & Hary considerd a post-synaptic mechanism involving cAMP was implicated in the spasmolytic effect. The authors also warned against the use of all three oils during childbirth based on the in vitro observations on the effects of the essential oils on rat uterus, where they caused a decrease in the force of the spontaneous contractions.
Anti-oxidant effect.
Lis-Balchin et. al. (2000) noted anti-oxidant effects for manuka oil. Anti-oxidant & free radical quenching abilities of various manuka honeys have been investigated (Henriques et al. 2006).
Effects against proteases.
Carr (1998) reported that Manuka can be effective against cysteine proteases implicated in muscle wasting diseases, such as muscular dystrophy, viral replication, tumour invasion etc., building on previous enzyme inhibitory properties shown by manuka (Carr 1991).
Cosmetic uses.
Beta-triketone fractions of manuka oil have been incorporated, with other active ingredients, as components of an anti-dandruff shampoo, based on the alleged fungiostatic properties of the manuka fractions towards Malassezia (:lipophilic yeast) species which proliferate in the scalp sebum.
Manuka is used in fragrances for toiletries in New Zealand's domestic market.
Insecticidal uses.
Leptospermone has previously been shown to have anti-helmintic properties, and to have some synergistic insecticidal properties. A patent has been filed concerning the use of manuka oil against arthropods (Watanabe Keisuke & Sugano Masayo 2003).
References – see Manuka Biblio.
Extra References in Manuka Biblio above:
Allan, H. H. (1961). Flora of New Zealand, Vol. 1. Wellington: DSIR.
Best E. (1905) Polynesian Society Journal 13, 213.