A literature review for Manuka Oil (Leptospermum scoparium)

Manuka oil is the steam distilled oil from the leaves of Leptospermum scoparium (J. R. et G. Forst.) (Stephens et al, 2005). The main commercial suppliers of the high value oil are NZ Manuka and Tairawhiti Pharmaceuticals (Jacobs, 1998; www.naturesremedies.co.nz; Porter and Wilkins (1999); Crop & Food Broadsheet (2000)). The oil is high in a group of terpenoid compounds known as triketones (see the Chemistry Section). The Cawthron Institute tested East Cape manuka oil and concluded that ‘East Cape manuka oil was 20- 30 times more active than Australian tea tree oil for Gram positive bacteria (for example, minimum inhibitory concentration (MIC) Manex oil against for example Staphylococcus aureus 0.078%w/v) ’ (Shell.world/Crest report, testing by Cawthron (Cooke and Cooke, 1994)). Harkenthal et al (1999) also found that manuka oil was more active than tea tree oil (TTO) against Gram positive bacteria.

Manuka oil was more active than TTO, with MIC and MBC (minimum bactericidal concentration) values of 0.06-0.25% and 0.12-0.5%, respectively. The activity of TTO was one or two dilution steps lower than those of manuka oil’.

It has also been confirmed that the East Cape oil is effective in combating bacteria including those associated with acne, foot and body odour (Shell.world/ Crest report). This manuka oil has also been shown to possess other biological activities: antiseptic, antifungal, anti-inflammatory, antihistamine and anti-allergenic properties (Jacobs, 1998).

The article by Barber and Fairhurst, ‘Unlocking the Potential of Manuka (Leptospermum scoparium)’, is a good background summary on manuka covering historical information, the plant, oil, uses in aromatherapy, chemotypes, suggested properties and uses.

Traditional uses: Traditionally ‘tea tree’ had many uses. For example: The leaves were used in vapour baths, for urinary and internal complaints and to reduce fever; a decoction of bark was used for skin diseases etc. (Brooker et al, 1987). Historically, there has been some confusion over the identification of manuka and the closely related kanuka; also the Maori name for manuka is ‘kahikatoa’ which is similar to ‘kahikatea’ (Dacrycarpus dacrydioides), so further confusion may have arisen (Brooker et  al, 1987). Cook’s voyages used manuka leaves to make tea. The seeds were also used e.g. they were chewed for colic and dysentery and a poultice was applied to dry an open wound or running sore (Brooker et al, 1987). The Crop and Food Broadsheet (2000) notes that the leaves were used in vapour baths and to scent toilet oils and the pulped seed capsules were applied as wound dressings. Infusions from the leaves were drunk as a tea supplement for various internal complaints.

Stephens et al (2005) have reviewed the known information about the biology of manuka and concluded that further ‘study of the genetically based differences between the New Zealand populations and the affinity of these populations to Australian populations and other closely allied Australian species’ is required. They also noted that ’improved understanding of the species’ variation will assist both in its conservation roles and economic uses. The need to sustain genetically distinct varieties is emphasised. They note that ‘a revised systematic treatment of L. scoparium would resolve many questions surrounding the species. The regional differences of essential oil profiles and honey non-peroxide antibacterial activity may relate to genetic differences between populations, but this awaits experimental confirmation’. In Stephens and Molan (2008) there is further discussion about the conclusions from a study of morphological characteristics, chemotaxonomic essential oil profiles and population genetics of L. scoparium populations. They list the varieties of L. scoparium that have been determined along with their distribution. The variety that grows on the East Cape is thought to be associated with L. scoparium var. myrtifolium. It is this unnamed variety that has the high levels of triketones (which give the antibacterial properties). The honey associated with this variety is one with medium UMF activity. High levels of triketones are also found in oil from L. scoparium growing in the Marlborough Sounds.

Manuka and kanuka often grow together and are difficult to differentiate morphologically (Porter and Wilkins, 1998; Crop and Food Broadsheet, 2000); Lis-Balchin (2006) suggested that the existence of manuka chemotypes has led to the availability of commercial manuka oils of variable quality (Lis- Balchin (2006) after Perry et al (1997a), Porter and Wilkins (1998)).

Kanuka (Kunzea ericoides) is a similar plant to manuka but chemically they are distinct (Porter and Wilkins, 1998). Both manuka and kanuka grow widely throughout most of NZ (Crop & Food Broadsheet, 2000) but occupy different habitats. Manuka and kanuka are found in lowland to low alpine regions–up to 1800m; but kanuka is not found naturally south of Kawarau Gorge/ Dunedin, in Westland/Fiordland or in Taranaki. It doesn’t like waterlogged soils, whereas manuka is tolerant of these conditions (Perry et al, 1997b; DOC, 2006). Manuka and kanuka probably compete for sites with kanuka failing to grow in the wetter sites whilst on dry sites it is the manuka that eventually dies out as kanuka grows taller and overtops it (Great Barrier Island Trust).

The essential oil yield varies depending upon the location in which it is grown (Perry et al, 1997a). Manex manuka oil has strong activity against Gram positive bacteria whereas kanuka oil has low activity against these bacteria. This has been conclusively proven to be due to the presence of the triketones in some manuka oil (Crop & Food Broadsheet, 2000). Australian tea tree oil (from Melaleuca alternifolia) has many similar properties/uses to manuka oil, but it has a stronger odour and there are differences in the chemistry (it is high in monoterpenes as is kanuka oil whereas manuka oil is high in sesquiterpenes (Lis-Balchin et al, 2000)). Low levels of monoterpenes are found in manuka oil, particularly in the East Cape oil (Crop & Food Broadsheet, 2000).

Oil production

 The Crop & Food Broadsheet (2000) discusses the process of oil production. The green leaves and small branches (<10mm) are cut and then left to wilt (this reduces the moisture content thus making distillation more efficient). The plant material is cut to < 100mm long and packed into the steam distillation vessel. The oil is prepared by steam distillation of the leaves and branchlets (Lis-Balchin (2006) after Arctander (1960)).

Manuka may require a lengthy distillation period to ensure that the maximum amount of β- triketones is extracted. The Crop & Food Broadsheet (2000) notes that ‘between 2 and 6 hours distillation time to extract 80-90% of the oils from manuka because of the heavy oil components (sesquiterpenes). Kanuka, which contains mainly light oil components (monoterpenes), can be distilled in 20-40 minutes’.

Chemistry and Chemotypes

Chemistry

Several papers discuss the chemistry of manuka (and kanuka) oil; much of this work also covers the chemotype studies discussed below (for example: Porter and Wilkins, 1998; Perry et al, 1997a, b).

Three major groups of compounds have been identified in manuka oils: Monoterpenes, sequiterpenes and triketones (Crop & Food Broadsheet, 2000).

The monoterpenes include for example alpha- and beta-pinene. The sesquiterpenes include alpha- and beta- eudesmol and beta- caryophyllene, whilst three major triketones have been identified in manuka oil from East Cape: Flavesone, iso-leptospermone and leptospermone. Several other minor triketones have also been identified (Perry et al, 1997 a and b; Porter et al, 1998; Porter and Wilkins, 1998; Douglas et al, 2001 and 2004), but they are only present in < 0.03% of the Manex oil. One of the minor triketones is grandiflorone.

These beta-triketones (flavesone, leptospermone and isoleptospermone) are of most interest to researchers and aromatherapists (Maddocks-Jennings et al, 2005). A Crop & Food 12 month seasonal study of the triketone-rich East Cape manuka oils showed there were no significant seasonal changes in triketone levels (>20%)(Crop & Food Broadsheet, 2000; Douglas et al, 2004). “This means that commercial producers of antibacterial manuka oils could harvest throughout the year, as long as this was consistent with maximising foliage yield and regrowth of the plants” (Douglas et al, 2004).

Christoph et al (1999) analysed 16 different commercial manuka oils from New Zealand and identified 51 constituents corresponding to about 95% of the oils. Four samples differed distinctly from the others as they contained a lower percentage of triketones. These latter samples contained higher percentages of alpha-pinene compared with the others. Monoterpene hydrocarbons were present only in low concentrations in contrast to kanuka oil, which usually contains about 75% alpha- pinene. Also, terpinen-4-ol and 1,8-cineole which are the most important constituents of the Australian tea tree oil were only detected in negligible amounts in the manuka oils that they tested.

Porter et al (1998) studied the variability in the essential oil chemistry and plant morphology in a L. scoparium population raised from seed from a wild population. They found “no significant grouping of plants on the basis of oil composition, but identified differences between the essential oil contributing most to variation in oil composition in both young and mature plants. The dominant variables that they found were six sesquiterpene components in young plants, and three monoterpenes and two sesquiterpenes in mature plants. Levels of these components differed significantly at the population level between young and mature plants and also within and between seasons.”

Van Vurren et al (2014) studied manuka, kanuka and Leptospermum petersonii that had all been grown in South Africa. They report that the major compounds in manuka were eudesma-4(14)-11 diene, alpha-selinene and (E)-methyl cinnamate. This is a different result to the NZ results (for example Perry et al, 1997a; Porter et al, 1998; Porter and Wilkins,1998; Douglas et al, 2001) but could be similar to one chemotype mentioned in Douglas et al (2004). However, Van Vurren et al concluded that of the eleven chemotypes in Douglas et al (2004), none closely resembled that in their study. The triketones aren’t mentioned in the Van Vurren et al’s list of compounds isolated from manuka. Van Vurren et al refer to Costa et al (2010) who reported the major compounds in manuka as alpha-copaene (36%) and (E)-caryophyllene (13.1%). This illustrates even more variation in composition.

Chemotypes

There are a number of different chemotypes of manuka that have been assigned on the basis of the oil content.

Porter and Wilkins (1998) reported “four groupings based on the presence of distinguishing oil components at significant levels:

  • Triketone rich g. Manex type from East Cape;
  • Linalool and eudesmol rich g. Kaiteriteri, Nelson;
  • Pinene rich, Otago and Woodstock, Canterbury
  • Triketone, linalool and eudesmol deficient, found in a range of North and South Island sites”.

They also proposed a fifth chemotype but considered it to be a variant of the linalool-eudesmol chemotype. They found that density can be used to predict the level of antimicrobial activity in manuka oils.

Crop & Food’s Broadsheet (2000) defined 3 predominant manuka chemotypes throughout NZ (Perry et al, 1997a):

  1. In the far north, the oil has a high pinene content (monoterpene rich)
  2. In the East cape and Marlborough sounds regions, a high triketone chemotype
  3. Oils containing a complex of sesquiterpenes are found over the rest of

They found that in a survey of 43 South Island manuka sites there was much variability both within and between sites, due principally to differences in the ratio of total monoterpenes to total sesquiterpenes. They found that oil yield will vary throughout the year but its composition remains relatively stable.

Douglas et al (2001) described four major chemotypes (plus one minor one) in their North Island study. They are:

  1. Monoterpene rich oils (dominated by alpha- and beta- pinene, myrcene, 1,8 cineole and linalool).
  2. Sesquiterpene rich oils –common chemotype (predominant compounds are gamma-ylangene, alpha copaene, B-caryophyllene, alpha- humulene and alpha- muurolene).
  3. Triketone enriched oils (East Cape) within the sequiterpene rich
  4. Monoterpene- sequiterpene mixed oils –more common north of
  5. Methyl cinnamate enriched oils -2 sites in the western North

Douglas et al (2004) also found that the high triketone chemotype was localised on the East Cape although oils with triketone levels up to 20% were also found in the Marlborough Sounds area. They used cluster analysis and found ten further chemotypes:

  1. Alpha-pinene;
  2. Sesquiterpene –rich with high myrcene;
  3. Sesquiterpene rich with elevated caryphyllene and humulene
  4. Sesquiterpene –rich with and unidentified sesquiterpene hydrocarbon
  5. High geranyl acetate
  6. Sesquiterpene- rich with high gamma-langene+ alpha –copaene and elevated triketones
  7. Sesquiterpene rich with no distinctive components
  8. Sequiterpene rich with high trans-methyl cinnamate
  9. High linalool
  10. Sesquitepene-rich with elevated elemene and selinene

Some of these chemotypes contained aroma compounds at relatively high levels, with a geranyl acetate –rich oil being the most notable. They propose possible origins for this complex array of chemotypes.

Maddocks-Jennings et al (2005) have summarized much of the knowledge to date concerning the significant geographic variation that affects the composition of the oils.

Senanayake (2006) described six major groups of volatile (steam distillable) compounds

  1. Monoterpenes,
  2. Sequiterpene hydrocarbons,
  3. Oxygenated sequiterpenes (excluding eudesmols),
  4. Eudesmols,
  5. Triketones,
  6. Nor-triketones

This study also discusses the presence of 3 groups of non-volatile or semi-volatile compounds (flavonoids, grandiflorone and nor-grandiflorone) which were recognised in the leaf oil study as part of the thesis. Four of these groupings (monoterpenes, sesquiterpene hydrocarbons, oxygenated sequiterpenes and triketones) correspond to those found in conventional steam distilled oils. It was found that active manuka honeys do not appear to be derived uniquely or predominantly from a single leaf oil chemotype. However the high UMF honeys appear to correlate to the eudesmol and triketone groups.

Perry et al (1997a) collected seed from manuka around from the country and grew the plants at Lincoln, thus removing environmental variation from the results. The manuka oil composition was shown to be largely genetically controlled since oil compositions of plants grown at the study site were similar to those plants growing at the seed source site (Douglas et al, 2004).

Antimicrobial Properties of Manuka Oil

Much work has been done on the antimicrobial properties of manuka oil. The following discussion summarises some of this work.

In the Shell .world/ Crest article the results from Cooke and Cooke (1994) are discussed. The article lists 15 bacteria tested against Tairawhiti manuka oil and activity was found in liquid culture against Gram positive bacteria (Staphylococcus aureus 147, S. aureus MRSA NS, S. epidermis, S. faecalis, S. agalactiae, Micrococcus luteus, Sarcina lutea, Bacillus subtilis and Listeria monocytogenes). The oil was not active against the Gram negative bacteria (E. coli, Pseudomonas aeruginosa 997, Klebsiella pneumonia, Proteus vulgaris 996, Vibrio furnissi 2605). There was some activity against Legionella pneumophila. Rhee et al (1997) also showed that manuka oil possesses a selective antibacterial activity against Staphylococcus aureus KCTC 1916. From their results they comment that manuka oil can be used as a potent antibacterial agent against S. aureus KCTC 1916 (MIC 3.05ug/ml). They found that the manuka oil was active against Gram positive bacteria (S. aureus and Micrococcus luteus) but that there was negligible activity against Gram negative bacteria such as Pseudomonas aeruginosa, Escherichia coli (E. coli), Klebsiella pneumonia or Proteus vulgaris. Harkenthal et al (1999) also got similar results and showed that manuka oil was more active against Gram positive bacteria than Tea tree oil. Both Tea tree oil (Melaleuca alternifolia) and manuka oil demonstrated a very good antimicrobial efficacy against various antibiotic-resistant Staphylococcus species. They also showed that Pseudomonas aeruginosa was resistant to these oils.

Williams et al (1998) found that ‘a selected manuka oil’ had strong antimicrobial activity against S. aureus. They tested a variety of essential oils and noted that the relative antimicrobial activity varied depending upon the microorganism under test

Lee et al (2013) comment that ‘there are many differences among experiments, such as in the methods (agar, diffusion or broth dilution), the performance units of the minimum inhibitory concentration (MIC), and in minimum bactericidal concentration (MBC) values (percentage or mg/ml). However, there is no systematic assessment method available to explore the antibacterial effects of each component’. This highlights the difficulties associated with comparing results from different laboratories. Harkenthal et al, (1999) also raise the problem of testing essential oils in aqueous media and offer some solutions.

The Crop & Food Broadsheet (2000) (also, Lis-Balchin, 2006) reports that the antimicrobial activity of manuka oil is determined largely by the proportion of triketones in the oil. Porter and Wilkins (1999) tested the Manex oil and found that it inhibited the growth of a range of test microorganisms including Staphylococcus aureus (and S. aureus MRSA), E. coli, Pseudomonas aeruginosa and Candida albicans. They showed that the polar fraction consisted largely of three components (comprising > 90%) and retained all of the biological activity. This supports what Perry et al (1997a) reported. The lack of activity in the non- polar fraction, which contains the remaining components of the oil (the non-polar fraction contained mainly sesquiterpene hydrocarbons), suggests that the activity is associated with the polar triketones (Porter and Wilkins, 1999). Perry et al (1997a) showed that the oil from the East Cape region had the strongest antimicrobial activity against Bacillus subtilis and Trichophyton mentagrophytes.

Van Klink et al (2005) synthesised the triketones and tested the activity of the pure chemicals. The natural mix of triketones is known to be responsible for the antimicrobial activity (i.e. against Gram positive bacteria and also dermatophytes), including activity against MRSA (Porter and Wilkins, 1998; Rhee et al, 1999; Kim et al, 2011; Christoph et al, 1999, Christoph et al, 2000). Van Klink et al (2005) showed that leptospermone and isoleptospermone had similar activity against MRSA and were more active than flavesone. They found that another natural minor triketone present in manuka (grandiflorone) was the most active of the natural triketones. Their synthetic analogues were active against MRSA, VRE (vancomycin-resistant) Enterococcus faecalis and MDR-TB (multi-drug resistant Mycobacterium tuberculosis). As with the natural compounds, the synthetic analogues were only active against Gram positive bacteria. They found that the compounds could disrupt the cytoplasmic membrane, but the mechanism is unknown. They suggest that the cell membrane of the Gram negative bacteria may serve as a barrier to triketones.

Christoph et al (2001a) found that manuka oil and the β-triketone complex lacked bactericidal properties. They concluded that their effectiveness against Gram positive bacteria is due to bacteriostatic effects.

In Lis-Balchin et al’s (2000) study they compared Melaleuca, manuka and kanuka oils. However the source of the manuka oil is not detailed; therefore its chemotype is unknown. They found considerable variation in the manuka results (and kanuka), whereas Melaleuca remained more constant. They also found that whilst alpha- terpineol had very low antibacterial activity as did alpha terpinene (one potent activity), terpinen-4-ol showed the highest activity against the bacteria tested. 1,8- Cineol showed reduced activity against most of the species. They saw variability in the manuka and kanuka oils and so cautioned their usage. This shows the importance of the identity /source of the oil when assigning, for example, antibacterial properties to the oils. Lis-Balchin’s review of manuka oil (2006) notes that the oil to be particularly effective against Gram positive bacteria e.g. Staphylococcus and Streptococcus.

Sojka’s (2005) results support the conclusion that manuka oil and the triketones exhibit strong antimicrobial efficacy (bacteriostatic) against Gram positive bacteria. No effects were observed against Gram negative bacteria. She suggests that it is the lipophilic character of the triketones that seems to be decisive for the intensity of the antimicrobial activity, this is because a direct correlation between the length of the side chain and the intensity of the antimicrobial activity could be found.

Jeong et al (2009) showed that the essential oil from manuka seeds (obtained from Tairawhiti Pharmaceuticals, NZ) inhibited the growth of Clostridium difficile and Clostridium perfringens but didn’t inhibit the growth of Bifidobacterium breve, B. longum and E. coli or Lactobacillus casei. They showed that the active component of the seeds was leptospermone. They also tested derivatives of leptospermone and found that one, 1,2,3-cyclohexanetrione-1,3-dioxime exerted strong inhibition against C. perfringens and moderate inhibition against C. difficile. From their results they suggest that cyclohexane containing the 1,3-dihydroxyl ketones is responsible for the antibacterial effects of L. scoparium. The results also indicate that the cyclohexanetrione group is required for triketone derivatives to inhibit the growth of C. difficile and C. perfringens. They suggest that these compounds could be useful in the development of new agents for the specific control of the harmful intestinal bacterial C. difficile and C. perfringens.

Costa et al (2010) used a “commercially available” manuka oil in their study and found it effective against B. subtilis. They also tested oils from Ravensara aromatica and Cinnamon camphora and found these oils to be more effective than the manuka oil against S. aureus. The manuka oil did contain leptospermone and isoleptospermone. Activity of terpinen-4-ol against S. aureus was low.

Sulaiman (2011) (PhD thesis (taken from the abstract)) found that Leptospermum scoparium essential oil showed a marked inhibitory activity against Gram positive bacteria, including S. aureus MRSA, Mycobacterium phlei and Bacillus subtilis. Gniewosz et al (2012) compared the antimicrobial activity of manuka and kanuka essential oils. They showed that manuka had more effective antimicrobial properties than kanuka oil. As above, both oils were active especially against Gram positive bacteria S. aureus and B subtilis.

Chen et al (2014) concluded from their work that manuka essential oil (and kanuka oil) had potent antimicrobial properties against Staphylococcus aureus, Streptococcus sobrinus, Streptococcus mutans and Escherichia coli. Chen et al (2014) do not discuss the chemistry of the compounds involved, so the reason for this observed activity is not known.

Van Vuuren et al (2014) also found that manuka and kanuka oil had antimicrobial properties but that they were slightly less effective than Leptospermum petersonii oil. The microorganisms tested in experiments where manuka oil showed potential activity were Mycobacterium smegmatis, Streptococcus pyogenes, Streptococcus agalactiae, Brevibacterium brevis, B. agri and B. laterosporum; Moraxella catarrhalis and Cryptococcus neoformans. In their trials the oil was less active against S. aureus, Staphylococcus epidermis, Enterococcus faecalis, Streptococcus pneumoniae, Klebsiella pneumonia, Pseudomonas aeruginosa and C. albicans. In particular, Van Vurren et al (2014) consider that the activity against Brevibacteria and S. agalactiae warrants further investigation.

Brevibacterium are not pathogenic, but are closely linked to microorganisms that are responsible for foot odour. The plants in this study were grown in South Africa.

Prosser et al’s (2014) results indicate the potential for the use of manuka leaf water soluble extracts as these were shown to inhibit pathogenic soil microorganisms. The extract significantly inhibited E. coli (lux biosensor), as well as inhibiting the growth of Salmonella typhimurium, E coli 0157, Clostridium perfringens, and Listeria monocytogenes.

An IRL report (2000) suggested the possible use of manuka oil in air filters for air conditioning systems to give antimicrobial / anti-infective properties and to stop the spread of airborne diseases.

Animal Health

Song et al (2013) found that manuka oil has the potential to be a useful therapeutic option for treating superficial infections caused by MRSP and MSSP (Methicillin-susceptible Staphylococcus pseudointermedius) in dogs. The manuka oil was from Honey Collection, Marlborough, NZ. It was also shown to inhibit biofilm formation by clinical Staphylococcus pseudointermedius strains. In vivo trials have not been done to determine whether there is clinical improvement or cure of dogs with bacterial skin infections. However, there is the potential for manuka oil to be used as a therapeutic antimicrobial agent for controlling Staphylococcus infections, including those due to MRSP.

Anti – acne

There are many references to anti-acne treatments in natural product promotional material (Manuka oil reviews; Sebocalm; manukaoil.de; manukanatural; Jacobs (1998) etc.) for manuka oil, but little scientific data is available. Magin et al (2006) note that tea tree oil and manuka honey have been used in skin treatment including for acne, but there is no reference to manuka oil. Yarnell and Abascal (2006) mention tea tree but not manuka oil for acne.

The only scientific paper studying acne that has been found is from Kim et al (2011) who studied the antimicrobial and anti-acne effects of manuka oil. From the abstract and tables in this paper (remainder of paper is in Korean) the results showed that there was significant improvement in the acne after treatment with manuka oil, compared with a control group. They suggest that manuka oil helps through its antibacterial activities and so it would be a useful ingredient for the development of anti-acne cosmetics. There was significant improvement in acne skin in both objective and subjective parameters (total microbial number in face, sebum content, erythema index, melanin index, pore index) compared with the control group. A thesis on the antimicrobial effects of manuka and kanuka honey against the growth of Propionibacterium acnes ATCC 6919 also showed that manuka tree oil had the bactericidal ability to kill the bacterium (Wu, 2011); bacteriostatic activity was also observed.

Carr (1998) writes that practical trials on manuka oil have shown that it is effective against acne (plus some other infective agents) but no details are given. Barber and Fairhurst suggest the use of manuka oil mixed with orange flower water, witch hazel, grapefruit juice and geranium oil as a treatment for very oily skin conditions and acne, and use as a daily skin toner.

Dental

Several papers have been written about the use of manuka oil in dental hygiene. Maddocks-Jennings et al (2009) evaluated the effects of an essential oil mouthwash on radiation induced mucositis of the oropharyngeal area during treatment for head and neck cancers. A gargle containing 1:1 manuka and kanuka oils was shown to have a delayed onset of mucositis and reduced pain and oral symptoms. Further work is required, but if successful it could be a ‘simple, yet effective treatment for a condition which causes considerable discomfort and for which there is currently no definitive treatment’. Lauten et al (2005) tested a mouth wash containing manuka oil, but further work is required to determine if it would be useful in a mouthrinse. Allaker and Douglas (2009) support the idea of using a plant-based product for dental plaque treatment. Based on results in Takarada et al (2004), manuka oil and tea tree oil (Melaleuca) were shown to inhibit Porphyromonas gingivitis and all of the oils they list inhibited the adhesion of Streptococcus mutans. The results from Takarada et al’s (2004) study showed that amongst the essential oils that they tested manuka oil and tea tree oil in particular had strong antibacterial activity against periodontopathic and cariogenic bacteria. In Takarada (2005) it is considered that essential oils, including manuka oil, can be used in oral health management as they reduce the level of the three major components of halitosis.

Filoche et al (2005) suggest the potential use of essential oils such as manuka oil to reduce the amount of chlorhexidine in mouth washes. The addition of essential oils reduced the amount of chlorhexidine needed by 4 to 10-fold to achieve an equivalent growth inhibition against biofilm cultures. So they conclude that there may be a role for essential oils in the development of novel anti-caries treatments.

Antifungal

Chen et al (2014) studied the effect of manuka (and kanuka) oil on four fungi (Trichosporon mucoides, Malassezia furfur, Candida albicans and Candida tropicalis). They found that kanuka oil was a more potent antifungal agent than the manuka oil and that growth of T. mucoides and M. furfur were the most effectively inhibited. As mentioned above, there is no discussion of the chemistry involved in this paper. The results in Gniewosz et al (2012) agree with those in Chen et al (2014) in that they found that kanuka oil showed a stronger fungistatic activity against Aspergillus niger than manuka oil did – with manuka oil showing low activity and a lack of antifungal properties in the studied concentration range. Costa et al (2010) also found that manuka oil did not inhibit the growth of Aspergillus niger but did inhibit Candida albicans (as did terpinen-4-ol). However, Lee et al (2009) found that manuka oil showed moderate activity against the phytopathogenic fungus Fusarium circinatum.

 Lihandra (2007) tested manuka oil for the antifungal activity against brown and Penicillium rot (blue and green mould) on fruit after harvest. Manuka oil was only effective at high concentrations, whereas lemon and lemongrass showed the greatest potential in the experiments carried out.

Cooke and Cooke (1994) (in Shell.world/Crest) tested manuka oil (Tairawhiti) against Candida albicans 1212, Aspergillus niger, Microsporum canis 90-359, Trichophyton rubrum 90-364 and Trichophyton mentagrophytes 90-196. Manuka oil was most active against Microsporum and Trichophyton. Gniewosz et al (2012) showed that manuka oil was less effective (fungistatic) than kanuka oil against Aspergillus niger.

Christoph et al (2000) attributed the antifungal (it was effective against dermatophytes) properties of manuka oil to the β-triketones. Sulaiman (2011) (PhD thesis (taken from the abstract)) found significant antifungal activity against Trichophyton terrestre with manuka oil.

Mould growth on houses is a big problem and Singh and Chittenden (2010) tested manuka oil against these pathogens, along with other compounds such as eugenol. They tested the manuka oil containing triketones, manuka oil without triketones and a triketone rich fraction. Manuka and its fractions exhibited selected activity against the test moulds. The fraction without triketones did not restrict the test mould. The manuka and triketones fractions were more successful, with manuka oil restricting the growth of Penicillium corylophilum, reducing the growth of Alternaria alternata and virtually stopping the growth of Cladosporium herbarum. The triketone fraction gave enhanced results however, eugenol, cinnamaldehyde, thymol and carvacrol oils also tested in the study inhibited all three fungi at the concentration used.

Gorski et al (2010) investigated the use of essential oils to control the growth of Trichoderma harzianum in cultivated mushroom growing which causes considerable losses to the crop. They concluded that ‘Natural manuka essential oil (Leptospermum scoparium) may be useful in the control of Trichoderma harzianum in the common mushroom (Agaricus bisporus) crop. Under laboratory conditions this oil strongly inhibited mycelium growth and sporulation of the pathogen, similarly as the compared organic fungicide Bravo 500 SC’. They found that manuka oil gave a strong gave a strong inhibitory action reducing the growth of T. harzianum. The manuka oil that they used was produced by Pollena Aroma Co in Warsaw and they do not list any ketones for this oil (they just mention monoterpenes, linalool , sesquiterpenes, Geranyl acetate , trans methyl cinnamate and 1, 8 cineole) or suggest which compounds are active.

Antiviral

Carr (1998) referred to her earlier work (Carr, 1991) where the research indicated that several components of manuka inhibited cysteine proteases which can be involved in muscle wasting diseases, such as muscular dystrophy, viral replication and tumour invasion and metastasis.

Reichling et al (2005) showed that manuka oil inhibited the Herpes simplex -1 and -2 virus when the viruses were pretreated with the oil one hour before cell infection. However, pretreatment of the host cells with the oil before viral infection did not affect plaque formation. After infection, only HSV- 1 replication was significantly inhibited. Flavesone and leptospermone inhibited the virulence of HSV-1 in the same manner as the oil. Manuka oil exhibited high levels of virucidal activity against HSV-1 and drug –resistant HSV-1 isolates in cell-suspension tests, but it was shown to be more cytotoxic than some other oils such as cajuput and clove oils (Schnitzler et al, 2008). Magsombol (2012) summarises the results of Reichling et al (2005) and Schnitzler et al (2008).

Sojka (2005) showed that flavesone and manuka oil were the most active substances against Herpes simplex virus type 1 when in direct contact with the virus, but all of the triketones showed strong antiviral activity. ‘The mechanism seems to be an interaction of the triketones with the virion envelope hindering the adsorption of the virus by the host cell’.

Aromatherapy

Manukaoil.com list many uses for manuka oil including aromatherapy. The oil is said to be beneficial for those who suffer from stress and anxiety, but also it is helpful for the skin especially itching, acne, badly healing skin, inclination to fungal infections, ulcers, bedsores and infections.

Lis-Balchin et al (1996, 2000) tested steam distillate/essential oils of manuka. Manuka oil showed a spasmolytic action on smooth muscle in contrast to the spasmogenic activity of kanuka and Melaleuca. Using the diaphragm, manuka and Melaleuca oils decreased tension and had a delayed contracture while kanuka oil had no effect at the same concentration. On the chick biventer muscle and uterus all 3 oils caused a decrease in the force of spontaneous contractions. From these results they suggest that manuka oil should not be used in pregnancy for aromatherapy. Lis-Balchin and Hart (1998) studied the mode of action of manuka and kanuka oils on guinea-pig ileum and found that both oils induced a spasmolytic effect, but that kanuka produced an initial contraction. They showed that this spasmolytic effect was the result of a post –synaptic mechanism with possibly manuka oil working through c AMP. The mode of action of the kanuka oil is not known. They suggest that these oils could be used as relaxants in aromatherapy. Lis-Balchin et al (2000) note that toxicity testing is also important as this has not been determined. Takarada et al (2004) tested manuka oil on cultured human umbilical vein endothelial cells and found that when the oil was tested at a concentration of 0.2%, it had little effect on cultured cells. Therefore, the oil seems a promising antibacterial substance for use in oral care at a concentration of 0.2% or lower, at which level it had little effect on human cells.

Anti-aging/ antioxidant properties

Kwon et al (2013) evaluated manuka oil for its effect against photoaging in UV-B irradiated hairless mice. They showed that a topical application suppressed the UV-B induced increase in skin thickness and wrinkle grading in a dose- dependent manner. Manuka oil also could suppress UV-B induced skin inflammation by inhibiting the production of inflammatory cytokines. They suggest that it is the presence of antioxidant chemicals and sesquiterpene compounds in manuka oil that underlie the attenuation of cutaneous photoaging. With these results they suggest that their work supports the use of manuka oil in the formulation of skin care and functional cosmetics. Their manuka oil was from Coast Biologicals ltd.

Sojka (2005) found that neither the β-triketones tested nor the triketone complex isolated from manuka oil showed any photo-toxicological properties.

Lis-Balchin et al (1996, 2000) found that the antioxidant activity of the ‘tea tree’ oils studied was variable but the manuka samples were more effective than either one of the kanuka samples or the manuka/kanuka mix. Of the individual compounds tested only gamma-terpinene and terpinen-4-ol showed antioxidant potential. The latter compound was also shown by them to be the most potent antibacterial agent and they suggest that this is possibly through the antioxidant action. No photosensitisation has been reported for manuka oil (Lis-Balchin, 2006).

In a comprehensive review of the biological effects of essential oils, Bakkali et al (2008) note that for a long time ‘essential oils have been used widely but recent work shows that in eukaryotic cells, es- sential oils can act as pro-oxidants affecting the inner cell membranes and organelles such as mito- chondria’. In Section 7: Medicinal and future medical applications they write ‘The cytotoxic capacity of the essential oils based on a pro-oxidant activity can make them excellent antiseptic and antimi- crobial agents for personal use, i.e. for purifying air, personal hygiene, or even internal use via oral

consumption, and for insecticidal use for the preservation of crops or food stocks. A big advantage of essential oils is the fact that they are usually devoid of long-term genotoxic risks. Moreover, some of them show a very clear anti-mutagenic capacity which could well be linked to an anti-carcinogenic activity’.

Anti-inflammatory

Chen et al (2014) investigated suppression or inhibition of inflammatory reactions mediated by manuka and kanuka oils. They found that where the inflammation was caused by microorganism infection both oils had anti-inflammatory effects on LPS (lipopolysaccharide) induced release of TNF- alpha, but had no influence on IL-4. They also found that neither of the oils had stimulatory effects on cytokine release in untreated THP-1 macrophages. Because TNF-alpha release from monocytes/macrophages regulates Th-1 mediated inflammatory responses, short term non-toxic

dosages for the 2 oils may be effective in treating inflammation. In THP-1 cells, both oils lowered tumour necrosis factor –alpha released after lipopolysaccharide stimulation. They suggest that the oils may be effective for treating lesions caused by insect bites or for repairing infected wounds. The fact that the oil did not significantly affect IL-4 release suggests that, in addition to the anti- inflammatory properties, the oils have potential applications as anti-allergenic agents and be effective in human epidermal- related products. Because of their anti-inflammatory properties and their absence of adverse allergic reactions resulting from cytokine release, kanuka and manuka oils may also be effective in human epidermal-related products (Chen et al, 2014).

As mentioned above, Kwon et al (2013) showed that manuka oil could suppress the UV-B induced skin inflammation by inhibiting the production of inflammatory cytokines. Calcabrini et al (2004) showed that Melulacea oil (tea tree) and terpinen-4-ol were able to interfere with the growth of human melanoma cells. Low levels of terpinen-4-ol are present in manuka oil (Manex) (Porter and Wilkins, 1998); however much higher levels were present in kanuka oil (Kanex) so this is not likely to be a role for manuka oil.

Enzymes

Kellam et al (1992) list enzyme activity for Leptospermum scoparium along with the activity for many other native plants that they screened. Carr (1998) refers to work by Kellam et al (pers. comm.) that showed that extracts of manuka showed variable inhibition of several enzymes.

Herbicides

Dayan et al (2007, 2009) have studied the mechanism of action of the β-triketones and found that the triketone fraction and leptospermone were more active than the manuka oil (2007). They have also found that the target site of β-triketone herbicides is p-hydroxyphenylpyruvate. Dayan et al (2011) tested manuka oil (obtained from Clean & Green Trading Co, California), and synthetic leptospermone and showed that the oil exhibited good post growth herbicidal action and also pre- emergent activity. They also showed that leptospermone possesses some soil persistence properties. Further work is needed to explore the bioactivity and selectivity of this as a herbicide. Owens et al (2013) showed that leptospermone is readily taken up by the roots and translocated to the molecular target site. This could also explain the anecdotal observation of allelopathic suppression of plant growth under β-triketone-producing species.

Insecticides

Based on earlier work by others, Brooker et al (1987) reported that leptospermone is an insecticide like valone. They also reported its anthelmintic properties (referred to in Lis-Balchin (2006)).

Some other studies have been carried out to determine if manuka oil and other essential oils are potential insecticides, for example Crook et al (2008) and Domingue et al (2013). Hanula and Sullivan (2008) and Hanula et al (2011) found that manuka oil and its calamenene and alpha-copaene constituents were attractive to redbay ambrosia beetle. Kendra et al (2012) found that while manuka oil was effective against the redbay ambrosia beetle, Phoebe oil was more effective and also stayed active longer than the manuka oil lures. The manuka oil lure remained active for 2-3 weeks compared to 10-12 weeks for Phoebe oil lures. They think that the sesquiterpenes alpha-copaene, alpha- humulene and cadinene could be the primary kairomones used by host seeking females.

Further work by Hanula et al (2013) showed that more research is required to confirm the effect of manuka oil.

Grant et al (2010) showed that manuka oil, when effective, attracted both male and female emerald ash borers; while Domingue et al (2013) found that manuka oil traps caught more adult emerald ash borer beetles than unbaited traps, but other oils such as Phoebe oil were more effective.

Acaricidal activity (Mites): Jeong et al (2009) refer to Jeong’s thesis (Jeong, 2008) in which it was shown that the triketone derivative from L. scoparium seeds exhibit acaricidal activity against Dermatophagoides farinae, D.pteronyssinus and Tyrophagus putrescentiae (Jeong, 2008; Jeong et al, 2009). George et al (2009) also studied the effect of essential oils including manuka oil on poultry mite. Results suggested that the oils were toxic to the mite by fumigant action. George et al (2010) found that manuka oil (the oil was from New Zealand but no chemistry detailed/or source of oil given) was effective against both juveniles and adult mites, but did not have ovicidal activity.

Patents: Many patents have been written that include manuka oil in the composition. Usually manuka oil is one of a selection of essential oils that is being tested. A list of patents is in Appendix 1.

Key bioactive compounds

Triketones: Antimicrobial against Gram + bacteria; antihelmintic; acaricidal; herbicidal; effective against mould on walls; antiviral.

alpha- Terpineol , alpha-terpinene and terpinene-4-ol showed some antibacterial activity especially terpinen-4-ol (Lis-Balchin et al, 2000) – but these compounds are present in low levels in manuka oil. Costa et al (2010) found that Candida albicans was inhibited by terpinen-4-ol. Gamma-terpinene and terpinen-4-ol showed antioxidant potential. Terpinen-4-ol was able to interfere with the growth of human melanoma cells. Low levels of terpinen-4-ol are present in manuka oil (Manex oil). Lis-Balchin et al (1996, 2000) found that the antioxidant activity of the ‘tea tree’ oils studied was variable but the manuka samples were more effective than either one of the kanuka samples or the manuka/kanuka mix. Of the individual compounds that they tested only gamma-terpinene and terpinene-4-ol showed antioxidant potential. The latter compound was also shown by them to be the most potent antibacterial agent in their study and they suggest that this is possibly through the antioxidant action (Lis-Balchin et al, 2000).

Gorski et al (2010) investigated the use of essential oils to control the growth of Trichoderma harzianum in cultivated mushroom growing which causes considerable losses to the crop. They found that manuka oil gave a strong gave a strong inhibitory action reducing the growth of T. harzianum. The manuka oil that they used was produced by Pollena Aroma Co in Warsaw and they do not list any ketones for this oil or suggest which compounds are active (they just mention monoterpenes, linalool, sesquiterpenes, Geranyl acetate, trans methyl cinnamate and 1, 8 cineole).

Potential uses

  1. Blending manuka oil or its triketone fraction with other oils g. Lema oil and tea tree oil (Christoph, 2001a and b).
  2. Manuka oil and particularly the triketone rich oils may have potential use in anti- acne treatment as well as many other antimicrobial roles, anti-viral products, anti-inflammatory products, dental –mouth washes, herbicides and insecticides – especially anti-mite treatments.
  3. Jacobs (1998) listed flu inhalation treatments, antiseptics, sunburn creams, anti-Candida preparations and for the control of Legionnaires disease in air conditioning plants as possible future products that they were currently testing. At that time they had soaps, shampoos, skin care creams, acne treatments, toothpastes, lip creams, cow udder lotions, foot deodorant creams, athletes foot preparations, pet skin care products, muscle and joint pain relief products and aromatherapy preparations on the
  4. Douglas et al (2001) conclude their report noting that “apart from the triketone rich oils, there is a need to ascertain the commercial potential of oils with limited biological They may have potential productive uses as teas or fragrance or aroma extracts and compounds”. They note that geranyl acetate and methyl cinnamate oils have commercial potential as they are used in the flavour industry.

 

Conclusions

Manuka oil, from the leaves and stems of Leptospermum scoparium, is a complex mixture of mono- and sesquiterpenes. In the past, there was some confusion over the identification of manuka and kanuka (Kunzea ericoides), however there are several morphological differences between these plants and the chemistry of the oils is different. Their oils have different properties, for example, manuka oil is a better antibacterial product against Gram positive bacteria whereas kanuka oil seems more effective against some fungi.

Most of the information in the literature associated with the activity of manuka oil mentions the β- triketones. These compounds are found predominantly in the East Cape manuka oil and also in oil from the Marlborough Sounds. It is these compounds that give the oil its antibiotic properties amongst others.

The chemistry of manuka oil has been found to be variable and much research has gone into understanding this observation; between four and ten chemotypes have been described for manuka growing in New Zealand. It has been noted that the quality of Australian tea tree oil has generally been stabilized, mainly by the selection of clones and the blending of different essential oils to conform to the Australian Standards.

In terms of the relationship between the oils and the active manuka honey it has been found that the active manuka honeys do not appear to be derived uniquely, or predominantly, from a single leaf oil chemotype; however, the high UMF honeys appear to correlate to the eudesmol and triketone groups.

There has been much research in this field, but frequently the research has not tied the chemistry of the oil in with the biological effects, so it is not always clear if the triketones are responsible for most of the activity or if the other sesquiterpenes and/or monoterpenes (and other compounds) are also active principles. Unless the origin of the oil is known (e.g. Tairawhiti manuka oil), then without a chemical analysis, the chemistry of the oil being tested is also unknown.

The oil is associated with a wide variety of bioactivity, although the most common is the antimicrobial activity (especially vs. Gram +ve bacteria). Other activities discussed above include anti-inflammatory, anti-aging, insect activity, anti-acne, dental use and antiviral activity.

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