Compendium of the most significant studies with reference to properties, intake, effects.

Terpinen-4-ol is a constituent of the essential oil of Melaleuca alternifolia, has remarkable antimicrobial and anti-inflammatory properties, accelerates the healing process of wounds and exhibits anti-skin cancer activity (1).

Another application is to combat parasitic lice infection in large animals (2).

Regarding its activity against skin tumours, we have this detailed study with topical use tested on animals in 2012 (3).

Its efficacy against psoriasis was also proven in a 2012 study (4).

It is therefore an oil that has many applications in different areas due to its anti-inflammatory properties.

Mondello F, Fontana S, Scaturro M, Girolamo A, Colone M, Stringaro A, Vito MD, Ricci ML. Terpinen-4-ol, the Main Bioactive Component of Tea Tree Oil, as an Innovative Antimicrobial Agent against Legionella pneumophila. Pathogens. 2022 Jun 14;11(6):682. doi: 10.3390/pathogens11060682. 

Abstract. Legionella pneumophila (Lp), responsible for a severe pneumonia called Legionnaires' disease, represents an important health burden in Europe. Prevention and control of Lp contamination in warm water systems is still a great challenge often due to the failure in disinfection procedures. The aim of this study was to evaluate the in vitro activity of Terpinen-4-ol (T-4-ol) as potential agent for Lp control, in comparison with the essential oil of Melaleuca alternifolia (tea tree) (TTO. Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of T-4-ol were determined by broth micro-dilution and a micro-atmosphere diffusion method to investigate the anti-Lp effects of T-4-ol and TTO vapors. Scanning Electron Microscopy (SEM) was adopted to highlight the morphological changes and Lp damage following T-4-ol and TTO treatments. The greatest antimicrobial activity against Lp was shown by T-4-ol with a MIC range of 0.06-0.125% v/v and MBC range of 0.25-0.5% v/v. The TTO and T-4-ol MIC and MBC decreased with increasing temperature (36 °C to 45 ± 1 °C), and temperature also significantly influenced the efficacy of TTO and T-4-ol vapors. The time-killing assay showed an exponential trend of T-4-ol bactericidal activity at 0.5% v/v against Lp. SEM observations revealed a concentration- and temperature- dependent effect of T-4-ol and TTO on cell surface morphology with alterations. These findings suggest that T-4-ol is active against Lp and further studies may address the potential effectiveness of T-4-ol for control of water systems.

Di Campli E, Di Bartolomeo S, Delli Pizzi P, Di Giulio M, Grande R, Nostro A, Cellini L. Activity of tea tree oil and nerolidol alone or in combination against Pediculus capitis (head lice) and its eggs. Parasitol Res. 2012 Nov;111(5):1985-92. doi: 10.1007/s00436-012-3045-0. 

Abstract. Head lice infestation is an emerging social problem in undeveloped and developed countries. Because of louse resistance increasing, several long-used insecticidal compounds have lost their efficacy, and alternatives, such as essential oils, have been proposed to treat this parasitic infestation. The present study investigated the efficacy of two natural substances: tea tree (Melaleuca alternifolia) oil and nerolidol (3,7,11-trimethyl-1,6,10-dodecatrien-3-ol) against lice and its eggs. Products were used alone and in combination (ratio 1:1 and 1:2) from 8 % dilution. The in vitro effect of natural substances at different concentrations were evaluated against 69 head lice (adults and nymphs) and 187 louse eggs collected from school children in Chieti-Pescara (Central Italy) over a 6-month period. The lice mortality was evaluated for 24 h by a stereo light microscope. The ovicidal activity was monitored by microscopic inspections for 15 days. Tea tree oil was more effective than nerolidol against head lice with 100 % mortality at 30 min and 1 % concentration. On the contrary, nerolidol expressed a more pronounced ovicidal activity inducing the failure of 50 % of the eggs to hatch at 1 % concentration after 4 days; the same effect was achieved by using a twice concentration of tea tree oil. The association of the two substances both in ratios 1:1 and 1:2 combined efficaciously their insecticidal and ovicidal effect; in particular, the ratio 1:2 (tea tree oil 0.5 % plus nerolidol 1 %) acted producing both the death of all head lice at 30 min and the abortive effect of louse eggs after 5 days. These results offer new potential application of natural compounds and display a promising scenario in the treatment of pediculosis resistant cases. The development of novel pediculicides containing essential oils could be, in fact, an important tool to control the parasitic infestation.

Carson CF, Hammer KA, Riley TV. Melaleuca alternifolia (Tea Tree) oil: a review of antimicrobial and other medicinal properties. Clin Microbiol Rev. 2006 Jan;19(1):50-62. doi: 10.1128/CMR.19.1.50-62.2006.

Abstract. Complementary and alternative medicines such as tea tree (melaleuca) oil have become increasingly popular in recent decades. This essential oil has been used for almost 100 years in Australia but is now available worldwide both as neat oil and as an active component in an array of products. The primary uses of tea tree oil have historically capitalized on the antiseptic and anti-inflammatory actions of the oil. This review summarizes recent developments in our understanding of the antimicrobial and anti-inflammatory activities of the oil and its components, as well as clinical efficacy. Specific mechanisms of antimicrobial and anti-inflammatory action are reviewed, and the toxicity of the oil is briefly discussed.

Silva RAD, Silva NBS, Martins CHG, Pires RH, Röder DVDB, Pedroso RDS. Combining Essential Oils with Each Other and with Clotrimazole Prevents the Formation of Candida Biofilms and Eradicates Mature Biofilms. Pharmaceutics. 2022 Sep 5;14(9):1872. doi: 10.3390/pharmaceutics14091872.

Abstract. Fungal infections by Candida spp. are opportunistic and most often occur in individuals with some predisposing factor. Essential oils (EO) have anti-Candida potential, being a therapeutic alternative to be explored, especially for superficial and mucosal candidiasis. The objective was to analyze the synergistic potential between the EO of Citrus limon, Cupressus sempervirens, Litsea cubeba and Melaleuca alternifolia, and each of them with clotrimazole, to inhibit in vitro the formation and eradication of Candida spp. biofilms. Added to this, the survival of Caenorhabditis elegans was evaluated after exposure to EO, clotrimazole and their synergistic combinations. Anti-Candida activity was determined by microdilution for the substances alone and in EO-EO and EO-clotrimazole combinations. The combinations were performed by the checkerboard method, and the reduction in the metabolic activity of biofilms was determined by the viability of MTT/menadione. C. elegans larvae survival was evaluated after 24 h of exposure to EO, clotrimazole and synergistic combinations. The minimum inhibitory concentration (MIC) of EO ranged from 500 to >4000 µg/mL. The lowest MIC (500 µg/mL) was for C. sempervirens and L. cubeba on a C. krusei isolate; for clotrimazole, the MIC ranged from 0.015 to 0.5 µg/mL. Biofilm inhibition and eradication both ranged from 1000 to >4000 µg/mL. The lethal concentration (LC50) of C. limon, L. cubeba and M. alternifolia was 2000 µg/mL for C. elegans, while for C. sempervirens and clotrimazole, it was not determined within the concentration limits tested. In combination, more than 85% of the larvae survived M. alternifolia-clotrimazole, M. alternifolia-L. cubeba, C. sempervirens-clotrimazole and C. sempervirens-C. limon combinations. This study is the first, to our knowledge, to present a synergistic relationship of EO-EO and EO-clotrimazole combinations on Candida spp. biofilms.

References______________________________________________________________

(1) Pazyar N, Yaghoobi R, Bagherani N, Kazerouni A. A review of applications of tea tree oil in dermatology. Int J Dermatol. 2013 Jul;52(7):784-90. doi: 10.1111/j.1365-4632.2012.05654.x.

(2) Ellse L, Burden FA, Wall R. Control of the chewing louse Bovicola (Werneckiella) ocellatus in donkeys, using essential oils. Med Vet Entomol. 2013 Dec;27(4):408-13. doi: 10.1111/mve.12004. 

Melaleuca Alternifolia Tea Tree oil (TTO) is an essential oil steam-distilled from a plant that originates in Australia, Melaleuca alternifolia belonging to the Myrtaceae family.

It appears as a yellowish oily liquid.

It has been shown to be non-toxic, non-irritating and non-corrosive to humans only at the doses set by medical warnings (1).

Human health

TTO exhibits a wide spectrum of anti-bacterial, anti-viral, and anti-inflammatory activities. Its wide range of antimicrobial activity is mainly due to its diversity of components, especially its volatile constituents. The volatile compounds in TTO are mainly 1,8-cineole, terpinen-4-ol, and α-terpilenol (2).

In particular, 1,8-cineole has been shown to destroy the cell menbrane of E. coli, while terpinen-4-ol exhibits strong antibacterial, disinfection, and anti-corrosive effects. α-terpilenol has been demonstrated to feature suitable permeability and exhibits a killing effect on common pathogens such as Staphylococcus aureus, E. coli, pseudomonas aeruginosa, candida aibicans, etc.(3).

TTO is not harmful to the human body and has a suitable, natural anti-bacterial effect; thus, it may be used as an anti-bacterial agent in food.
Several studies have suggested the uses of TTO for the treatment of acne vulgaris, seborrheic dermatitis, and chronic gingivitis. It also accelerates the wound healing process and exhibits anti‐skin cancer activity (4).

Agriculture

Botrytis cinerea, one of the most destructive fungal pathogens, causing gray mold rot in a wide range of fresh fruits and vegetables. Although chemical fungicides are widely used to control the incidence of the disease, this practice potentially introduces harmful substances into the food chain, and also selects for B. cinerea strains with increased drug resistance. This study found that important metabolic pathways, including glycolysis, the TCA cycle, and purine metabolism, were compromised by TTO treatment, while Cytochrome c (a hemoglobin located in the inner mitochondrial membrane, and is responsible for transferring electrons between mitochondrial electron transport chain complexes) increased. We conclude that the disruption of energy metabolism by TTO contributes to its antifungal activity against B. cinerea (5).

The most relevant studies on this ingredient have been selected with a summary of their contents:

Melaleuca Alternifolia studies

CTypical commercial product characteristics Melaleuca Alternifolia Tea tree oil

• Molecular Formula      C₂₈H₆₀O₄P₂S₄Zn
• Molecular Weight 716.352 g/mol
• CAS:   68647-73-4
• EC Number: 232-293-8     
• UNII     VIF565UC2G
• DSSTox Substance ID  DTXSID6028789      DTXSID60100611      DTXSID30874014
• IUPAC  zinc;diheptoxy-sulfanylidene-sulfido-λ⁵-phosphane
• InChl=1S/2C14H31O2PS2.Zn/c2*1-3-5-7-9-11-13-15-17(18,19)16-14-12-10-8-6-4-2;/h2*3-14H2,1-2H3,(H,18,19);/q;;+2/p-2
• InChl Key      ZKAQFYDDTYGBBV-UHFFFAOYSA-L
• SMILES  CCCCCCCOP(=S)(OCCCCCCC)[S-].CCCCCCCOP(=S)(OCCCCCCC)[S-].[Zn+2]
• MDL number  MFCD00132409
• PubChem Substance ID    
• RTECS  RJ3697600
• NCI    C74295
• C74295     69627 
• FEMA Number: 3902

Synonyms:

• Melaleuca alternifolia Oil
• Phosphorodithioic acid, O,O-di-C1-14-alkyl esters, zinc salts
• Oil, Melaleuca alternifolia
• TEA TREE OIL
• Oil, Tea Tree
• Tea Tree Oil
• Dialkyl(C1-C14)dithiophosphoric acid, zinc salt
• SCHEMBL164653
• zinc diheptoxy-sulfido-thioxo-$l^{5}-phosphane
• Zinc Dialkylphosphorodithiloate
• AN-38072

References__________________________________________________________________

(1) Hammer KA, Carson CF, Riley TV. In vitro activity of Melaleuca alternifolia (tea tree) oil against dermatophytes and other filamentous fungi. J Antimicrob Chemother. 2002 Aug;50(2):195-9. doi: 10.1093/jac/dkf112.

Abstract. The in vitro activity of Melaleuca alternifolia (tea tree) oil against dermatophytes (n = 106) and filamentous fungi (n = 78) was determined. Tea tree oil MICs for all fungi ranged from 0.004% to 0.25% and minimum fungicidal concentrations (MFCs) ranged from <0.03% to 8.0%. Time-kill experiments with 1-4 x MFC demonstrated that three of the four test organisms were still detected after 8 h of treatment, but not after 24 h. Comparison of the susceptibility to tea tree oil of germinated and non-germinated Aspergillus niger conidia showed germinated conidia to be more susceptible than non-germinated conidia. These data demonstrate that tea tree oil has both inhibitory and fungicidal activity.

(2) Lin G, Chen H, Zhou H, Zhou X, Xu H. Preparation of Tea Tree Oil/Poly(styrene-butyl methacrylate) Microspheres with Sustained Release and Anti-Bacterial Properties. Materials (Basel). 2018 May 1;11(5):710. doi: 10.3390/ma11050710.

Abstract. Using butyl methacrylate (BMA) and styrene (St) as monomers and divinylbenzene (DVB) as a crosslinking agent, P(St-BMA) microspheres were prepared by suspension polymerization. Tea tree oil (TTO) microspheres were prepared by adsorbing TTO on P(St-BMA) microspheres. The structure and surface morphology of P(St-BMA) microspheres and TTO microspheres were characterized by Fourier transformed infrared spectroscopy (FTIR), optical microscopy, and Thermogravimetric analysis (TGA). In doing so, the structural effect of P(St-BMA) microspheres on oil absorption and sustained release properties could be investigated. The results show that the surface of the P(St-BMA) microspheres in the process of TTO microsphere formation changed from initially concave to convex. The TTO microspheres significantly improved the stability of TTO, which was found to completely decompose as the temperature of the TTO increased from about 110 &deg;C to 150 &deg;C. The oil absorption behavior, which was up to 3.85 g/g, could be controlled by adjusting the monomer ratio and the amount of crosslinking agent. Based on Fickian diffusion, the sustained release behavior of TTO microspheres was consistent with the Korsmeyer-Pappas kinetic model. After 13 h of natural release, the anti-bacterial effect of the TTO microspheres was found to be significantly improved compared to TTO.

(3) Thomas J, Carson CF, Peterson GM, Walton SF, Hammer KA, Naunton M, Davey RC, Spelman T, Dettwiller P, Kyle G, Cooper GM, Baby KE. Therapeutic Potential of Tea Tree Oil for Scabies. Am J Trop Med Hyg. 2016 Feb;94(2):258-266. doi: 10.4269/ajtmh.14-0515. 

(4)  Pazyar N, Yaghoobi R, Bagherani N, Kazerouni A. A review of applications of tea tree oil in dermatology. Int J Dermatol. 2013 Jul;52(7):784-90. doi: 10.1111/j.1365-4632.2012.05654.x.

(5) Xu J, Shao X, Wei Y, Xu F, Wang H. iTRAQ Proteomic Analysis Reveals That Metabolic Pathways Involving Energy Metabolism Are Affected by Tea Tree Oil in Botrytis cinerea. Front Microbiol. 2017 Oct 12;8:1989. doi: 10.3389/fmicb.2017.01989.

Abstract. Tea tree oil (TTO) is a volatile essential oil obtained from the leaves of the Australian tree Melaleuca alternifolia by vapor distillation. Previously, we demonstrated that TTO has a strong inhibitory effect on Botrytis cinerea. This study investigates the underlying antifungal mechanisms at the molecular level. A proteomics approach using isobaric tags for relative and absolute quantification (iTRAQ) was adopted to investigate the effects of TTO on B. cinerea. A total of 718 differentially expression proteins (DEPs) were identified in TTO-treated samples, 17 were markedly up-regulated and 701 were significantly down-regulated. Among the 718 DEPs, 562 were annotated and classified into 30 functional groups by GO (gene ontology) analysis. KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis linked 562 DEPs to 133 different biochemical pathways, involving glycolysis, the tricarboxylic acid cycle (TCA cycle), and purine metabolism. Additional experiments indicated that TTO destroys cell membranes and decreases the activities of three enzymes related to the TCA cycle. Our results suggest that TTO treatment inhibits glycolysis, disrupts the TCA cycle, and induces mitochondrial dysfunction, thereby disrupting energy metabolism. This study provides new insights into the mechanisms underlying the antifungal activity of essential oils.