Backhousia citriodora

Backhousia citriodora (Lemon Myrtle): A Comprehensive Reference

1. Identity and Botanical Classification

1.1 Nomenclature and Taxonomy

Backhousia citriodora F. Muell. was formally named in 1845 by botanist Ferdinand von Mueller; the genus name honors the English botanist James Backhouse, and the species epithet derives from the distinctively strong lemon scent of the foliage. The genus Backhousia, belonging to the family Myrtaceae, is endemic to eastern Australia and is a close relative of the genus Choricarpia, with which it forms the Backhousia alliance.

The primary common name "Lemon-scented myrtle" was shortened to "lemon myrtle" for the native foods industry to market the leaf for culinary use; "Sweet Verbena Myrtle" and "Lemon Ironwood" are also common names. The species is a flowering plant in the family Myrtaceae, native to the subtropical rainforests of central and south-eastern Queensland, Australia, with a natural distribution from Mackay to Brisbane.

The genus Backhousia comprises trees and shrubs that predominantly grow in eastern Queensland and New South Wales; there are now 13 described species, including two previously designated to the genus Choricarpia, and excluding B. anisata, which has been shifted to the Syzygium genus on the basis of phylogenetic evidence. The leaves of all Backhousia species are rich in essential oils and their aromatic profiles have been reported, though other than B. citriodora, none have attracted much attention from aromatherapists or the bushfood industry.

1.2 Chemotypes

B. citriodora has two chemotypes, and a distinction must be made between the citral chemotype and the L-citronellal chemotype. The chemotype high in L-citronellal (80% or more) does not conform to the Australian standard and therefore cannot be marketed as lemon myrtle oil.

1.3 Morphology

The species can reach 6 m (20 ft) in height, but is often smaller. The leaves are evergreen, opposite, lanceolate, 5–12 cm (2.0–4.7 in) long and 1.5–2.5 cm (0.6–1.0 in) broad, glossy green, with an entire margin. The flowers are creamy-white, 5–7 mm (0.20–0.28 in) in diameter, produced in clusters at the ends of the branches from summer through to autumn.

1.4 Standardization

In 2001, Standards Australia's CH21 Essential Oil Committee elaborated a monograph entitled "Oil of Backhousia citriodora, citral type (lemon myrtle oil)", AS 4941-2001. The leaf oil of B. citriodora was first described by the firm of Schimmel & Co of Miltitz, Germany in 1888, and was reported to be almost entirely (95%) citral. This was confirmed in 1923 by Penfold, and it has since become the source of a commercial industry for supply of geranial/neral.

1.5 Common Preparations and Dosage Forms

The leaf is often used as dried flakes or in the form of an encapsulated flavour essence for enhanced shelf life. It has a range of uses, such as lemon myrtle flakes in shortbread, flavouring in pasta, whole leaf with baked fish, infused in macadamia or vegetable oils, and made into tea including tea blends. The leaf of the lemon myrtle is steam distilled to produce the essential oil (EO). The essential oil and hydrosol are used in a wide range of household and toiletry products, including hand and body washes, hair and oral care, deodorant, foot spray, surface disinfectant, detergent and spritzer.

2. Traditional and Historical Use

2.1 Aboriginal Australian Use

Aboriginal Australians have long used lemon myrtle, both in cuisine and as a healing plant. Lemon myrtle (Backhousia citriodora), native mint (Mentha australis), wattleseed (Acacia spp.), and tea tree (Melaleuca alternifolia) have been integral to Indigenous communities. These plants were not only consumed for their nutritional value but also applied in various medicinal and ceremonial contexts.

In Aboriginal cuisine, lemon myrtle leaves were added to fish wrapped in paperbark to add a citrus flavour and aroma to the meat. Indigenous communities traditionally employed this aromatic plant species for culinary and medicinal purposes due to its robust lemon flavour and health benefits. Crushed leaves were used as a topical treatment for wounds, while brewed teas served as remedies for respiratory ailments.

2.2 Botanical Documentation and Commercial History

In 1989, B. citriodora was investigated as a potential leaf spice and commercial crop by Peter Hardwick, who commissioned the Wollongbar Agricultural Institute to analyse B. citriodora selections using gas chromatography. In 2001, a standard for oil of B. citriodora was established by The Essential Oils Unit, Wollongbar, and Standards Australia. Lemon myrtle is one of the well-known bushfood flavours and is sometimes referred to as the "Queen of the lemon herbs."

3. Key Constituents and Active Compounds

3.1 Essential Oil — Primary Constituents

Lemon myrtle essential oil (LMEO) contains 80–98% citral, a mixture of two main isomeric aldehydes — geranial and neral — which is higher than that found in other lemony essential oils, such as citrus (3–10%) and lemongrass (75%). This terpene aldehyde is a mixture of the two geometric isomers neral (IUPAC Name: (2E)-3,7-dimethylocta-2,6-dienal) and geranial ((2Z)-3,7-dimethylocta-2,6-dienal), also known as citral a and citral b, respectively, in the ratio of 1.2–1.5.

Across multiple GC-MS analyses of Australian material, BCEO was found to be composed of geranial (46.1% to 60.7%) and neral (32.0% to 40.9%) as the major compounds, followed by iso-geranial (1.0% to 4.2%), iso-neral (0.6% to 2.7%), linalool (0.3%–1.0%), 6-methyl-5-hepten-2-one (0.1% to 2.5%), citronellal (0.1% to 0.9%), and myrcene (0.1% to 0.7%). The yield of essential oil from fresh leaves of lemon myrtle varies from 1.1 to 3.2%.

3.2 Non-Volatile Phenolic Compounds

Beyond the volatile fraction, B. citriodora emerges as a rich repository of diverse non-volatile compounds, with the non-volatile extract specifically abundant in phenolic acids, flavonoids, and proanthocyanidins. Quantitative HPLC-DAD analysis of the phenolic fraction identified luteoloside, rutin, ellagic acid, gallic acid, catechin, and quercetin as key compounds. A notable ellagitannin — casuarinin — has also been identified in water extracts of B. citriodora. Casuarinin, an ellagitannin, was identified as the active compound in lemon myrtle extract involved in skeletal muscle satellite cell activation.

3.3 Mechanisms of Action

The biological activities of B. citriodora are attributable to multiple distinct mechanisms operating across the essential oil and polyphenolic fractions:

• Membrane disruption (antimicrobial): The strong antimicrobial and antibiofilm effect of BCEO against Gram-positive bacteria may be due to the high amount of hydrophobic citral. The outer membrane of Gram-negative bacteria containing hydrophilic lipopolysaccharides creates an effective barrier for hydrophobic compounds such as those found in essential oils; the structure of Gram-positive bacteria cell walls enables hydrophobic molecules to penetrate and act on both the cell wall and the cytoplasm.
• Anti-inflammatory signalling: In a study using LPS-activated murine macrophages, B. citriodora 80% methanol extracts reduced the expression of inflammatory enzymes iNOS and COX-2, accompanied by a decrease in nitric oxide (NO) and prostaglandin E2 (PGE2) levels; the tannins gallic acid and ellagic acid were identified as the active constituents.
• Antioxidant radical scavenging: Antioxidant effects of lemon myrtle are closely associated with polyphenol content, derived from both aqueous and solvent extractions.
• Satellite cell activation: Skeletal muscle satellite cell activation by lemon myrtle extract and casuarinin was associated with upregulation of interleukin-6 mRNA expression, which is essential for satellite cell activation and proliferation.

4. Scientific Evidence by Area of Use

4.1 Antimicrobial Activity

In Vitro Evidence

The antimicrobial and toxicological properties of lemon myrtle (Backhousia citriodora) have been investigated, and lemon myrtle oil was shown to possess significant antimicrobial activity against the organisms Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans, methicillin-resistant S. aureus (MRSA), Aspergillus niger, Klebsiella pneumoniae, and Propionibacterium acnes, comparable to its major component citral.

The antibacterial activity of BCEO showed stronger activity against Gram-positive bacteria (Staphylococcus aureus and Staphylococcus epidermidis) than against Gram-negative organisms. BCEO exhibited a significantly stronger ability to eradicate pre-formed biofilms of S. aureus (70.92% to 81.28%), S. epidermidis (83.56% to 90.73%), and E. coli (74.62% to 85.80%) than streptomycin at equivalent concentrations.

At the 1:100 dilution level, lemon myrtle completely inhibited Clostridium sp. and Aspergillus sp., and Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, Mycobacterium phlei, and Candida albicans had minimal growth.

Methanolic extracts of B. citriodora demonstrated potent antibacterial and antifungal effects using a disc diffusion and growth time course assay, establishing susceptibility of a broad range of microbes; both Gram-positive and Gram-negative bacteria were equally susceptible, though the extracts were weaker than the reference antibiotics tested.

Evidence strength: The body of in vitro antimicrobial data is substantial, reproducible across multiple independent laboratories, and covers a broad pathogen spectrum. These findings are in vitro only; no large-scale clinical trials in humans have been conducted for general antimicrobial indications.

Clinical Evidence — Molluscum Contagiosum

The most significant human clinical evidence for B. citriodora relates to the topical treatment of molluscum contagiosum. Thirty-one children (mean age 4.6 ± 2.1 years) with the diagnosis of molluscum contagiosum (mean length of time with condition 8.6 ± 5.3 months) were treated with once-daily topical application of a 10% solution (v/v) of essential oil of Australian lemon myrtle or vehicle (olive oil). At the end of 21 days, there was greater than 90% reduction in the number of lesions in 9/16 children treated with lemon myrtle oil, while 0/16 children met the same criteria for improvement in the vehicle group (P < 0.05). No adverse events were reported.

Evidence strength: This is a small, single randomized controlled trial (n=31) with a statistically significant primary outcome. It remains the sole published human clinical trial for a medicinal use of B. citriodora. The small sample size, single study center, and short 21-day duration substantially limit the strength of conclusions. Replication in larger, multi-center trials has not been published.

4.2 Antioxidant Activity

Evaluation of DPPH radical scavenging activity and ferric reducing antioxidant power showed that BCEO exhibited strong antioxidant activity at IC₅₀ of 42.57 μg/mL and EC₅₀ of 20.03 μg/mL, respectively.

Lemon myrtle leaves were extracted with ethanol at different temperatures and times to examine the effect of extraction conditions on antioxidant activities. Under optimal extraction conditions (80°C and 6 hours), the DPPH and ABTS radical scavenging activities were 83.31% and 60.13% respectively, and total polyphenol content was 102.72 mg gallic acid equivalents (GAE/g dry basis).

The total polyphenol content of an ultrasonic-assisted extract was 134.72 mg GAE/g, underscoring the botanical's abundance in phenolic compounds.

Evidence strength: Antioxidant data for B. citriodora are robust at the in vitro level, derived from multiple assay methods and consistent across laboratories. There are no human clinical trials evaluating antioxidant biomarkers following consumption of B. citriodora preparations.

4.3 Anti-inflammatory Activity

In a cell model study using LPS-activated murine macrophages (RAW 264.7 cells), B. citriodora 80% methanol extracts reduced the expression of inflammatory enzymes iNOS and COX-2 and decreased nitric oxide (NO) and prostaglandin E2 (PGE2) levels; the tannins gallic acid and ellagic acid were identified as the active constituents. These results were replicated in a similar study in which the activated macrophages were treated with both aqueous and 80% ethanolic extracts of lemon myrtle. The anti-inflammatory effect was further confirmed in lipopolysaccharide-stimulated RAW 264.7 cells, which showed inhibited production of inflammatory mediators such as nitric oxide.

Evidence strength: Anti-inflammatory activity is well-supported at the cell/in vitro level across multiple independent replications. All data are from cell-based assays; no human clinical evidence exists for anti-inflammatory outcomes.

4.4 Antifungal Activity

Lemon myrtle essential oil has broad-spectrum antimicrobial activity and is very effective against fungi, increasing the potential of using the EO in food preservation and treatment of postharvest diseases in fruits. BCEO is a rich source of citral, which possesses versatile biological activities including antimicrobial, antifungal, antiviral, and antioxidant activities.

Evidence strength: Antifungal data are entirely from in vitro studies. No human clinical data for fungal infections have been published.

4.5 Skeletal Muscle / Sarcopenia Research

The aqueous extract of B. citriodora leaves promoted proliferation of skeletal muscle satellite cells (SCs), with casuarinin determined to be one of the active compounds involved in the effects; myoblasts were unaffected. Both the extract and casuarinin upregulated IL-6 expression in the satellite cells, which is essential for their activation, proliferation, and subsequent muscle hypertrophy. These results were further supported by oral administration of the extract and casuarinin in rats.

Oral administration of lemon myrtle extract or casuarinin to rats showed significant activation of SCs in skeletal muscle (p < 0.05), suggesting that lemon myrtle and casuarinin may serve as novel nutritional interventions for improving sarcopenia through activating SCs.

Evidence strength: Preliminary. Evidence is limited to in vitro cell assays and a rodent model. No human clinical trials on muscle mass, sarcopenia, or related outcomes have been conducted.

4.6 Anti-adipogenic / Metabolic Activity

Lemon myrtle essential oil, a popular food-flavoring and perfume, has anti-inflammatory and antioxidant properties and contains citral in abundance. Citral-enriched essential oils have exhibited anti-obesity effects; however, the effect of LMEO on adipogenesis and the precise mechanism of citral had not been fully elucidated. One study evaluated the anti-adipogenic activity of commercial LMEO and its components, geranial, neral, and citral (a mixture of geranial and neral), in 3T3-L1 cell differentiation.

Evidence strength: Very preliminary — cell-model only. No human data are available.

4.7 Ectoparasite Repellency

Essential oil extracted from the leaves of B. citriodora demonstrated potent and long-term (3-hour) repellent activity against Rhipicephalus sanguineus, a tick species believed to be a vector of a number of human pathogens, including Rickettsia species, which causes spotted fever. Repellent activity at 1 hour after application was significant (P=0.005). These findings are from a preclinical experimental assay; human repellency trials have not been reported in the published literature.

5. Body Systems Associated with Backhousia citriodora

• Integumentary system: Topical antimicrobial and antiviral applications (molluscum contagiosum, skin infections); demonstrated skin barrier effects at high concentrations.
• Immune system: In vitro inhibition of pro-inflammatory mediators (iNOS, COX-2, NO, PGE2) via polyphenolic constituents.
• Musculoskeletal system: Preclinical skeletal muscle satellite cell activation via casuarinin and IL-6 upregulation, with potential relevance to sarcopenia.
• Metabolic system: Citral-mediated anti-adipogenic effects demonstrated in cell models.
• Respiratory system: Historical use by Aboriginal Australians as a tea for respiratory ailments; no formal clinical evidence.
• Flavour/Fragrance/Aromatherapy: Widely applied as a flavouring and scenting agent; lemon myrtle oil has been used for citral applications and added as a flavouring and scenting agent to foods, cosmetics, and aromatherapy massage oils.

6. Dosage Forms and Reported Dosages

No universal therapeutic dosage recommendations have been established for B. citriodora in any dosage form. Clinical trial data are lacking to provide dosing recommendations. The following specific dosages appear in the scientific literature:

• Molluscum contagiosum (topical, clinical trial): A 10% solution (v/v) of essential oil in olive oil, applied once daily topically over 21 days.
• Antibacterial (in vitro): Bacteria were inhibited at 0.0625% v/v.
• Methanolic leaf extract (in vitro antibacterial): One gram of Backhousia leaves extracted in 50 mL of methanol; the 15.7 mg/mL extract was found to be effective against two Gram-positive bacteria.
• Satellite cell activation (in vitro): Lemon myrtle extract was tested at final concentrations of 1–10 µg/mL; casuarinin at 13–400 nM.
• Antioxidant (ethanolic extract, in vitro): Optimal extraction conditions were 80°C and 6 hours, yielding 102.72 mg gallic acid equivalents per gram dry basis of total polyphenol content.
• Formulated topical product (skin absorption study): When a formulated product containing 1% lemon myrtle oil (0.18 mg/cm²) was applied to human skin discs (4.9 cm²) at 8 hours' exposure, the total absorption of citral in the full-thickness skin disc was 5.12 ± 0.60 × 10⁻⁴ mg/cm².

7. Safety Considerations

7.1 Acute Toxicity

The safety of citral itself has been thoroughly evaluated. Acute toxicity is low in rodents; the oral or dermal LD₅₀ values were more than 1000 mg/kg. Citral was concluded to be a skin sensitiser and otherwise non-toxic, and skin sensitization only occurs at high concentrations and is not caused by consumer products (as evaluated by the OECD).

7.2 Skin Sensitization and Irritation

In vitro cytotoxicity testing indicated that both lemon myrtle oil and citral had a very toxic effect against human cell lines: HepG2 (a hepatocarcinoma-derived cell line), F1-73 (a fibroblast cell line derived from normal skin), and primary cell cultures of human skin fibroblasts. This cytotoxic effect is relevant primarily to undiluted applications; it underscores that the oil should not be used neat on skin.

Although the absorption of lemon myrtle oil components was limited in a human skin disc study, histological staining showed significant losses in cellular functioning of skin including losses of integrity and solubilisation of the stratum corneum, cellular necrosis (to 15%) and cellular vacuolation (to 25%) on comparison to control skin, when the neat oil was applied. When assessed using a diluted 1% formulation over 8 hours, the histopathological assessment indicated limited damage to epidermal cells.

Due to the high citral content of the essential oil, there is a risk of sensitization when applied topically, so it should always be used diluted, with care on sensitive, diseased, or damaged skin. The potential for skin irritation can be reduced if used in conjunction with high d-limonene content essential oils such as lemon, mandarin, and sweet orange. The recommendation by the IFRA is 80% citral quenched with 20% d-limonene for skin applications.

RIFM data provided citral a No Expected Sensitization Induction Level (NESIL) of 1400 μg/cm² for the skin sensitization endpoint; citral is not expected to be phototoxic or photoallergenic based on available data and ultraviolet spectra evaluation.

7.3 Ocular and Special Population Cautions

Citral-containing oils are contraindicated in patients with glaucoma. Contact with the eyes should be avoided. Avoidance of use on children under 2 years of age is recommended.

Information regarding safety and efficacy in pregnancy and lactation is lacking; caution is warranted with topical B. citriodora oil use during pregnancy.

7.4 Drug and Herb Interactions

Information regarding the interaction of Backhousia with drugs, foods, or herbs is lacking in the published literature.

7.5 Blending to Reduce Sensitization Risk

Research by Hayes and Markovic demonstrated that a potent antimicrobial blend of 4:1 tea tree/lemon myrtle oil increases the antimicrobial effect of tea tree oil while reducing the sensitization risk associated with lemon myrtle.

7.6 Myrtle Rust Threat to Supply

A significant fungal pathogen, myrtle rust (Uredo rangelii), was detected in lemon myrtle plantations in January 2011. Myrtle rust severely damages new growth and threatens lemon myrtle production.

8. Regulatory and Commercial Status

In 2001, Standards Australia's CH21 Essential Oil Committee elaborated a formal monograph entitled "Oil of Backhousia citriodora, citral type (lemon myrtle oil)", AS 4941-2001. More recently, lemon myrtle leaves are used as a dry or fresh herb in food applications and the essential oil is used as a flavouring agent in food and beverages. The leaves and flowers of lemon myrtle are used in tea blends and beverages, biscuits, breads, confectionery, pasta, syrups, liqueurs, flavoured oils, packaged fish (salmon), and dipping and simmer sauces.

9. Summary of Evidence Strength

• Antimicrobial (in vitro): Strong, reproducible in vitro data across multiple organisms and preparations. One small positive human RCT for molluscum contagiosum (n=31); inadequate for broader clinical conclusions.
• Antioxidant (in vitro): Robust in vitro evidence. No human trial data.
• Anti-inflammatory (cell-based): Multiple replicated cell studies. No human data.
• Skeletal muscle / sarcopenia: Preliminary — in vitro and animal data only.
• Anti-adipogenic: Very early stage — single cell-model study.
• Ectoparasite repellency: Preclinical experimental data; no human trials.

References

• Southwell I. Backhousia citriodora F. Muell. (Lemon Myrtle), an Unrivalled Source of Citral. Foods. 2021;10(7):1596. PMC8305781.
• Lim AC, et al. Chemical Composition, Antioxidant, Antibacterial, and Antibiofilm Activities of Backhousia citriodora Essential Oil. Molecules. 2022;27(15):4895. PMC9370046.
• Yamamoto A, et al. Lemon Myrtle (Backhousia citriodora) Extract and Its Active Compound, Casuarinin, Activate Skeletal Muscle Satellite Cells In Vitro and In Vivo. Nutrients. 2022;14(5):1078. PMC8912364.
• Kim et al. Antioxidant and anti-inflammatory activities of phenolic compounds extracted from lemon myrtle (Backhousia citriodora) leaves at various extraction conditions. PMC7492333.
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• Backhousia citriodora. Wikipedia.
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