A systematic review of the ethnopharmacology and phytochemistry of Callistemon citrinus “Myrtaceae”: implication for phytomedicine future prospect

Abstract

Background

The increase in bacterial infection, combined with antibacterial resistance, remains a global threat, fueling the popularity of medicinal herbs as people seek alternatives to synthetic medicine. This review synthesizes scientific evidence regarding the medicinal uses of lemon bottle-brush (Callistemon citrinus (Curtis) (C. citrinus)) to inspire future research prospects.

Methods

Scientific evidence was thoroughly synthesized from databases (PubMed, Web of Science, and Scopus).

Results

One hundred articles reported the medicinal use of C. citrinus, while 39 (39%) provided evidence of its ethnopharmacology and phytochemistry. The C. citrinus medicinal plant research progresses at an annual growth rate of 6.41%. Most prolific authors were from India and Iran countries. The leaf extracts and essential oil are the most studied part of the plant for microbial sensitivity, respiratory disorders, hemorrhoids, and anti-inflammatory among several illnesses. Phytochemistry and phytomedicinal analysis of C. citrinus revealed several bioactive compounds, including 1,8-cineole, α-pinene, and β-pinene, and gas chromatography-mass spectrometry and high-performance liquid chromatography as the most used method of identification/characterization. Most of the included studies omitted information about the standardization preparation methods or formulation composition. Poor ethnobotanical studies, toxicological profile, and efficacy studies of C. citrinus have hindered the possible translational progress. Also, despite its pharmacological implications, no study of human subjects and clinical trials has been done on the plants.

Conclusion

This systematic review shows the research progress and provides evidence of details of phytochemical constituents and ethnopharmacology benefits of C. citrinus. It highlights research gaps and future engagement in advancing the medicinal use of C. citrinus.

Background

The emerging and reemerging bacterial infection coupled with antibacterial resistance remain a concern to public health systems [1–3]. However, medicinal herbs are becoming more mainstream as more people seek alternatives to synthetic chemicals as remedies for their health challenges [4–6]. Callistemon citrinus (Curtis) Skeels (C. citrinus), also known as lemon bottle-brush is a shrub from the Myrtaceae family that is indigenous to Australia, where it grows wild [7]. Due to its hardiness and adaptability, it is now widely utilized as a decorative garden plant worldwide. Lemon bottle-brush C. citrinus is an ornamental plant with a red/purple coloration and is found worldwide due to its widespread distribution [7]. A growing number of food and pharmaceutical applications incorporate anthocyanins because of their unique structural/chemical properties. Callistemon citrinus is employed in ethnopharmacology for its therapeutic benefits in combination to serving as an ornamental plant. Numerous conditions, including hemorrhoids, dysentery, rheumatism, tuberculosis, bronchitis, urinary incontinence, heavy menstruation, or mucosal discharge, have been reported to be treated with C. citrinus in traditional medicine [8–13]. Own to the bioactivities of phytochemicals such as α-glucosidase, present in C. citrinus recent studies have shown a promising effect against multi-resistant food-borne diseases, and the MCF-7 cancer cell line [14], Listeria monocytogenes [15]. According to the phytochemical screening, the plant’s components, including the leaves, stem backs, flowers, and seeds, have been found to contain polyphenols, alkaloids, monoterpenoids, aliphatic acids, tannins, sesquiterpenes, triterpenoids, and steroids [9, 16–19].

Botany and “ethno” (relating to people, culture, language, food, belief, aesthetic knowledge, and practice) together to form ethnobotany provide a deep understanding of how plants have influenced and been influenced by human societies [20]. The ethnobotanical knowledge includes domesticated plants of agricultural origin and wild plant species. This corpus of knowledge has developed over many centuries, acting as a storehouse of traditional wisdom and practical insights into the various uses of plants in various contexts. Plants’ diverse roles in human cultures are illuminated by ethnobotanical studies, which cover everything from food and medicine to ritual and cultural traditions.

There is a lack of post-scientific facts and pertinent bioactive translational advancements of C. citrinus based on phytochemistry and ethnopharmacology. In order to encourage researchers, policymakers, and funders to focus on research advances in sourcing alternative medicine [1, 5, 21], we examined the breadth and development of research information on C. citrinus for possible translational progress. However, the summaries of the earlier reviews on C. citrinus are insufficient as they lack detailed information on structure–function usefulness and no report on the toxicological profile, tradition preparation, or ethnopharmacology.

By our mainstream search, there are only two reviews on this plant, including [13] reporting the traditional uses, phytoconstituents, and pharmacological properties and [22] that focuses on phytochemistry and biological activities of C. citrinus. Therefore, it is essential to systematically review studies on C. citrinus to get a more thorough and current viewpoint. Our review of C. citrinus will promptly offer an in-depth analysis of the traditional applications, phytochemistry, ethnopharmacology, and toxicology profile, thereby identifying research gaps and highlighting future perspectives. Also, to discuss the gaps in the compound information description.

Methods and research design

Search strategy

The Preferred Reporting Items guidelines for the Systematic Reviews and Meta-Analyses (PRISMA) [23] were adopted for the data generation in this study. The search terms (Ethnobotany AND Ethnopharmacology AND Phytochemistry AND Callistemon citrinus “Myrtaceae” Melaleuca citrina OR Red bottle-brush OR Crimson bottle-brush OR Lemon bottle-brush) and (Ethnobotany AND Ethnopharmacology AND C. citrinus “Myrtaceae”) were used to retrieve dataset from PubMed, Scopus and web of science, respectively.

Ethnopharmacology and phytochemistry

Inclusion and exclusion criteria

Only articles that contain any of the following word(s); (C. citrinus “Myrtaceae” Melaleuca citrina OR Red bottle-brush OR Crimson bottle-brush OR Lemon bottle-brush) were retrieved from the databases from January 1969 to June 2021 and update to December 2023.

Study selection

Data collection, mining, and analysis

The data analysis was performed using a bibliometrix R-package [24, 25] on Rstudio versions 4.0.5 [24, 26] and Excel 2019 upon removal of duplicated and normalization of variables using ScientoPy R-package [27]. The productivity trend analyses by (sources/journal and country) and the yearly production, expressing the mean citations/articles indices of the consistent dataset, were done in the R programming environment. However, the top most relevant journals and countries’ performance-based impact and productivity were reported with their H-index and rate of citations, respectively. The annual growth rates of scientific articles were determined using the calculator (CAGR) at www.investopediacom.com/calculator.

The metadata comprising ethnomedicine indices were extracted using a data collection tool into Microsoft Excel (Microsoft Corporation, USA). Relevant indices including local name(s), part(s) used, mode of preparation, method of extraction, method of study and administration, efficacy, phytochemistry/phytochemical screening of crude/solvent extracts, pure compounds isolated and toxicological meta-data were extracted from the included articles by individual authors (OH, CN NES), then a zoom meeting was organized to agree and checked for completeness before analysis.

Prior to analysis, the bioactive structural elucidation and incomplete local names were retrieved from the Google search engine. PubChem and botanical databases (The Plant List, International Plant Names Index, NCBI taxonomy browser, and Tropicos), respectively.

The clustered metric networks analyzed were constructed by adopting the visualization of similarities (VOS) approach using an optimized algorithm of VOSviewer 1.6.13 [28]. Descriptive statistical methods were used to analyze the extracted data, while the results were articulated as ranges, percentages, and distribution/frequencies and presented in tables and charts using Microsoft Excel.

Results and discussion

Literature search summary

Using the electronic database search, 209 articles were identified. The total number of articles was decreased to 125 after elimination and de-duplication. There were 100 papers after filtering the titles and abstracts, and 39 articles were meta-synthesis for C. citrinus publication progress, publishing Journal metrics, ethnobotany, ethnopharmacology, and phytochemistry, and toxicology profile (Fig. 1).

Research progress on C. citrinus

The findings show a relatively low research publications output on the plant and a fluctuating production level overall between 1959 and 2023. Nevertheless, the most productive year was 2019 with 16 (14.5%) and 2020 with 11 (10%). The breadth and development of research on the C. citrinus show an annual growth rate of 6.4% (Fig. 2).

The Journal of Essential Oil Research, BMC Complementary and Alternative Medicine had the most published articles on research on C. citrinus detailed in Table 1. The Journal of Essential Oil Research, BMC Complementary and Alternative Medicine, Scientia Horticulturae and American Journal of Applied Sciences showed the most cited publishers.

Table 2 shows the most productive countries related to C. citrinus research. The researchers from India (22 articles, (28.2%), and Iran 6 articles (7.7%)) were the most productive countries. Also, the Indian nation (137, 6.2%) and South Africa (111, 22.2%) top the scientific evidence by total citation and average article citations, respectively. Argentina, Benin, and Cameroon were the least productive in C. citrinus research and had the least total citations and average article citations.

The keywords analysis shows four clusters of thematic themes (CTT) of research landscapes on C. citrinus, including CTT#1 (red cluster) involving the research on the bioactive compound on cancerous cells in animal models. The CTT#2 research focuses mostly on the plant leaves, extracts, phytochemicals, and their effects or sensitivity to resistant bacteria. The CTT#3 studies were based on the repelling/anti-helmet effect of the plant extracts on intestines parasite, schistosoma mansol, and liver, in mice model as a drug therapy or in combination with other drugs. The CTT#4 studies focus on C. citrinus volatile/essential oil, antibacterial activities, and its antioxidant effects, and hct1 16 cells (Fig. 3).

Ethnobotany and traditional application of C. citrinus

The C. citrinus is primarily used in India, Mexico, and Egypt and is relatively applied in Cameroon, Spain, and Zimbabwe as the folkloric plant. Regardless of the region, the common name used was Bottlebush (Table 3). The leaves were mostly used to characterize the phytochemical constituents and ethnopharmacological potential of C. citrinus (Fig. 4).

Decoction was the primary folkloric preparation method to administer the plant constituent to treat illness. The most prevalent pharmacological implications of C. citrinus were gastrointestinal protection, antioxidant, anti-inflammatory, wound healing, antibacterial, antifungal, and antiviral properties (Table 4).

Bioactive components and phytomedicinal analysis of the C. citrinus

The phytochemistry and phytomedicinal analysis reveal more than 22 bioactive compounds studied for potential clinical application in phytomedicine, as detailed in Fig. 5. The gas chromatography-mass spectrometry (GC–MS) and high-performance liquid chromatography (HPLC) have frequently been applied to identify bioactive components. The common phytochemicals present in the leaves and flowers C. citrinus are 1,8-cineole, α-pinene, β-pinene, limonene, and linalool, as detailed in Table 5.

Discussion

There has been a growing push for the use of bioactive substances from folkloric and medicinal plants in local treatments of illness, emerging, and reemerging infections and bacterial resistance [3, 4, 6, 20, 56]. Indigenous plants are an important source of unique compounds needed to create innovative, safe, and effective medications and combat resistance concerns. Our systematic review focuses on research progress and the meta-synthesis phytomedicine impact of 29 articles on C. citrinus for future engagement. The low research publications output and an annual growth rate of 8.4%, with the most prolific journals publishing only three articles, indicate a relatively poor research interest in C. citrinus for finding effective phytomedicine compounds. In comparison to other plants such as Moringa oleifera [57], Aloe megalacantha [58], and many more, C. cirtrinus has not gained much research attention, possibly due to its poor ethnopharmacology documentations.

Few authors in India have indicated interest on the C. citrinus globally. However, C. Citrinus has the potential of research landscapes for exploring the bioactive compounds on cancerous cells in animal models, the effects or sensitivity to resistance bacterial of its leaf’s extracts, repelling/anti helmet, and the implication on intestines parasites, the high content of volatile/essential oil of antibacterial activities. In addition to the numerous phytochemicals and antioxidant properties of C. citrinus, it has very poor global research relevant and post-translation application. However, many authors have spotted the pharmacological properties of C. citrinus and use it worldwide.

Research in ethnobotany provides priceless information about the medicinal and pharmacological qualities of native plants. Ethnobotanists add to our knowledge of indigenous healing traditions and the promise of plant-based medicines by recording traditional knowledge about medicinal plants. This multidisciplinary strategy informs contemporary scientific research and drug discovery initiatives while protecting traditional knowledge systems. However, C. citrinus still lacks detailed advances in ethnobotanical literature for future research engagement.

Clinical implication and prospect of C. citrinus

The phytochemical and bioactive constituents of C. citrinus against various diseases have been reported in various studies, indicating potential for clinical possibilities. Among the phytochemistry includes Acylphloroglucinol, Monoterpene galloylglucosides, Eucalyptol, Cyanidin-3,5-O-diglucoside (Cyanin), Peonidin-3,5-O-diglucoside (Peonin), Cyanidin-3-O-glucoside, Alpha-pinene, Pulverulentone A, 8-desmethyleucalyptin, Eucalyptin β-pinene Limonene, Linalool, Myrcene, Menthylacetate α-terpineol α-Thujene, Linalyl acetatep-cymene, Spathulenol, (E)-β-terpineol as identified by GC–MS and HPLC. The synthesis and purification of these active principles will be a great source of novel medicine and increase market values. Our findings show serval pharmacological uses, like the Eucalyptol, also known as 1,8-cineole, cajeputol, 1,8-epoxy-p-menthane, 1,8-oxido-p-menthane, and so on. It has a fresh camphor-like smell and a spicy, cooling taste. It is insoluble in water but miscible with ether, ethanol, and chloroform, which implies its relevance as a food additive. Furthermore, eucalyptol has been reported to possess anti-microbial and anti-inflammatory effects and cardiovascular benefits [59]. Specifically, eucalyptus oil was found to have respiratory protective effects through anti-inflammation, mucosa excretion, and improved airway mucociliary movement at low concentrations/doses, but the high dose is potentially harmful [60, 61] Also, eucalyptol has been reported to have antifungal and antiviral potential, though at doses toxic to liver cells [62]. Few studies have attribute the pharmacological mechanisms effect of C. citrinus to be antioxidant and anti-inflammatory activities [49, 51]. All of these findings point to the need for additional research on the dose–response, mechanism of action, and toxicity of eucalyptol and other bioactive compounds found in eucalyptus oil, such as eucalyptin, to gain a better understanding of these compounds’ effectiveness.

Cyanidin-3,5-O-diglucoside is also known as cyanin, an anthocyanin. Cyanidin 3-O-glucoside (C3G) is responsible for anthocyanins’ bioactive potential by exerting pharmacological properties such as antioxidant, anti-inflammatory, anti-microbial, neurodegenerative protection, anti-cancer, antidiabetic, cardiovascular protection, and so on [63–65]. However, while some studies indicate that anthocyanin metabolism in the gastrointestinal tract reduces their bioavailability, others have shown that the interaction between anthocyanins and gut microbiota increases their biological benefits. As a result, more research on the interactions between C3G and other anthocyanins and the microbiota is needed to reconcile these arguments. Future studies focusing on their metabolism would be beneficial in providing insight into their positive and negative benefits. Experimental studies have shown α-pinene, β-pinene, ρ-cymene, myrcene, and linalool have antioxidant, antiviral, anti-inflammatory, antitumor, anti-microbial, antifungal, antibiotic resistance modulation, anticoagulant, antimalarial, anti-Leishmania, neurodegenerative protection, and analgesic effects [60, 66–69]. Though biologically beneficial, these compounds have been reported to have low bioavailability, lasting for a short time at low concentrations before being rapidly metabolized and eliminated from the body. Although several in vivo and, more recently, few clinical studies have assessed their pharmacological effects, especially concerning safe and toxic doses. Hence, more studies, particularly clinical trials, are required on their bioavailability, protective doses and toxicities to provide more insight into their pharmacological relevance. Metabolomics research could also reveal new future bacterial and fungal targets for anti-microbial pharmaceutical and agrochemical products. Despite the various biological benefits of sputhalenol, α-terpineol, and β-terpineol, more research is needed from experimental to clinical applications and justification.

Efficacy and toxicological profile of C. citrinus.

Phytomedicines (plant-determined medications) have a longtime practice worldwide for counteracting and treating infections/diseases due to their vast array of bioactive components. However, toxicity, chemical compositions, and biological barricades, including bioavailability, insolubility, and hydrophobicity, hinder such imperious phytomedicines. At the same time, minimum inhibitory concentration (MIC) is one viable laboratory measurement indication for examining the anti-microbial activities against an organism. A lower MIC value implies that less of the drug is needed to inhibit the organism’s growth; therefore, the lower the MIC values/scores, the better the anti-microbial effects of the specific agent.

Our synthesis shows a dearth of reports on the effectiveness and safety of C. citrinus and their consequences. The application of bioactive molecules is indigenous, with parallel usage as supplements, and often, no substandard median lethal dose (LD₅₀) is reported. It limits the toxicological profile and clinical applications as potential candidate drugs despite reports of their antibacterial and MICs being documented. Our findings show that only one author reported the half maximal inhibitory concentration (IC50) of leave extracts of C. citrinus on human adipocyte lipid-binding protein FABP4 (3P6H) and human nitric oxide synthase (3E7G), which possess a moderate antibacterial activity against MRSA (IC₅₀ 22.2 µM), strong antibacterial activity against VRE (IC₅₀ 15.9 µM) on HepG2, LLC-PK1, and Vero cells model [30]. Also, the solo author reported the aerial part essential oil MIC assay against microbial strains: B. cereus at 2.00 mg/ml, S. typhimurium at 4.50 mg/ml, S. mutans at 2.50 mg/ml, C. albicans at 4.00 mg/ml, and S. cerevisiae at 4.50 mg/ml concentrations [29, 42] reported the methylene chloride-methanol extract of C. citrinus of MIC values (MSSA 62.5 μg/ml and MRSA 125 µg/ml), MBCs against MSSA and MRSA were 64 and 128 µg/ml, 62.5–500 µg/ml respectively. Three authors reported the flowers to have an LD₅₀ value of 7.4 μg/ml [7, 34] essential oils MIC of 8.4/13.0 μg/ml. The study of [35] reveals the flowers’ essential oils IC₅₀ values of 1.40 mg/ml and 1.77 mg/ml, and the pure compound was 5.6 μg/ml. The study of Ref. [14] reports EC₅₀ values of 0.474 ± 0.03 and 0.787 ± 0.15 ml for pure compounds. Again, the study of [46] showed the essential oil MIC of pure extracts to be 84.4 μl/l on the insects model, while essential oils LC₅₀ values were 12.88 and 84.4 μl/l for male and female mice [45]. Furthermore, the MIC assay revealed the anti-microbial potential of C. citrinus essential oils against microbial strains B. cereus at 2.00 mg/ml, S. typhimurium at 4.50 mg/ml, S. mutans at 2.50 mg/ml, C. albicans at 4.00 mg/ml, and S. cerevisiae at 4.50 mg/ml concentrations [29]. Overall, eight studies documented the toxicological profile of C. citrinus, indicating a sufficient research gap. This underscores the need to use folkloric herbs in medical situations for organized research, and systematic study on their safety and effectiveness using accepted scientific methods is required. This stance was taken in the WHO report, demonstrating that only 10% of the native medicinal plant flora has been used for its medicinal relevance [20, 70]. As a result, their potential as preferred medications and as novel drug leads for the treatment and management of illness has been limited globally.

Furthermore, our findings indicate that very little is known about the antioxidants profiling of C. citrinus. Among the antioxidants activities studied in C. citrinus is the radical scavenging activity, which showed an IC₅₀ value of 16.71 µg/ml. In contrast, the protein denaturation assay showed an IC₅₀ value of 21.19 µg/ml and 19.53 µg/ml in protease inhibitor activity for the aerial part essential oil [29]. The 2, 2-diphenyl-1-picrylhydrazyl (DPPH center dot) radical scavenging and beta-carotene/linoleic acid assay [42]. The ethanol, methanol, and n-hexane plant extracts yield NO-92.42%, HO•-94.03%, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS)•+-97.41%, DPPH-93.74%, super oxide-93.80% [71]. Antioxidants been a vital pivot in phytomedicine has not been well articulated in C. citrinus research. Antioxidants have been categorized into primary antioxidants, which primarily serve as scavengers or free radical terminators; secondary antioxidants, which are significant preventive antioxidants that work by delaying the initiation of chains; and tertiary antioxidants, which are focused on the repair of damaged biomolecules. Therefore, categorizing the antioxidants found in C. citrinus could impact the advancement of potential drug development.

The strength of this systematic review is the synthesized evidence, a combination of data from several studies in vitro and in vivo, which is crucial for future investigations. It identifies gaps for future directions and encourages conventional use. The limits in the primary studies that were included in this analysis are the source limitations. Nevertheless, it is important to note that no efficacy study of C. citrinus has been done on human subjects, neither clinical trials nor a well-defined protocol for conducting them. Also, most research omitted information about the preparation methods, standardization processes, and formulation composition.

Conclusion

This systematic review reveals the research progress and provides evidence of details of phytochemical constituents and ethnopharmacology benefits of C. citrinus from published articles. The study identified research gaps and highlighted future perspectives that could be exploited to advance the medicinal use of C. citrinus. The information presented in this review would be useful to researchers and policymakers seeking research advances in alternative medicine.

Acknowledgements

None declared.

Author contributions

O.H. conceived and designed the study. O.H., N.C., S.O.N., S.E.N., and S.P.N.B. carried out the study, analysed, and interpreted the data. O.H. and S.E.N. drafted the manuscript and revised the manuscript. All authors read and make the final corrections.

Conflict of interest

None declared.

Funding

None declared.

Data availability

The datasets used for this study are available from the corresponding author on a reasonable request.

Ethical approval

This study reviewed relevant documents using the International Prospective Register of Systematic Reviews (PROSPERO) and can be accessed at their website (https://www.crd.york.ac.uk/prospero/display_record).

Consent for publication

All the authors have read and agreed to the final copy of the findings as contained in the manuscript.

References