E. globulus belongs to the family of Myrtaceae; an evergreen broadleaf tree, with a straight trunk, indigenous to Australia, the genus Eucalyptus comprises of more than 700 species. It is known as “the blue gum” or Tasmanian blue gum. E. globulus Labill. has precious bioactive constituents, antioxidants, antimicrobials, and phytoremediation, and herbicidal activities, which will pave the way to the development of new pharmaceuticals and agrochemicals, as well as food preservatives. They may also provide potential commercial applications to counteract the limitations of synthetic antioxidants.
1. Introduction
E. globulus is extensively cultivated worldwide [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] because of its easy adaptability to environmental conditions, ease of cultivation, fast growth rate, and increase in the woody biomass. Furthermore, it can be planted in contaminated areas [17]. Obviously, Eucalyptus can adapt to biotic and abiotic stresses by selectively releasing defense compounds mainly of mono- and sesquiterpenes, as well as some hydrocarbons and quinones [18]. E. globulus has attracted the attention of researchers as well as environmentalists worldwide, due to it commonly being used in the pulp industry as a fast-growing source, and for the essential oil extracted from its leaves. Commercially, it is widely used for different purposes.
The essential oil of Eucalyptus spp. is among the 18 most frequently traded essential oils throughout the world. On account of this, there is an increasing amount of attention paid to their advantages as raw materials, which can be used in food, pharmaceuticals, and cosmetics, both in scientific and industrial research [1,19]. Among them E. globulus Labill. is the main source of the Eucalyptus leaf oil used globally [3]. Its leaves are alternate, with yellowish petioles, precisely lanceolate, 10–30 cm long, 2.5–5 cm wide, shiny dark green on both surfaces, and are a rich source of essential oil [7,10,20,21,22]. The oil is typically extracted by hydrodistillation, as the greatest amount of extracted chemical compounds can be obtained by utilizing this method [4,15,21]. The essential oil yield ranges from 1–3% [1,3,9,19,20,23]. The chemical composition of the Eucalyptus leaf essential oil is a complex mixture of substances, commonly containing 20 to 54 components with varying concentrations [4,8,13,15,24]. The oil comprised particularly of oxygenated monoterpenes (80.65–87.32%) as well as monoterpene hydrocarbons (18.33–12.45%) [9,25].
Eucalyptus leaf essential oil has drawn the attention of numerous researchers, as it represents a wide range of biological potentials, such as antibacterial efficacy toward both Gram-positive and Gram-negative bacteria [28,29] and considerable antibacterial potency against periodontal diseases [20,30]. Therefore, it can be incorporated into dental care products, as it is an anti-inflammatory agent [10,31]; has antifungal [16,28,32,33] and antidiabetic potential [34,35]; insecticidal [36,37,38,39], acaricidal, and repellent activities; is a biodegradable pesticide [40]; has bio-nematicide efficacy [7], phytopathogen control [8,23], and anthelmintic activity [41]; and it is a natural agent for food preservation [25]. Furthermore, this leaf essential oil has been previously applied in “folk medicine, in the treatment of respiratory problems, including, cold, cough, runny nose, sore throat, asthma, nasal congestion, bronchitis, and sinusitis” [42].
Leaves and root extracts possess antioxidant, anti-inflammatory, and antitumor activities [
35,
45,
46,
47], which correlate with the elevated amount of plant phytoconstituents. Additionally, leaf extracts possess fungicidal potency [
48], allelopathic potential [
49,
50,
51], anti-hyperglycemic potential [
52], a potential anticancer activity [
53], antibacterial activity [
30,
54,
55], and neuroprotective activity [
56]. In addition, leaf extracts are presently utilized in cosmetic formulations and food additives [
35]. The
E. globulus stump that is defined as “the basal part of the tree”, including the near-the-ground stem portion, and the woody roots that remain after “stem felling”, possesses antioxidant and antimicrobial activities [
57].
The aim of the current review is to present recent information on the phytochemical, antioxidant, and antimicrobial aspects of the E. globulus plant parts, besides the factors influencing the essential oil composition of the plants, and also its eco-friendly applications. This may help gather fruitful information for the potential use of the plant, as a precious source of diverse ingredients, in multiple applications.
2. Bioactive Components
In most of the studies, 1,8-cineole is the dominant component in the essential oils of leaves, with various percentages (
Table 1), except for a few findings [
26,
27,
58,
59]. Furthermore, phytochemical analysis of the leaf extracts proved the presence of tannins, saponins, terpenoids, glycosides, alkaloids, phenolic compounds, steroids, cardiac glycosides, terpenes, reducing sugars, carbohydrates, resins, acidic compounds, and flavonoids [
19,
30,
52,
60]. In addition, the high phenolic content in the
Eucalyptus plant parts (
Table 1) that correlated with positive health influences against oxidative stress has also been formerly documented [
2,
46,
56,
57,
61]. Additionally, triterpene compounds that possess significant pharmaceutical applications have been previously identified in
Eucalyptus leaves, bark, and fruits [
62,
63,
64].
The anthelmintic activity of the essential oil from extracted leaves has been previously reported by Taur et al. [
41]. This has been ascribed to the occurrence of valuable phytoconstituents in the oil, such as borneol, linalool, cineol, geranyl acetate, anethol, and saffrol. Additionally, phytochemical and fingerprint analyses of the essential oil from leaves displayed a higher content of flavonoids, as compared to phenols, whereas the Fourier Transform Infrared Spectrophotometer proved the existence of polyphenolic compounds, such as rutin, tannic acid, vanillic acid, and ascorbic acid, which is vastly utilized in the food industry.
Pyrolysis gas chromatography-mass spectrometry analysis of the
Eucalyptus leaves’ aqueous ethanol extract identified the presence of 21 compounds and the key characterized compounds were β-eudesmol (12.84%), γ-eudesmol (3.36%), globulol (2.87%), and alloaromadendrene (3.80%); in contrast, the presence of α-selinene and α-gurgujene was observed in low concentrations [
54]. Furthermore, the detection of guaiacol, syringol, o-cymene, catechol, and phenol could be associated with the presence of small fragments of lignin and lignin-derived monomeric products in the leaf extracts [
54]. The major triterpenic acids identified in the
E. globulus leaf extracts were betulinic, betulonic, oleanolic, ursolic, 3-acetyloleanolic, and 3-acetylursolic acids, which were considered to be valuable pharmaceutical compounds [
62]. Gas chromatography-mass spectrometry analysis (GC-MS) of the methanolic extract identified the existence of nine compounds, 9,10-secocholesta-5,7,10(19)-triene 1, 3-diol,25 [(trimethylsily) oxy] (,3β, 5Z,7E)-, 1-Heptatriacotanol, Morphinan-4,5 epoxy3,6diol, 6 [7nitrobenzofurazan-4-yl] amino, 10-Heptadecen-8-ynoic acid, methyl ester, [E]-, Ethyl Iso-allocholate, 2-Myristynoyl pantetheine, Cholesta-8,24-dien-3-ol,4-methyl-, [3β, 4α]-, Cyclopropanedodecanoic acid, 2-octyl-, methyl ester, 9,12,15-Octadecatrienoic acid, and 2-phenyl-1,3-dioxan-5-yl ester [
52]. Interestingly, hydrosol, obtained as a by-product of leaf hydrodistillation, exhibited an effective insecticidal activity toward the mealybug, which nominated it as a potential biodegradable insecticidal compound, instead of a synthetic pesticide [
68].
Regarding the chemical composition of the bark, Domingues et al. [
64] reported the exploitation practicality of using the large quantities of bark generated by pulp industries as a by-product, instead of burning it for energy production, as it was rich in valuable triterpenoid compounds (betulonic, betulinic, 3-acetylbetulinic, ursolic, 3-acetylursolic, oleanolic, and 3-acetyloleanolic acids) in addition to β-amirine, β-sitosterol, palmitic acid, and aliphatic alcohols. Additionally, triterpenoid acids were chiefly concentrated in the surface layers of the trunk and the bark branches. The triterpenoid content was 1.2 g/kg in fruits and 121.1 g/kg in the surface layers of the bark branch residues.
Although the bark is an industrial waste product, its aqueous extract contains a natural source of antioxidant that is attributed to an abundant amount of gallotannins and some other phenolic components [
67]. Similarly, the
E. globulus bark has a plethora of polyphenolic compounds that exhibit anti-proliferative activity against carcinoma cells [
69]. Moreover, De Melo et al. [
63] stated that the deciduous bark of
E. globulus contains ursolic and 3-acetylursolic acids, as the most abundant components, representing 52% of triterpenic acids, while hexacosan-1-ol and octacosan-1-ol from aliphatic alcohols represent approximately 7% of the total extract, even as the sterols fraction represents 4.24% of the total extract and β-sitosterol represents the most of this group (Sterols) (4.05% vs. 4.24%).
In recent times, Lourenço et al. [
70], for the first time, identified using GC-MS, eight polyhydroxy triterpenoid acids from the milled wood of mature
E. globulus mature trees (40 years), the dichloromethane extract, which included “hederagenin, (4α)-23-hydroxybetulinic acid, maslinic acid, corosolic acid, arjunolic acid, asiatic acid, caulophyllogenin, and madecassic acid, with 2,3, and 4 hydroxyl substituents”, which represented 10.4% of the wood extract. These characterized compounds are known to have interesting pharmaceutical and medical applications.
3. Antioxidant Activity of E. globulus Labill.
The leaf essential oil of Nigerian-grown E. globulus exhibited a low antioxidant capacity via its potential to scavenge DPPH radicals, with elevated IC50 values (136.87 µL/mL) as compared to the standard antioxidant ascorbic acid. This may be attributed to the absence of some components such as 1,8-cineole in the leaf oil as well as the potential antagonistic impact between other components in Eucalyptus oil [27]. Conversely, Luis et al. [13] reported that the essential oil of E. globulus exerted a remarkable antioxidant efficacy through its ability to scavenge DPPH radicals with an IC50 value (2.90 ± 0.35 v/v), with respect to the IC50 value (4.56 ± 0.70 v/v) for Eucalyptus radiata. This may be related to the existence of 1,8-cineole as the main constituent only in E. globulus essential oil, as well as the synergistic effect between other oil components. Moreover, the essential oil of E. globulus showed great activity to inhibit the lipid peroxidation with an IC50 value (2.72 ± 0.01 v/v) inferior to the activity of the synthetic antioxidant BHT, with an IC50 value of 3.58 ± 0.02 w/v, in the β-carotene bleaching test. This was considered a promising result that supported the E. globulus essential oil as a potential natural substitute, to overcome the adverse side effects of synthetic antioxidants, especially for food preservation.
5. Antimicrobial Activity
5.1. Antibacterial Activity
The extracted essential oil from fruits exerted pronounced antibacterial potency against tested multidrug-resistant bacteria. Furthermore, the combination of 1,8-cineole and aromadendrene from fruit oils produced a higher inhibition through an additive and synergistic effect against methicillin-resistant Staphylococcus aureus, Streptococcus pyogenes, and Bacillus subtilis, as compared to using a single compound [43]. The antibacterial efficacy was ascribed to the highest percentage of oxygenated monoterpenes (87.32%) in Eucalyptus leaf oil, and the synergism also resulted from other minor components [9]. The antimicrobial effects of the methanolic extract from leaves, against S. aureus and B. subtilis, could be attributed to the existence of tannins and saponins [60]. Similarly, leaf extracts proved the anticariogenic activity, due to the existence of sesquiterpene alpha-farnesene that would lead to an advancement of effective drugs for the treatment of dental caries [19].
The greatest antibacterial activity was obtained from the synergism between E. globulus essential oil or leaf extracts and antibiotics toward P. aeruginosa [2]. Additionally, Goldbeck et al. [1] observed a synergism effect as a result of the combination between E. globulus and E. urograndis essential oils against Streptococcus mutans. Furthermore, the highest antibacterial activity was correlated with the elevated concentration of 1,8-cineole (71%) in E. globulus, as compared to E. urograndis (36%), which supported the potential usage of E. globulus essential oil, through its incorporation into biodegradable films, as an environmentally benign strategy to control S. mutans, as an important oral pathogen.
5.2. Antifungal Activity
E. globulus essential oil showed a potent anti-candidal efficacy, indicating that the oil is a potential candidate for mouth wash applications [
32]. Similarly,
Eucalyptus essential oil surpassed the antifungal nystatin (a drug utilized to control fungal infections on the skin, mouth, vagina, and intestinal tract) twice over, in the antifungal efficacy against
C. albicans, which could likely be ascribed to the high content of 1,8-cineole (85.8%) in the leaf essential oil [
9]. Nanoemulsions containing
E. globulus essential oil that was commercially acquired possessed antifungal and antibiofilm activities against
C. albicans, which were the main microorganisms responsible for initiating fungal infections globally [
12]. These findings were due to the nanoencapsulation-enhanced functionality of the essential oil via the protection of essential oil components, as well as the reduced size of the nanoemulsions that resulted in rapid penetration.
The essential oil from the Tunisian
Eucalyptus aerial parts proved to have a potent antifungal efficacy against
C. albicans that was greater than the antifungal (Amphotericin B), especially the essential oil obtained in the fruiting stage, rather than the essential oil obtained in the vegetative and full flowering stages. This was probably because of the defense mechanisms of the plant during fruit formation or the variation in oil components during the different growth stages, as the oil contains considerable amounts of α-pinene and
p-cymene with a high essential oil yield during the final developmental stage. Furthermore, the combined application of
Eucalyptus essential oil with amphotericin B showed a great decline in the MIC value of
Eucalyptus essential oil alone against
C. albicans, from 1000 to 31.25 μg/mL in the case of a combination [
72]. Similarly, Bogavac et al. [
16] showed the potent antifungal efficacy of essential oil as a promising alternative against vaginal
C. albicans strains that were multidrug-resistant to conventional antifungals.
7. Future Perspectives
For a deep understanding of all the bioactive compound mechanisms involved in all the studied bioactivities, further trials should be implemented to explore the synergistic or antagonistic interactions among the complex mixtures of essential oils. In addition, the mechanism of action after the combination of oils and conventional antibiotics, which may influence multiple targets at the same time, should be investigated more thoroughly.
Additional studies are warranted to counteract the poor penetration of natural antimicrobial agents into the microorganism biofilm matrix, which can be achieved through nanocarriers. However, in the case of using biosynthesized nanoparticles, further investigation will be needed to demonstrate the impact of their cytotoxicity, with emphasis on optimizing the separation processes of the phenolic compounds, with detailed chemical characterization for possible extract quality improvement. Taking into consideration implementation of the available information on the effect of Eucalyptus essential oil and its extracts on the antimicrobial potency, it will be considered a great alternative to combat the resistance problem. Furthermore, the potent antimicrobial, herbicidal, and insecticidal activities of essential oil and plant parts should be transformed into commercial products.
This entry is adapted from the peer-reviewed paper 10.3390/horticulturae7110450
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