Review
ABSTRACT
Essential oil from plants belonging to several species has been extensively used as natural resources in the prevention and treatment of a large number of diseases. The medicinal and aromatic plants of genus Etlingera produces high percentages of essential oil from its every organ and is recommended for a variety of health problems by traditional systems of medicine in south-east Asia. Essential oils from this genus demonstrated promising antimicrobial properties. As antibiotic resistance is emerging at an alarming rate, many infectious diseases have become difficult to treat. Thus, the genus Etlingera can be considered as a good source of essential oils, and extensive studies of biological activities of these oils may lead to the identification of new compounds which can be used in modern medicine, cosmetics and pharmaceutical industry, primarily as antimicrobial agents. This review summarizes the characteristics of essential oil of Etlingera species with particular attention to the chemical composition and antimicrobial activities from the data in the recent literature.
Key words: Etlingera, essential oil, chemical composition, antimicrobial activity.
INTRODUCTION
Etlingera is a genus of medicinal plants native to the Indo-Pacific region. It has been used by indigenous communities for its flavor, culinary and medicinal properties since antiquity. The well-established traditional uses may be explained by the presence of biologically active volatile components in this genus. However, despite the increasing scientific interest in this field, there is a lack of summarized data on herbal medicine composition, therapeutic applications and risks associated to their consumption. Therefore, the purpose of this article is to provide an overview of the published data results regarding chemical composition and antimicrobial activities of essential oil of Etlingera species.
Essential oils (EOs), also known as volatile or ethereal oil or aetherolea, are concentrated hydrophobic liquid containing volatile aroma compounds obtained from aromatic plants. An oil is “essential” in the sense that each plant oils contains characteristic “essence” of the plant's fragrance from which it is derived. Essential oils are generally highly odorous, volatile with penetrating taste. Although, their consistency is more like water than oil, it is lighter than water and give transparent to pale yellow color. It is a complex mixtures of various compounds containing about 20-100 components at quite different concentrations. Chemically, they are derived from terpenes and their oxygenated compounds and also include non-terpenic compounds such as alcohols, aldehydes, ethers, ketones, phenols, oxides and phenylpropanoids. Many essential oils have limited solubility in aqueous solutions but are soluble in alcohol, non-polar or weakly polar solvents, waxes and oils.
The term ‘essential oil’ was used for the first time in the sixteenth century by the Swiss reformer of medicine, Paracelsus von Hohenheim, who named the effective component of a drug as Quinta essential (Guenther, 1948). They are not essential for health but have been used to relieve a wide variety of human maladies including bronchitis, pneumonia, pharyngitis, diarrhea, periodontal disease, wounds and numerous other illnesses for thousands of years (Boire et al., 2013). Today, the term ‘essential oils’ is used to describe complex mixtures of low molecular weight (usually less than 500 daltons) compounds extracted from aromatic plants using conventional techniques. At present, around 3000 essential oils are known, of which 300 are commercially important especially used as flavoring agents in food products, drinks, perfumeries, pharmaceuticals and cosmetics.
Morphology of Etlingera species
The genus Etlingera belonging to Zingiberaceae family consists of more than 100 species. They are perennial herbs mainly grown in perhumid forest (Poulsen, 2007). The leafy shoots of some of the Etlingera species can be up to eight m tall and the bases of these shoots are so stout as to seem almost woody. Many of the Etlingera species grow as clumps of leafy shoots, while others have such long creeping rhizomes that each of their leafy shoots can be more than a meter apart. True, Etlingera is characterized by its unique and distinctive flowers which have exceptional ornamental value. The inflorescence shoots are found short and do not emerge from the ground. The flowers are characterized with prominent bright red petal-like structures (labella) radiating outward, with the flower tubes and ovaries being below ground level. The brightly colored flowers are thought to be pollinated by insects. Fruits ripen below ground, and the seeds are thought to be dispersed by wild pigs. These plants are very aromatic because of the high content of essential oil in its leaves, stems, flower, fruit and rhizomes.
Distribution
Etlingera is dominant in Indo-pacific terrestrial, native to India, Bangladesh, Burma, China, Laos, Cambodia, Vietnam, Thailand, Malaysia, Singapore, Indonesia, Philippines, Brunei, Papua New Guinea, Queensland and several Pacific Islands, predominantly close to the equator between sea level and 2500 m (Poulsen, 2006; Wojdyło et al., 2007). Etlingera are also naturalized in other warm places such as Hawaii, Puerto Rico, Trinidad, Central America, Mauritius and the islands of the Gulf of Guinea (Govaerts et al., 2011). These species are also cultivated in gardens, especially in Mexico and western parts of Indonesia (Java) as an ornamental plant and a source of condiment and spice. A total of 155 names of Etlingera species have been accepted in the World Checklist of Selected Plant Families facilitated by the Royal Botanic Gardens at Kew, England (Govaerts et al., 2011). Borneo rainforest is exceptionally rich and presently, at least forty species are known in this forest (Poulsen, 2006). Of these species, eighty-five percent of the Etlingera species in Borneo are endemics and thirty-three percent are found in Brunei Darussalam. Three new species of Etlingera: Etlingera rubromarginata (from Sabah, Sarawak and Brunei), Etlingera belalongensis (from the Temburong District of Brunei), and Etlingera corrugata (presently only known from Danum Valley, Sabah) from northern Borneo have recently been described (Poulsen et al., 1999). Etlingera kenyalang from Sarawak and Etlingera palangkensis from Central Kalimantan have also been reported (Poulsen and Christensen, 2003; Takano and Nagamasu, 2006). Fifteen Etlingera species in Malaysia Peninsular, nine species in Java, Indonesia and only three species (one endemic and one introduced) have been recorded in China (Chen and Boufford, 2000).
Traditional application
Plants of Etlingera have been used since ancient times as spice and vegetable as well as for medicinal purposes. The common traditional uses of different Etlingera species are shown in Table 1.
More specialized uses of few species include perfume (rhizome of Etlingera baramensis), shampoo (fruit of Etlingera elatior and Etlingera pyramidosphaera) and spice (rhizome of Etlingera punica) (Chan et al., 2013). Several other species of Etlingera also have been used as medicine in the prevention and therapy of diseases (e.g. rheumatism: Etlingera foetens; jaundice, fever, urinary ailments: E. belalongensis; stomach-ache: E. pyramidosphaera; snake bite: Etlingera sessilanthera, diarrhea: E. pyramidosphaera) (Poulsen, 2006; Sabli et al., 2012; Sirirugsa, 1997).
Essential oils in commercial preparations
Several European countries have developed some essential oil-based industry in last few decades. Carvon, the principal constituent of the dill and caraway seeds EO, is currently marketed as Talent® in the Netherlands.
The preparation is used to inhibit the growth of storage pathogens and to suppress sprouting of potatoes in the warehouse (Hartmans et al., 1995). Soil Technologies Corporation (USA) has developed two natural products named Fungastop™ and Armorex™ which are commercially available for the control of various plant diseases in agriculture (Dubey et al., 2012). Eugenol based formulations (eugenol-Tween®; eugenolethoxylate) showed potent inhibitory effect against four apple pathogens (Phlyctema vagabunda, Penicillium expansum, Botrytis cinerea and Monilinia fructigena) and thus used in post-harvest disease management of apple fruit (Amiri et al., 2008). Cinnamite™ (cinnamon), Valero™ (rosemary), Promax™ (thymus) and several other essential oil based pesticides are already commercially available (Prakash et al., 2015). EOs or their components (α-bisabolol, geraniol, elemene, d-limonene, diallyl trisulfide (DATS) and Eucalyptol) have been shown to exhibit cancer suppressive activity against glioma, colon cancer, gastric cancer, human liver tumor, pulmonary tumors, breast cancer, leukemia and others (De Angelis, 2001). Essential oils rich in terpinolene and/or eugenol have shown antioxidative activity against low density lipoprotein (LDL) oxidation thereby reducing the chance of atherosclerosis (Edris, 2007). Essential oils and their components are exploited for antibacterial properties in diverse commercial products as dental root canal sealers, antiseptics and feed supplements for lactating sows and weaned piglets (Burt, 2004). A few preservatives containing EOs are already available in the market, such as DMC Natural base, which comprises 50% essential oils (Speranza and Corbo, 2010). Beside these, essential oil and their individual constituents exhibited antiviral, antimycotic, antiparasitic and insecticidal properties (Bakkali et al., 2008; Dubey et al., 2010).
Extraction Procedures of Essential Oil
Extraction of essential oils can be achieved by various methods such as distillation, solvent extraction, effleurage, aqueous infusion, cold or hot pressing, supercritical fluid extraction, solvent free microwave extraction (SFME) and phytonic process. The method of extraction is normally dependent on what type of botanical material is being used. It is the key step that determines the quality of the oil as wrongly executed extraction method can damage the oil and alter the chemical signature of the essential oil. Today, hydro-distillation (with a collecting solvent that is then removed under vacuum) and steam distillation are widely used for extracting essential oils from plants. Volatile components in these methods can be distilled at temperatures lower than their individual boiling points and are easily separated from condensed water. Losses of some volatile compounds, low extraction efficiency, degradation of unsaturated or ester compounds of these widely used conventional methods have led to the consideration of the use of new “green” technique in essential oil extraction, which typically use less solvent and energy, such as supercritical fluids, ultrasound and microwave extraction. Berka-Zougali et al. (2012) describes a new innovative method, solvent free microwave extraction (SFME), which yields an essential oil with higher amounts of more valuable oxygenated compounds and allows substantial savings of costs, in terms of time, energy and plant material. However, essential oils obtained by SFME were quantitatively (yield) and qualitatively (aromatic profile) similar to those obtained by conventional method (hydro-distillation), while SFME is highly effective for reducing extraction time (30 min for SFME against 180 min for hydro-distillation) (Périno-Issartier et al., 2013). In Etlingera species, hydro-distillation technique is extensively used for extracting essential oils except from the flower of E. elatior, where steam distillation is used.
Yield and chemical composition of essential oil of Etlingera
Essential oils are derived from almost all parts of plant (leaf, stem, flower, peduncle, bark, rhizome, seed and fruit) in Etlingera species. Total oil content in this genus was found very low and rarely exceeds 1% by mass. For example, the essential oil yields in leaf, stem, flower, peduncle, rhizome and whole plant of Etlingera species were found in the range 0.031 - 1.94%, 0.001 - 0.02%, 0.014 - 0.9%, 0.005 - 0.1% 0.006 - 1.4% and 0.004 - 0.07% (w/w) respectively as shown in Table 1. Leaves showed the highest yield, while least percentage was obtained from stems. The ranking was in the order: leaf>rhizome>flower>peduncle>whole plant>stem. Essential oil yields of the same plant parts can also vary on the time of collection (Vahirua-Lechat et al., 2010). Leaves of Etlingera cevuga showed the highest percentage of yield than other Etlingera species. The most extensively studied plant in Etlingera species is E. elatior and the plant part is rhizome.
Essential oils and their components have gained wide acceptance by consumers because of promising biological activities, safety, and exploitation for potential multi-purpose uses. The chemical composition of the Etlingera (around 21 different species including 2 varieties) essential oil has been described by many authors that has been summarized in Table 2.
Table 2 shows that different species Etlingera are dominated by different chemical components. Vahirua-Lechat et al. (2010) analyzed the essential oil of E. cevuga by capillary gas chromatography and combined GC/MS. Thirty-one components were identified where methyl eugenol (40.9-45.7%) and (E)-methyl isoeugenol were the major constituents. The author also reported that the percentage of major chemical constituents of E. cevuga varied with the location of plant. Chemical compositions of essential oil for the Etlingera species were found different in different parts of the same plant (Jaafar et al., 2007; Khaleghi et al., 2012a, b). Many reports demonstrated that the fragrance and chemical composition of essential oils can vary based on geography (soil type, climate, altitude, amount of water available, harvesting season) and method of preparation (Luna, 2002). Essential oil concentration is also fluctuated during its different stages of ripening (Wannes et al., 2009). These variations in chemical composition lead to the notion of chemotypes, which are generally defined as a chemically distinct entity in a plant with different compositions of the secondary metabolites(Djilani and Dicko, 2012). An essential oil chemotype can distinguish Etlingera of different origins, as well as seasonal variations throughout the vegetative cycle of plants.
Because chemotypes are defined only by the most abundant secondary metabolite, Etlingera brevilabrum may be assigned to two different chemotypes, depending on dominant component of the essential oil; elemicin and eucalyptol. Such chemotypes may be indicated as E. brevilabrum ct. elemicin, or E. brevilabrum ct. eucalyptol, although such indication has no taxonomic standing. Individuals of these chemotypes have vastly different chemical profiles, varying in the abundance of the kind of the next most abundant chemical. This can be a very qualitative assessment of an individual's chemical profile, under which may be hiding significant chemical diversity. Plant genotype is another important factor that can also affect the changes of the chemical composition of essential oils (Djilani and Dicko, 2012). Therefore, genetic and epigenetic factors influence the biochemical synthesis of essential oils in a given plant resulting in different chemical compositions and therapeutic activities.
Essential oil compositions of Etlingera were also varied on drying condition of the plants (Abdelmageed et al., 2011a; Mahdavi et al., 2012). The major constituents found in fresh rhizomes of E. brevilabrum were elemicin (35.6%) and methyl isoeugenol (19.2%) whereas eucalyptol (27.6%) and β-pinene (13.4%) were abundant in air-dried rhizomes of same species collected from Sabah, Malaysia (Mahdavi et al., 2012). Abdelmageed et al. (2011a) reported that fresh and dried of same plants parts (leaves, pseudostems, flower and rhizomes) of E. elatior produced different oil composition with increasing drying time. The author also found that different drying periods had an effect on the percentage of the main compounds and resulted in slight losses in volatile compounds compared with the fresh herb. Some authors also reported that the concentrations of various volatile substances increased after air drying (Díaz-Maroto et al., 2002; Faridah et al., 2010). This might be due to chemical transformation (breakdown of glycosylated forms, dehydration reactions, or oxidation reactions) or loss of compounds during drying process or due to the rupture of the plant cells in which the volatiles are stored.
The major constituents of Etlingera leaf essential oil are 1,8-cineole, 1-dodecanol, dodecyl acetate, elemicin, eucalyptol, methyl eugenol, (E)-methyl isoeugenol, α-pinene, β-pinene, and thujene. Stem of the plants contains mainly (E)-5-dodecene, 1,1-dodecanediol diacetate, 1,8-cineole, limonene, β-pinene and α-phellandrene, whereas cyclododecane, 1,1- dodecanediol diacetate, dodecanal, n-dodecyl acetate, 1-decanol and 1-hexadecanol are dominant in flowers. The main essential oil components in the rhizome are aromadendrene oxide, trans-anethole, 1,8-cineole, 1-dodecanol, n-dodecyl acetate, decanoic acid methyl chevicol, lauryl acetate, (E)-methyl isoeugenol as well as other compounds including cyclododecane, dodecanoic acid, linalool, α-pinene, β-pinene and α- phellandrene. Cyclododecane, dodecanol, dodecanal, (E)-2-tetradecene and caryophyllene oxide are the dominant component in peduncle essential oil and it is followed by α-pinene n-dodecyl acetate and cyclotetradecane. Analysis of whole plant essential oil composition showed that caryophyllene oxide n-dodecyl acetate, α-pinene, β-pinene, nerolidol, phellandrene, cyclododecane and 1-dodecene are abundant in Etlingera species.
Classification of Etlingera essential oil
Essential oil compounds can be classified into three main categories: terpenes (monoterpene hydrocarbons and sesquiterpene hydrocarbons), terpenoids (oxygenated monoterpenes and oxygenated sesquiterpenes) and non-terpenic compounds. Terpenes are hydrocarbons derived from five carbon atoms attached to eight hydrogen atoms regarded as isoprene units (C5H8). They form structurally and functionally diverse classes of organic compounds. They are synthesized in the cytoplasm of plant cells, where two molecules of acetic acid are combined to form mevalonic acid (C6H12O4) (Dhifi et al., 2016). Terpenes are usually grouped according to the number of isoprene units in the molecule, which can be rearranged into cyclic structures by cyclases, thus forming monocyclic or bicyclic structures. Monoterpenes (C10H16) contain two isoprene units; three sesquiterpenes (C15H24); four diterpenes (C20H32); six triterpenes (C30H48); and eight tetraterpenes (C40H64). Etlingera essential oils are abundant with monoterpenes and sesquiterpenes but longer chains also exist. Themonoterpene hydrocarbons found in Etlingera essential oils include α-pinene, β-pinene, α-phellandrene and limonene as seen in Table 3. On the other hand, cadinene, caryophyllene, (E)-farnesene and tetradecadiene are the major sesquiterpene hydrocarbons observed in Etlingera oil.
Terpenoids can be thought of as modified terpenes which undergo biochemical modifications via enzymes that add oxygen molecules and move or remove methyl groups (Williams et al., 1989). It can be sub-divided according to the number of isoprene units: monoterpenoids (2 isoprene units), and sesquiterpenoids (3 isoprene units) etc. The principle monoterpenoids found in Etlingera essential oil are 1,8-Cineole, 1,1-dodecanediol diacetate, eucalyptol, linalool and α-terpineol while caryophyllene oxide and nerolidol are major sesquiterpenoids.
Terpenic compounds are dominant in all parts of Etlingera species except E. fulgens and Etlingera venusta. Monoterpene hydrocarbon was found in greater percentage in essential oil of Etlingera species followed by oxygenated monoterpene, sesquiterpene and oxygenated sesquiterpene. Among different plant parts (leaves, stems, flowers, peduncle and rhizomes), leaves are found to contain highest proportion of monoterpene hydrocarbon than any other parts.
Phenylpropanoids (non-terpenic compounds) are a wide-spread class of plant-derived natural products synthesized from the amino acid precursor, phenylalanine. They serve as essential components of a number of structural polymers, provide protection from ultraviolet light, defend against herbivores and microbial attack and acts as signaling molecules (Korkina, 2007).
In Etlingera essential oils, most thoroughly studied phenylpropanoids are elemicin, eugenol, methyl eugenol, (E)-methyl isoeugenol and methyl chavicol. Out of 16 different Etlingera species, they were identified only in 8 species such as Etlingera bravlabrum (R), E. cevuga (L,R), Etlingera linguiformis (R), E. linguiformis (R) Etlingera littoralis (L,R), E. pavieana (R), E. punica (R), Etlingera sphaerocephala (R), E. venusta (L) (Bhuiyan et al., 2010; Khaleghi et al., 2012a; Tachai et al., 2014; Vahirua-Lechat et al., 2010; Vairappan et al., 2012; Wong et al., 2010; Yahya et al., 2010).
Various other groups of non-terpenic compounds such as alcohol, aldehyde, ketone, esters, carboxylic acid and hydrocarbon were also observed in Etlingera species. Examples of this group of compounds are dodecanoic acid, decanoic acid, dodecyl acetate, dodecanol, cyclododecane and (E)-2-tetradecene. E. fulgen, Etlingera fimbriobracteata and E.venusta are characterized by high proportion of non-terpene hydrocarbon, alcohol and esters (Chiang et al., 2010; Khaleghi et al., 2012a; Ud-Daula et al., 2016).
However, E. bravilabrum, E. cevuga, E. coccinea, E. elatior (S, R), E. linguiformis, E. littoralis, E. pyramidosphaera and E. Sphaerocephala var. grandiflora are devoid of these compounds (Jaafar et al., 2007; Kithan and Daiho, 2014; Mahdavi et al., 2016; Vahirua-Lechat et al., 2010; Vairappan et al., 2012).
Antimicrobial Activity
Test of antimicrobial assays of essential oils in Etlingera
An overview of the literature reporting antimicrobial assays of Etlingera EOs is presented in Table 4.
Determination of antimicrobial activity of Etlingera EOs was generally done by agar disk diffusion, agar dilution and broth micro dilution method. Screening of antimicrobial properties of EOs is generally by the agar disk diffusion method, where a sterile paper disk impregnated with EO is laid on top of an inoculated agar plate.
This method is not considered an ideal method for essential oils, normally used as a preliminary check for antimicrobial activity prior to more detailed studies. Agar dilution or broth microdilution methods are most widely used methods to determine minimum inhibitory concentration (MIC) (Rios et al., 1988).
MIC is generally regarded as the most important parameter to confirm resistance of micro-organisms to an antimicrobial agent and also to monitor the activity of new antimicrobial agents (Andrews, 2001). In addition to MIC, some author also stated minimum bactericidal concentrations (MBCs) and bacteriostatic concentration (Susanti et al., 2013).
Antimicrobial effects of Etlingera EOs
Researchers expedite their relentless venture to discover and synthesize new antimicrobial agents due to emergence of antibiotic resistance, increased population with lower immunity, undesirable effects of current drugs, etc. Plant molecules are well known for their antimicrobial properties; especially many essential oils and their components are known to possess significant activity against wide range of microorganisms (Bakkali et al., 2008; Bassolé and Juliani, 2012). Therefore, detail study of the plant EOs could be helpful to identify novel drugs and targets for controlling the infectious diseases worldwide. The antibacterial activity of Etlingera essential oils against gram-positive and gram-negative bacteria were reported in few studies and obtained results is presented in Table 4.
Tadtong et al. (2009) indicated antimicrobial effect of E. punicea rhizome essential oil on some pathogenic bacteria, particularly Staphylococcus aureus, Escherichia coli and Salmonella albany, and also possesses fungicidal activity against Candida albicans. Essential oils from rhizomes of E. pyramidosphaera, E. megalocheilos, E. coccinea, E. elatior and E. brevilabrum were tested against four strains of clinical bacteria (S. aureus, Staphylococcus sp., Streptococcus pyrogenes and Salmonella enteritidis), where E. megalocheilos, E. coccinea and E. elatior inhibited all the four tested bacteria with MIC values of less than 10 µg/mL, and the other two Etlingera species, E. pyramidosphaera and E. brevilabrum also showed interesting selective inhibition against S. aureus and S. pyrogenes, with MIC values ranging from 40.0 to 120.0 µg/mL (Vairappan et al., 2012). The author also claimed that the dominant presence of borneol (28.3%) and the availability of camphor (3.8%) could possibly be responsible for potent antibacterial activity.
In the study of Chiang et al. (2010), the oils from leaves of E. elatior, E. fulgens and E. maingayi, and E. rubrostriata showed inhibition against gram-positive bacteria of B. cereus, M. luteus and S. aureus with MIC values ranging from 6.3-100 mg/ml and ranking was in the order: E. maingayi > E. rubrostriata > E. elatior > E. fulgens. Of the gram-positive bacteria, M. luteus was the most susceptible with all four Etlingera species having MIC of 6.3 mg/ml, whereas no activity was observed on Gram-negative bacteria of E. coli, P. aeruginosa and S. choleraesuis (Chiang et al., 2010).
Dodecanoic (lauric) acid and decanoic (capric) acid which constitute more than 87% of E. maingayi oil content might be responsible for strong antibacterial activity. The oils (leaves, aerial stems, basal and rhizomes) from E. fimbriobracteata exhibited moderate to potent broad-spectrum antimicrobial activity against gram positive (Bacillus subtilis, Bacillus spizizenii. and S. aureus), gram negative (E. coli) and fungi (C. albicans and Saccharomyces cerevisiae) (Ud-Daula et al., 2016). (Reference) E. sayapensis essential oils also displayed a broad spectrum of antimicrobial activity against gram positive (B. subtilis, B. thuringiensis, MRSA, S. aureus), gram-negative bacteria (A. hydrophila, E. aerogenes, E. coli, P. mirabilis, S. marcescens, S. sonnei, V. parahaemolyticus) and fungi (C. albicans and C. parapsilosis) (Mahdavi et al., 2017).
Abdulwahab et al. (2010) reported that essential oils from whole plant of E. elatior failed to inhibit gram positive (B. subtilis) and gram-negative (S. choleraesuis and P. aeruginosa) bacteria. However, Susanti et al. (2013) demonstrated the activity of E. elatior flowers essential oil against Gram positive bacteria of S. aureus, Bacillus cereus with zone of inhibition of 13 and 12.3 mm, respectively but not sensitive against P. aeruginosa and E. coli. The oil also showed potent inhibitory effects against fungi with mean inhibition zones 10 mm for C. albicans and 20 mm for C. neoformans. C. neoformans was found as the most susceptible among all four microorganisms with MIC value of 0.05 µL/mL. antifungal property of Etlingera is suspected to be associated with their high contents of monoterpene hydrocarbon and phenylpropanoids.
It has been generally reported that gram-negative bacteria are more resistant than gram-positive bacteria. Similar observations were also found in Etlingera species where Gram-negative bacteria (E. coli and P. aeruginosa) showed less susceptibility to the Etlingera essential oil than Gram positive bacteria (Abdelwahab et al., 2010; Susanti et al., 2013). Of the gram-negative bacteria, P. aeruginosa was found resistant to the action of essential oil. This could be due to highly restricted outer membrane of these bacteria that slows down the passage of essential oils, whereas lacking of outer membrane enables Gram positive bacteria to be more susceptible to Etlingera essential oils.
Mode of antimicrobial action
Different modes of action are involved in the antimicrobial activity of essential oils, because of the variability of quantity and chemical profiles of essential oil. Antimicrobial action of essential oils and their components may be attributed by any or a combination of six possible mechanisms which include: (1) disintegration of cytoplasmic membrane, (2) damage of membrane proteins (ATPases and others), (3) degradation of cell wall with the release of lipopolysaccharides, (4) leakage of ions and other cell content, (5) coagulation of cytoplasm and (6) inhibition of enzyme synthesis (Bakkali et al., 2008; Bouhdid et al., 2010; Burt, 2004). Until now, the mechanisms of antimicrobial action of Etlingera essential oil and their components are not demonstrated in any published paper. Precise mechanisms of antibacterial activity of few individual pure essential oil components from different genera were reported. Carvacrol and thymol are able to disintegrate the outer membrane, increasing membrane fluidity, which in turn increases the permeability of the cytoplasmic membrane to ATP (Ultee et al., 1999). Eugenol - a major component (approximately 85%) of clove oil binds with membrane protein of both gram positive and negative bacteria altering their structure and increases permeability. Farnesol, nerolidol and plaunotol has been regarded as a cause for the loss of potassium and sodium ions in S. aureus (Carson and Hammer, 2011). p-cymene, terpinen-4-ol, 1,8-cineole, terpinolene and γ-terpinene increase membrane fluidity that breaches membrane integrity and allows small intracellular components such as hydrogen, potassium and sodium to pass through the cell membrane and ultimately causes cell death.
Areas of Future Research
All the literature in the last 15 years mainly focuses on the chemical composition of essential oil of Etlingera species. Only three literatures demonstrated the antimicrobial properties of essential oil in which two papers reported antifungal properties. Also, the oil from E. punicea and E. elatior showed potent antifungal properties than bacteria. However, some limitations have also been identified in the investigation of antimicrobial activity (only 2 species of fungi were used). Fungi are eukaryotic cells and consequently most agents that are toxic to fungi are also toxic to the host, hence development of antifungal agents has lagged behind that of antibacterial agents. Fungi grow slowly and many of them have multicellular forms which complicates in developing new antifungal agents and in understanding the existing ones. The action of essential oil and their components against bacterial and fungal cell is not fully identified and is a focal area for future research. Thus, elucidation of mechanism of action of essential oil and their components would provide insights that may lead to identification of new antibiotic target and exploitation of novel biochemical pathways.
CONCLUSION
This review summarizes and characterizes the importance of essential oils obtained from different parts of Etlingera species which comprise diverse chemical constituents. The essential oil of Etlingera showed promising antimicrobial properties that could be an alternative source of synthetic antibiotics in order to combat emerging drug resistance. Since, essential oil and their components possess many important medicinal activities, seeking new drugs from aromatic and medicinal plant like Etlingera is crucial. Many studies have demonstrated the chemical composition of this plant but only few researchers have investigated the antimicrobial properties of essential oil. Thus, more studies are needed to understand and elucidate the mechanism of action of essential oil and their constituents.
CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.
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