Chemical composition and antimicrobial activity of Micromeria hedgei Rech. f. oil from Iran

Abstract Micromeria hedgei belongs to the Lamiaceae family and is a rare endemic and endangered species that has been used in traditional medicine in Iran. In this regard, essential oil composition and antimicrobial activity of wild and cultivated M. hedgei was reported for the first time. Essential oils isolated via hydro distillation from the aerial parts of M. hedgei were analysed by a combination of capillary GC and GC–MS. The major constituents were geranial (18.04 and 22.68%), neral (13.81 and 15.99%), geraniol (13.15 and 10.74%), nerol (7.69 and 6.02%), E-caryophyllene (6.52–3.80%), carvacrol (6.20 and 5.27%), geranyl acetate (5.79 and 3.06%), caryophyllene oxide (4.73 and 3.88%), thymol (3.13 and 3.63%) and α-humulene (3.27 and 3.27%) in wild and cultivated M. hedgei. Antimicrobial activity of essential oils was investigated by disc diffusion method. Essential oil showed good antimicrobial activity against five medically important pathogens compared with standard antibiotics.


Introduction
The genus Micromeria belongs to the family of Lamiaceae and includes 70-90 herbs, sub-shrub and shrubs distributed in temperate area (Chater & Guinea 1972). This genus is represented in flora of Iran by four species: Micromeria cristata subsp. cristata, Micromeria hedgei, Micromeria myrtifolia and Micromeria persica which M. hedgei is endemic (Jamzad 2013). The essential oil and extract of Micromeria species has been reported as medicinal plant, for example, the leaves have been reported to possess anti-inflammatory, antimicrobial (Stojanović et al. 2006), and antioxidant activity (Öztürk et al. 2009; Šamec et al. 2015), and some are used in folk medicine for other human ailments as inflamed eyes, wounds, skin infections, stomach ache, chest pain, colds, fevers and others (Palevitch & Yaniv 1991;Vuko et al. 2012). The chemical composition of the essential oils and biological activities of different Micromeria species have already been reported (Tzakou & Couladis 2001;Chalchat et al. 2005;Mastelić et al. 2005;Slavkovska et al. 2005;Kostadinova et al. 2007). Although the M. hedgei plant and its extract are used extensively in the folk medicine among native people, no scientific evidences are available in the literature to support its medical uses. The knowledge on biology of this rare and endemic species is essential for future protection and conservation measures. The current study focuses on essential oil composition and antimicrobial activities of wild and cultivated M. hedgei oil on some pathogenic microorganisms which can cause infection disease in human. As far as we know, the composition and antimicrobial activity of the essential oil of wild and cultivated M. hedgei has not been the subject of previous study, and this is the first report on these properties of M. hedgei.

Antimicrobial activity
The antimicrobial activity of M. hedgei essential oils against microorganisms which are considered in this study was assessed by evaluating the presence of inhibition zones (Iz) and minimum inhibitory concentrations (MIC) values. The antimicrobial activity of the oils had varying degrees of inhibition against the tested microorganisms. All essential oils exhibited a significant antimicrobial activity against all five microorganisms tested, compared with the positive standard antibiotics (Table S4), the values of Iz and MIC being in the ranges of 17-25 mm and 0.78-3.12 μg/mL, respectively. Significant differences were observed between the antimicrobial activities of essential oils obtained at different habitats. Generally, the antimicrobial activity of the essential oils of the cultivated plants (Estahban region) was higher than those of the samples collected from natural habitat (Bokhoon region). Also, the bacteria appear to be more sensitive compared to the fungi (Candida albicans), which could be explained by their different cell organisation. The highest antibacterial activity was observed against Staphylococcus aureus and Bacillus cereus with the strongest Iz (25 mm) for the essential oil from the cultivated plants of Estahban region, while the essential oil from the natural habitat of Bokhoon region exhibited weak antimicrobial activity against C. albicans (17 mm). The mechanisms by which the essential oil can inhibit microorganisms vary. In some cases, it may be due to the hydrophobicity of the constituent in the essential oil which penetrates into the lipid bilayer of the cell membrane and makes the cells more permeable, leading to leakage of vital cell contents (Kim et al. 1995;Burt 2004). The essential oil components dissolve in the membrane, causing swelling and reducing membrane function, and lead to cell death (Holley & Patel 2005). The antimicrobial property of this species could be resulted from the relatively high amount of monoterpenes (geranial, geraniol, neral and nerol) and phenolic compounds (thymol and carvacrol) in the essential oils of M. hedgei. Moreover, the antimicrobial activity of the essential oils may be due to the presence of synergy between the major components and other constituents of the oils leading to various degrees of antimicrobial activity. This study reveals that the isolated essential oils have antimicrobial activities against important human pathogenic microorganisms. Hence, the present results support traditional and ethno medical uses of this plant against various infections, whose properties could be easily attributed to its essential oil.

Conclusions
In this study, the essential oil constituent and antimicrobial activity of wild and cultivated M. hedgei from Iran were reported for the first time. The main components of the oil were geranial, neral, geraniol, nerol, E-caryophyllene, carvacrol, geranyl acetate, caryophyllene oxide, thymol and α-humulene in wild and cultivated M. hedgei. Essential oil showed good antimicrobial activity against five medically important pathogens.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the Estahban Branch of the Islamic Azad University Research and Technology Council.