Chemical Composition and Biological Activities of the Essential Oils from Three Melaleuca Species Grown in Tunisia

The chemical composition of the essential oils of Melaleuca armillaris Sm., Melaleuca styphelioides Sm. and Melaleuca acuminata F. Muell., collected in Tunisia, was studied by means of GC and GC-MS analysis. In all, 46 compounds were identified, 38 for M. armillaris, 20 for M. acuminata and eight for M. styphelioides, respectively. The presence of a sesquiterpenic fraction (52.2%) characterized the oil from M. armillaris; M. sthypheliodes oil was rich in methyl eugenol, a phenolic compound (91.1%), while M. acuminata oil is mainly constituted by oxygenated monoterpenoids (95.6%). The essential oils were evaluated for their in vitro potentially phytotoxic activity against germination and initial radicle growth of Raphanus sativus L., Lepidium sativum L., Sinapis arvensis L., Triticum durum L. and Phalaris canariensis L. seeds. The radicle elongation of five seeds was inhibited at the highest doses tested, while germination of all seeds was not affected. Moreover, the essential oils showed low antimicrobial activity against eight selected microorganisms.


Chemical Composition of the Essential Oils
Hydrodistillation yielded 0.65%, 0.53% and 0.35% of essential oil (on a dry mass basis) for M. armillaris, M. acuminata and M. stypheliodes, respectively. Table 1 shows the chemical composition of the three Melaleuca oils; compounds are listed according to their elution order on a HP-5MS column. In all, 46 compounds were identified, 38 for M. armillaris, accounting for 99.3% of the total oil, 20 for M. acuminata, accounting for 99.7% of the total oil, and eight for M. styphelioides (92.4%), respectively. In the oil from M. armillaris, the oxygenated monoterpenoids amounted to 44.6%; on the other hand, the total sesquiterpenic fraction amounted to 51.6% of the total oil. The main compounds are the sesquiterpene cis-calamenene (19.0%) and the oxygenated sesquiterpene torreyol (15.1%). Other compounds, in a lesser amount, are dihydrocarveol (9.0%) and α-terpineol (7.7%), both oxygenated monoterpenoids. In the literature, Chabir and coworkers [8,9] reported 1,8-cineole (85.8%) as the most abundant compound in M. armillaris. The major compound in the essential oil of M. styphelioides was methyl eugenol (91.1%), a phenolic compound. In the literature, Farag and coworkers [12] reported that the essential oil of this species contained mainly caryophyllene oxide (43.8%), followed by (−)-spathulenol (9.7%). In another paper [13], the same authors reported that the essential oil of this species contained mainly caryophyllene (50.0%) and methyl eugenol (26.6%).
In the literature, only two references reported the chemical composition of the essential oil of M. acuminata: Smith and coworkers [14,15] reported that cineole was the main component of the essential oil of M. acuminata.

Phytotoxic Activity
The three essential oils were evaluated for their phytotoxic activity against germination and radicle elongation ( Table 2) of radishes and garden cress, two species frequently utilized in biological assays, and of wild mustard, wheat and canary grass, three weed species.

Sinapis arvensis Germinated seeds ± SD (cm)
Sinapis arvensis Radicle elongation ± SD (cm)  The oils seem to be ineffective against germination, but they affected the radicle elongation of the five tested seeds. The essential oil of M. styphelioides, at all doses tested, significantly inhibited the radicle elongation of garden cress. The radicle elongation of wild mustard and radish were inhibited by M. acuminata oil at the highest doses (2.5 μg/mL, 1.25 μg/mL) used. At doses of 1.25 and 0.625 μg/mL, the essential oil of M. acuminata significantly inhibited the radicle elongation of canary grass ( Table 2). The difference in biological activity of the oils could be attributed to their different chemical composition.

Doses
On the other hand, the oil of M. acuminata was rich in oxygenated monoterpenoids, trans-pinocarveol, dihydrocarveol, myrtenol and 1,8-cineole. trans-Pinocarveol was reported as one of the main components of the strong phytotoxic oil from a Cistus ladanifer L. population [18].
Yatagai and coworkers [19] reported that the leaf oil of Melelauca bracteata F. Muell. had the strongest germination and growth-inhibition activity against radish seeds.
The roots were probably more sensitive than shoots to the phytotoxic activity of the oil; the process of germination was active while the oil probably affected the elongation process. Such activity of the essential oils could help to explain the ecological role of the genus Melaleuca in the Mediterranean area.

Antimicrobial Activity
The Minimum Inhibitory Concentration (MIC) and the Minimum Bactericidal Concentration (MBC) values of the essential oils against eight selected microorganisms are reported in Table 3. The essential oils showed inhibitory activity against the gram-positive pathogens, among which is S. epidermidis. Among gram-negative bacteria, E. coli was affected by the oil of M. armillaris and M. acuminata. The essential oil of M. acuminata was more active than other oils and presumably this activity is related to the high amounts of oxygenated monoterpenoids.
Farag and coworkers [12,13] reported the effect of the essential oil from the leaves of M. armillaris, M. ericifolia (Smith), M. leucadendron (Linn.) and M. styphelioides against the growth of some microorganisms. The results demonstrated that the degree of the microbial inhibition is largely dependent on the species. M. ericifolia exhibited the highest inhibitory effects against Bacillus subtilis and Aspergillus niger. The antimicrobial properties of Melaleuca essential oil have been reported in several studies [20] Terpinen-4-ol is considered to be the principal active component of Melaleuca alternifolia Cheel (tea tree) oil [21][22][23]. Terpinen-4-ol could constitute an interesting alternative in the therapy of MRSA infections of the skin [5]. In the literature, it was reported that some members of the Myrtaceae family (Eucalyptus globulus and Melaleuca alternifolia) whose essential oil consisted mainly of monocyclic and bicyclic monoterpenes (e.g., 1,8-cineole and terpinen-4-ol) effectively inhibited the growth of drug-resistant bacterium strains [5].

Plant Material
Leaves of Melaleuca armillaris, M. acuminata and M. styphelioides Sm. were collected from the Botanical Garden of the National Institute of Researches on Rural Engineering, Water and Forests (Ariana, Tunisia) in April 2011. Five samples were collected from more than five different trees, mixed for homogenization and used in three replicates for essential oil extractions. Specimens were identified by Dr. H. Lamia, and voucher specimens were deposited at the herbarium of the Laboratory of Forestry Ecology at the National Institute of Research on Rural Engineering, Water and Forest (Tunisia).

Isolation of the Volatile Oils
One hundred grams of dried leaves of each Melaleuca species were ground in a Waring blender and then, subjected to hydrodistillation for 3 h according to the standard procedure described in the European Pharmacopoeia [24].
The oils were solubilized in n-hexane, dried over anhydrous sodium sulphate and dried under N 2 to remove hexane. Samples were stored at +4 °C in the dark until tested and analyzed.

GC-FID Analysis
The GC-FID analysis was carried out on a Perkin-Elmer Sigma-115 gas chromatograph equipped with a flame ionization detector (FID) and a data handling processor. The separation was achieved using an apolar HP-5 MS fused-silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness). Column temperature: 40 °C, with 5 min initial hold, and then to 270 °C at 2 °C/min, 270 °C (20 min); injection mode: splitless (1 μL of a 1:1000 n-pentane solution). Injector and detector temperatures were 250 °C and 290 °C, respectively. Analysis was also run by using a fused silica HP Innowax polyethylenglycol capillary column (50 m × 0.20 mm i.d., 0.25 μm film thickness). In both cases, helium was used as carrier gas (1.0 mL/min).

GC/MS Analysis
The GC/MS analysis was performed on an Agilent 6,850 Ser. II apparatus, fitted with a fused silica DB-5 capillary column (30 m × 0.25 mm i.d., 0.33 μm film thickness), coupled to an Agilent Mass Selective Detector MSD 5973; ionization energy voltage 70 eV; electron multiplier voltage energy 2000 V. Mass spectra were scanned in the range 40-500 amu, scan time 5 scans/s. Gas chromatographic conditions were as reported in the previous paragraph; transfer line temperature, 295 °C.

Identification of the Essential Oils Components
Most constituents were identified by gas chromatography by comparison of their Kovats retention indices (Ri) with either those of the literature [25,26] or with those of authentic compounds available in our laboratories. The Kovats retention indices were determined in relation to a homologous series of n-alkanes (C 10 -C 35 ) under the same operating conditions. Further identification was made by comparison of their mass spectra on both columns with either those stored in NIST 02 and Wiley 275 libraries or with mass spectra from the literature [25,27] and a homemade library. The components relative concentrations were obtained by peak area normalization. No response factors were calculated.

Biological Assay
A bioassay based on germination and subsequent radicle growth was used to study the phytotoxic effects of the three essential oils on seeds of Raphanus sativus L. cv. "Saxa" (radish), Lepidium sativum L. (garden cress) and the three weed species Sinapis arvensis L. (wild mustard), Triticum durum L. (wheat) and Phalaris canariensis L. (canary grass). The seeds of radish and garden cress were purchased from Blumen srl (Piacenza, Italy), while mustard, wheat and canary grass were collected from wild plants. The seeds were surface sterilized in 95% ethanol for 15 s and sown in Petri dishes (Ø = 90 mm) containing five layers of Whatman filter paper impregnated with distilled water (7 mL, control) or a tested solution of the essential oil (7 mL) at the different assayed doses. The germination conditions were 20 ± 1 °C with a natural photoperiod. The essential oils, in a water-acetone mixture (99.5:0.5), were assayed at the doses of 2.5, 1.25, 0.625, 0.25, 0.125 and 0.062 μg/mL. Controls performed with water-acetone mixture alone showed no appreciable differences in comparison with controls in water alone. Seed germination was observed directly in Petri dishes, each 24 h. A seed was considered germinated when the protrusion of the root became evident [28]. After 120 h (on the fifth day), the effects on radicle elongation were measured in centimeter. Each determination was repeated three times, using Petri dishes containing 10 seeds each. Data are expressed as the mean ± SD for both germination and radicle elongation. Data were ordered in homogeneous sets, and the Student's t test of independence was applied [29].

Determination of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC)
The antibacterial activity was evaluated by determining the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) using the broth dilution method [30][31][32]. Eight bacterial species, selected as representative of the gram-positive and gram-negative classes, were tested: Staphylococcus aureus (ATTC 25923), Streptococcus faecalis (ATTC 29212), Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853), Staphylococcus epidermidis (ATCC 12228), Klebsiella pneumoniae (ATCC 10031) and Proteus vulgaris (ATCC 13315). The strains were maintained on Tryptone Soya agar (Oxoid, Milan, Italy); for the antimicrobial tests, Tryptone Soya broth (Oxoid, Milan, Italy) was used. In order to facilitate the dispersion of the oil in the aqueous nutrient medium, it was diluted with Tween 20 at a concentration of 10%. Each strain was tested with a sample that was serially diluted in broth to obtain concentrations ranging from 100 μg/mL to 0.8 μg/mL. The sample was previously sterilized with a 0.20 μm Millipore filter. The sample was stirred, inoculated with 50 μL of physiological solution containing 5 × 10 6 microbial cells and incubated for 24 h at 37 °C. The MIC value was determined as the lowest concentration of the sample that did not permit any visible growth of the tested microorganism after incubation. The control, containing only Tween 20 instead of the essential oil, was not toxic to the microorganisms. Cultures, containing only sterile physiologic solution Tris buffer, were used as positive control. MBC was determined by subculture of the tubes with inhibition in 5 mL of sterile nutrient broth. After incubation at 37 °C, the tubes were observed. When the germs did not grow, the sample denoted a bactericidal action. Oil samples were tested in triplicate and the experiment was performed three times. The results are expressed as mean ± SD. Chloramphenicol was used as reference drug [17].

Conclusions
Data obtained in this paper could be useful in the chemotaxonomic knowledge of the genus Melaleuca that consists of about 230 species, the chemistry of M. armillaris, M. styphelioides and M. acuminata being little known. Data on phytotoxic activities could help to explain the ecological role of genus Melaleuca in the Mediterranean area. The antimicrobial activity of the essential oil is in agreement with the uses of other Melaleuca species as antimicronial drugs.