Chemical Composition of Kickxia aegyptiaca Essential Oil and Its Potential Antioxidant and Antimicrobial Activities

The exploration of new bioactive compounds from natural resources as alternatives to synthetic chemicals has recently attracted the attention of scientists and researchers. To our knowledge, the essential oil (EO) of Kickxia aegyptiaca has not yet been explored. Thus, the present study was designed to explore the EO chemical profile of K. aegyptiaca for the first time, as well as evaluate its antioxidant and antibacterial activities, particularly the extracts of this plant that have been reported to possess various biological activities. The EO was extracted from the aerial parts via hydrodistillation and then characterized by gas chromatography-mass spectrometry (GC-MS). The extracted EO was tested for its antioxidant activity via the reduction in the free radicals, 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). In addition, the EO was tested as an antibacterial mediator against eight Gram-negative and Gram-positive bacterial isolates. Forty-three compounds were identified in the EO of K. aegyptiaca, with a predominance of terpenoids (75.46%). Oxygenated compounds were the main class, with oxygenated sesquiterpenes attaining 40.42% of the EO total mass, while the oxygenated monoterpenes comprised 29.82%. The major compounds were cuminic aldehyde (21.99%), caryophyllene oxide (17.34%), hexahydrofarnesyl acetone (11.74%), ar-turmerone (8.51%), aromadendrene oxide (3.74%), and humulene epoxide (2.70%). According to the IC50 data, the K. aegyptiaca EO revealed considerable antioxidant activity, with IC50 values of 30.48 mg L−1 and 35.01 mg L−1 for DPPH and ABTS, respectively. In addition, the EO of K. aegyptiaca showed more substantial antibacterial activity against Gram-positive bacterial isolates compared to Gram-negative. Based on the minimum inhibitory concentration (MIC), the EO showed the highest activity against Escherichia coli and Bacillus cereus, with an MIC value of 0.031 mg mL−1. The present study showed, for the first time, that the EO of K. aegyptiaca has more oxygenated compounds with substantial antioxidant and antibacterial activities. This activity could be attributed to the effect of the main compounds, either singular or synergistic. Thus, further studies are recommended to characterize the major compounds, either alone or in combination as antioxidants or antimicrobial agents, and evaluate their biosafety.


Introduction
Human beings are putting increased pressure on the planet's various resources. Scientists and researchers are doing their best to explore new, green, eco-friendly natural    The identified chemical compounds are listed in detail, along with their retention times (Rt), and Kovats indexes (KI) in Table 1. These compounds can be categorized into seven classes of components, including monoterpenes (oxygenated and hydrocarbons), sesquiterpenes (oxygenated and hydrocarbons), diterpenes (oxygenated only), carotenoid derived components, and other hydrocarbons ( Figure 2). These data revealed that this oil is very rich in terpenoid compounds, which represented 75.46% of the total EO mass. From overall terpenoids, a relative concentration of 70.57% was found in oxygenated forms. The abundance of terpenoids, particularly the oxygenated ones, was in harmony with the published data for K. spuria EO [10].   Sesquiterpenes were identified with a relative concentration of 44.68% of the overall oil mass, including both the oxygenated sesquiterpenes (40.42%) and sesquiterpenes hydrocarbon (4.26%) forms. The dominance of the sesquiterpenes is also described in the EO of K. spuria [10]. Out of the 14 identified oxygenated sesquiterpenes, caryophyllene oxide (17.34%), ar-turmerone (8.51%), aromadendrene oxide (3.84%), and humulene epoxide (2.70%) represented the major ones ( Figure 3), while trans-nerolidol (0.25%) represented the minor. Sesquiterpenes were identified with a relative concentration of 44.68% of the overall oil mass, including both the oxygenated sesquiterpenes (40.42%) and sesquiterpenes hydrocarbon (4.26%) forms. The dominance of the sesquiterpenes is also described in the EO of K. spuria [10]. Out of the 14 identified oxygenated sesquiterpenes, caryophyllene oxide (17.34%), ar-turmerone (8.51%), aromadendrene oxide (3.84%), and humulene epoxide (2.70%) represented the major ones ( Figure 3), while trans-nerolidol (0.25%) represented the minor. Caryophyllene oxide was determined in a high concentration (8.90%) in the EO of K. spuria [10]. Some of the identified compounds were also described in the constituents of K. spuria EO, such as ar-curcumene, spathulenol, as well as the cis isomer of sesquisabinene hydrate. Caryophyllene oxide, ar-turmerone, aromadendrene oxide, and humulene epoxide are widely distributed compounds in the EOs of several plants, such as  Caryophyllene oxide was determined in a high concentration (8.90%) in the EO of K. spuria [10]. Some of the identified compounds were also described in the constituents of K. spuria EO, such as ar-curcumene, spathulenol, as well as the cis isomer of sesquisabinene hydrate. Caryophyllene oxide, ar-turmerone, aromadendrene oxide, and humulene epoxide are widely distributed compounds in the EOs of several plants, such as Centaurea species [20], Artemisia campestris [21], Cullen plicata [2], Chromolaena odorata [22], and Heliotropium curassavicum [23]. On the other side, five sesquiterpene hydrocarbons were assigned, including isocaryophillene (1.97%) and longicyclene (1.07%) as the main compounds.
Monoterpenes represented the second class of identified compounds (30.45%), which encompass oxygenated monoterpenes as the main compounds, with a relative concentration of 29.82%, along with 0.63% of monoterpene hydrocarbons. Seven compounds were assigned as oxygenated monoterpenes, in which cuminic aldehyde (21.99%) and pcymen-7-ol (2.17%) were determined as major compounds. The profile of the monoterpenes was totally different compared to that reported in K. spuria EO, except for eugenol [10]. This variation could be ascribed to the genetic differences in both species [24], and the environmental and climatic conditions have also been reported to affect the composition of the EO [23,[25][26][27]. Cuminic aldehyde is basically the main compound of Cuminum cyminum, with a concentration of 22.4-41.5% [28][29][30]. On the other hand, p-cymen-7-ol was described as a major compound in the EOs of several plants, for instance, Curcuma cf. xanthorrhiza [31], Eucalyptus largiflorens [32].
Diterpenes have been known as rarely described compounds in EOs of the aromatic plants; nevertheless, they were reported as a major compound in the EO of Lactuca serriola [33], Euphorbia mauritanica [34], Araucaria bidiwillii [35], Araucaria heterophylla [3,36]. The results of the current study agreed with the scarcity of diterpenes, identifying only one oxygenated diterpenoid, trans-geranylgeraniol (0.33%), with a complete absence of diterpene hydrocarbons.
In addition to terpenoid components, four carotenoid-derived compounds were determined in the EO of K. aegyptiaca (Table 1). Hexahydrofarnesyl acetone attained a remarkable concentration (14.23%) in the K. aegyptiaca (Figure 3). This compound was assessed as an abundant constituent of EOs of Launaea mucronata and Launaea nudicaulis [37], H. curassavicum [23], and Bassia muricata [38]. The other hydrocarbons were represented with a relative concentration of 7.67% and ten compounds were represented as a mixture of oxygenated and non-oxygenated compounds. Benzyl acetylacetate with a concentration of 2.32% represented the main non-terpenoids, while n-nonadecane (0.27%) represented the minor one. The presence of hydrocarbons in the EO of K. aegyptiaca is consistent with the published data of Iranian K. spuria [10].

Antioxidant Activity of K. aegyptiaca EO
The EO of K aegyptiaca showed a substantial antioxidant activity based on both DPPD and ABTS methods compared to the ascorbic acid as a standard synthetic antioxidant ( Figure 4). The scavenging activity increased with the increment of EO concentration. At a concentration of 20 mg mL −1 of K. aegyptiaca EO, the DPPH and ABTS colors were reduced by 39.85% and 33.16%, respectively, while ascorbic acid showed a reduction of 83.48% and 71.33%, respectively, at the same concentration ( Figure 4).
According to the IC 50 data, the K. aegyptiaca EO revealed IC 50 values of 30.48 mg L −1 and 35.01 mg L −1 for DPPH and ABTS, respectively. The standard antioxidant, ascorbic acid, showed IC 50 values of 9.45 mg L −1 and 12.61 mg L −1 , regarding DPPH and ABTS, respectively. The considerable antioxidant activity of K. aegyptiaca EO in the present study could be attributed to the effect of key compounds, such as cuminic aldehyde, caryophyllene oxide, hexahydrofarnesyl acetone, ar-turmerone, aromadendrene oxide, and humulene epoxide. These compounds could act in either singular or in synergistic ways [39,40]. The major compound (cuminic aldehyde) has been reported in a high concentration (52.56%) in C. cyminum, showing strong antioxidant activity [29,41]. On the other hand, the second major compound in the present study (caryophyllene oxide) has been reported to possess substantial antioxidant activity [2,42]. The carotenoid-derived compound, hexahydrofarnesyl acetone, has been reported in the EOs of various plants that showed strong antioxidant activity, such as Launaea species [37], H. curassavicum [23], and B. muricata [38]. The aromadendrene oxide-rich EO of Cleome amblyocarpa has been described to have allelopathic, antioxidant, and anti-inflammatory activities [43]. The K. aegyptiaca EO showed a higher antioxidant activity than the EOs of other reported plants, such as Persicaria lapathifolia [25], Cleome droserifolia [44], and Deverra tortuosa [45], while it showed a lower antioxidant activity than those reported for the EOs of E. mauritanica [34]. The EO of K aegyptiaca showed a substantial antioxidant activity based on both DPPD and ABTS methods compared to the ascorbic acid as a standard synthetic antioxidant (Figure 4). The scavenging activity increased with the increment of EO concentration. At a concentration of 20 mg mL −1 of K. aegyptiaca EO, the DPPH and ABTS colors were reduced by 39.85% and 33.16%, respectively, while ascorbic acid showed a reduction of 83.48% and 71.33%, respectively, at the same concentration ( Figure 4).

Antibacterial Activity of K. aegyptiaca EO
The EO extracted from K. aegyptiaca aerial parts displayed considerable antibacterial activity against Gram-positive and Gram-negative bacterial isolates ( Table 2). The EO showed varying inhibitory activity on various bacterial strains, but it did not reveal antibacterial activity against Streptococcus epidermis (Gram-negative strain). The antibacterial effect can be ordered as follows: Salmonella typhimurium > Bacillus cereus > Escherichia coli > Staphylococcus aureus > Pseudomonas aeruginosa > Staphylococcus xylosus > Staphylococcus haemolyticus ( Table 2).
The selected antibiotics showed varied activity against the bacterial strains, with a general trend that Gram-negative bacteria were more resistant than Gram-positive strains. This observation is consistent with most research [26,[45][46][47][48][49], where it is ascribed to the structure of the bacterial cells [46]. Cephradin showed the highest activity against S. haemolyticus, while it was inactive against P. aeruginosa and S. typhimurium at a dose of 10 mg mL −1 . The tetracycline exhibited maximum inhibition on S. epidermis, but did not show activity against P. aeruginosa. On the other hand, the antibiotic azithromycin showed maximum activity against S. aureus, S. epidermis, and B. cereus at a concentration of 10 mg mL −1 , while it did not show any activity against S. typhimurium. At a concentration of 10 mg mL −1 , ampicillin was detected as a powerful antibacterial agent against S. aureus, while it did not have any activity against P. aeruginosa and S. typhimurium (Table 2). Based on the data of the minimum inhibitory concentration (MIC), the EO activity was highest (0.031 mg mL −1 ) againt E. coli and B. cereus, while the activity against the other bacterial isolates can be sequenced as follows: S. typhimurium, P. aeruginosa, S. aureus, S. xylosus, and S. haemolyticus. However, S. epidermis was comopletely resistant to the EO of K. aegyptiaca. The antibacterial activity of the K. aegyptiaca EO in the present study was higher than those reported for the EO of D. tortuosa [45] and Teucrium polium [48], while it was lower than others, such as Thymus decussatus [48], Achillea fragrantissima, Artemisia Judaica, and Tanacetum sinaicum [47]. Table 2. Antibacterial activity of the essential oil extracted from K. aegyptiaca aerial parts, expressed by the diameter of the inhibition zone (mm) and minimum inhibitory concentration (MIC), as well as some selected reference antibiotics at a concentration of 10 mg mL −1 . The observed antibacterial activity of K. aegyptiaca could be attributed to the activity of the major compounds (cuminic aldehyde, caryophyllene oxide, hexahydrofarnesyl acetone, ar-turmerone, aromadendrene oxide, and humulene epoxide), either singly or synergistically. The insecticidal activity of Rosmarinus officinalis has been attributed to the synergistic interaction between camphor and 1,8-cineole [40]. A similar study by de Sousa, et al. [50] indicated that the combination of carvacrol and 1,8-cineole maximizes the inhibitory activity against bacterial strains associated with vegetable processing. In addition, cuminic aldehyde has been reported to have antibacterial effects [29,30]. In contrast to our results, the EO of C. cyminum, rich with cuminic aldehyde, did not show antibacterial activity against Pseudomonas species [29].

Microbes
Caryophyllene oxide has been reported as a strong antimicrobial agent against a wide range of microbes [51]. In addition, the EOs that were rich in caryophyllene oxide were reported to have a considerable antimicrobial activity, such as Satureja coerulea [52], Psidium guajava [53], and Pinus eldarica [54]. Moreover, EOs rich in hexahydrofarnesyl acetone have been described as possessing considerable antimicrobial activity [55,56]. In addition, ar-turmerone has been reported as antimicrobial agent in Artemisia integrifolia [57]. Our findings supported the potential uses of the EO of K. aegyptiaca in the food industries, as well as in the manufacturing of cosmetics and aromas, due to its potent antioxidant and/or antimicrobial significance.

Plant Materials
The K. aegyptiaca aerial parts were collected during the flowering season (April 2019) from different populations growing in Wadi Araba, Eastern Desert, Egypt (28.9781482N, 32.2019523E). The collected samples were healthy and flowering. Plant authentication was carried out following Boulos [13] and Tackholm [58]. From the collected sample, a voucher specimen was prepared and deposited in the Botany Department Herbarium at College of Science, Mansoura University, Egypt, with the code Mans.191101007 ( Figure 5).

Extraction of EO and GC-MS Analysis
The EO was extracted via the subjection of ~180 g of the air-dried K. aegyptiaca aerial parts to hydro-distillation for 3 h over the Clevenger apparatus. The separation of the EO layer was performed by n-hexane, dried by anhydrous Na2SO4 (0.5 g), and then saved at 4 °C in glass vials until further chemical and biological analyses. The extracted EO was chemically analyzed via gas chromatography-mass spectrometry (GC-MS). The characterization and identification of chemical constituents were performed with the same conditions and protocol as described previously [25,59]. The GC-MS apparatus was made up of TRACE GC Ultra-Gas Chromatographs (THERMO Scientific™ Corporate, Waltham, MA, USA) with a quadrupole mass spectrometer (Thermo Scientific ISQ™ EC, Waltham, MA, USA). The GC-MS column dimension was 30 m × 0.32 mm and i.d. 0.25 µm film thickness. At a flow rate of 1.0 mL per min, helium was used as a transporter gas, with a split ratio of 1 to 10. The temperature program was accustomed as follows: 60 °C for 1 min., raised to 240 °C with 4 °C/min. The diluted sample in n-hexane (1 µL) at a ratio of 1:10 (v/v) was injected into the instrument, where the injector and detector were adjusted at 210 °C. The mass spectra of compounds were charted by electron ionization (EI) at 70 eV, using a spectral range of m/z 40-450. The authentication and identification of the

Extraction of EO and GC-MS Analysis
The EO was extracted via the subjection of~180 g of the air-dried K. aegyptiaca aerial parts to hydro-distillation for 3 h over the Clevenger apparatus. The separation of the EO layer was performed by n-hexane, dried by anhydrous Na 2 SO 4 (0.5 g), and then saved at 4 • C in glass vials until further chemical and biological analyses. The extracted EO was chemically analyzed via gas chromatography-mass spectrometry (GC-MS). The characterization and identification of chemical constituents were performed with the same conditions and protocol as described previously [25,59]. The GC-MS apparatus was made up of TRACE GC Ultra-Gas Chromatographs (THERMO Scientific™ Corporate, Waltham, MA, USA) with a quadrupole mass spectrometer (Thermo Scientific ISQ™ EC, Waltham, MA, USA). The GC-MS column dimension was 30 m × 0.32 mm and i.d. 0.25 µm film thickness. At a flow rate of 1.0 mL per min, helium was used as a transporter gas, with a split ratio of 1 to 10. The temperature program was accustomed as follows: 60 • C for 1 min., raised to 240 • C with 4 • C/min. The diluted sample in n-hexane (1 µL) at a ratio of 1:10 (v/v) was injected into the instrument, where the injector and detector were adjusted at 210 • C. The mass spectra of compounds were charted by electron ionization (EI) at 70 eV, using a spectral range of m/z 40-450. The authentication and identification of the chemical compounds were performed using the Automated Mass spectral Deconvolution and Identification (AMDIS) software, NIST library database, Wiley spectral library collection, retention indices relative to n-alkanes (C 8 -C 22 ), or assessment of the mass spectrum with authentic standards compounds. The relative concentrations of the compounds were performed based on Tentatively Identified Compounds (TICs) of the EO.

Antioxidant Activity of the EO
To test the antioxidant activity K. aegyptiaca EO, two protocols were considered: (a) reduction in the radical 2,2-diphenyl-1-picrylhydrazyl (DPPH, Sigma-Aldrich, Darmstadt, Germany) and (b) reduction in the radical 2,2 -azinobis(3-ethylbenzothiazoline-6sulfonic acid) (ABTS, Sigma-Aldrich, Darmstadt, Germany). In the DPPH assay, the EO was prepared in a concentration range of 5-50 mg L −1 , using methanol as a solvent. This range was selected based on the scavenging activity that enabled us to determine the IC 50 (EO amount necessary to reduce the radical by 50%) [45]. According to Miguel [60], equal volumes of each concentration and DPPH (0.3 mM) were shaken vigorously and kept in dark conditions for 30 min. The absorbance was assessed at 517 nm via spectrophotometer, model Spectronic 21D, Milton Roy, CA, USA. On the other side, the ABTS assay was conducted according to Re, et al. [61]. In brief, about 0.2 mL of each concentration was mixed with 2 mL of freshly prepared ABTS and incubated in a dark condition for 6 min. The range of the EO concentration was similar to those of DPPH (5-50 mg L −1 ). The color absorbance was measured at 734 nm by Spectronic 21D spectrophotometer, Milton Roy, CA, USA. In addition, to refer the antioxidant activity to standard antioxidant, various concentrations (1-20 mg L −1 ) of ascorbic acid were prepared and their antioxidant activity was determined as previously described for the EO. The scavenging activity was calculated based on the following formula: Scavenging activity (%)= 100 × Absorbance sample − Absorbance sample Absorbance sample

Antibacterial Activity of the EO
The EO extracted from K. aegyptiaca aerial parts was tested for its antibacterial activity against four Gram-negative bacterial strains (E. coli (ATCC 10536), P. aeruginosa (ATCC 9027), S. typhimurium (ATCC 25566), and S. epidermis (ATCC 12228)) and four Gram-positive bacterial strains (B. cereus (EMCC number), S. aureus (ATCC 6538), S. haemolyticus (ATCC 29970), and S. xylosus (NCCP 10937)). The bacterial isolates were obtained from the Cairo Microbiological Resources Centre (Cairo MIRCEN), College of Agriculture, Ain Shams University, Egypt. The bioassay was performed using the agar diffusion method [62]. In brief, filter paper discs (Whatman no.1, 5 mm) were saturated with the EO of K. aegyptiaca at a concentration of 10 mg mL −1 in dimethyl sulfoxide. Petri dishes (90 mm) were prepared and filled with sterilized nutrient agar medium and inoculated with 10 6 colonyforming units (CFU)/mL of each bacterial strain. The filter paper disc was adjusted above the medium in the center of the Petri dish, and the 1080 plates were immediately sealed with Parafilm ® tape (Sigma, St. Louis, MO, USA) and incubated for 24 h at 37 • C. After incubation, the diameter of the inhibition zone (clear zone around disc without growth) was measured in mm at three random positions. The MIC was determined based on the dimensions of the inhibition zone for different EO concentrations. To compare the antibacterial activity of the EO with reference antibiotics, cephradin, tetracycline, azithromycin, and ampicillin were subjected to the same procedures.

Statistical Analysis
The experiments of both antioxidant activity and antibacterial activity were achieved three times with three replications for each treatment. The data were subjected to one-way analysis of variance (ANOVA), after Duncan's test using CoStat software program (version 6.311, CoHort Software, Monterey, CA, USA). The data were expressed as mean values with standard error. The IC 50 value for antioxidant assays was graphically calculated using MS-Excel 2016.

Conclusions
A GC-MS analysis of the extracted EO from the aerial parts of K. aegyptiaca revealed, for the first time, the presence of 43 compounds, mainly terpenes. Oxygenated compounds were predominant, particularly sesquiterpenes and monoterpenes. Cuminic aldehyde, caryophyllene oxide, hexahydrofarnesyl acetone, ar-turmerone, aromadendrene oxide, and humulene epoxide were identified as major compounds with a concentration of 66.02% of the total mass. The extracted EO showed considerable antioxidant activity as well as antibacterial activity. The major compounds have been reported to possess various biological activities, including antioxidant and antimicrobial activities. Therefore, further studies are recommended to evaluate the various biological activities of the major identified compounds, either alone or in combination, as well as to assess their biosafety.