Blessings in disguise: a review of phytochemical composition and antimicrobial activity of plants belonging to the genus Eryngium

Medicinal and edible plants play a crucial role in the prevention and/or mitigation of different human diseases from ancient times to today. In folk medicine, there are different plants used for infectious disease treatment. During the past two decades, much attention has been paid to plants as novel alternative therapeutic agents for the treatment of infectious diseases due to their bioactive natural compounds such as phenol, flavonoids, tannins, etc. The genus Eryngium (Apiaceae) contains more than 250 flowering plant species, which are commonly used as edible and medicinal plants in different countries. In fact, some genus Eryngium species are used as spices and are cultivated throughout the world and others species are used for the treatment of hypertension, gastrointestinal problems, asthma, burns, fevers, diarrhea, malaria, etc. Phytochemical analysis has shown that genus Eryngium species are a rich source of flavonoids, tannins, saponins, and triterpenoids. Moreover, eryngial, one the most important and major compounds of genus Eryngium plant essential oil, possesses a significant antibacterial effect. Thus, the objective of this review is to critically review the scientific literature on the phytochemical composition and antibacterial effects of the genus Eryngium plants. In addition, we provide some information about traditional uses, cultivation, as well as phytochemistry.


Background
Infectious diseases are known as one of the most important leading causes of long and short-term morbidity and mortality worldwide [1,2]. According to the World Health Organization, in 2011, infectious diseases were responsible for approximately 18 million deaths worldwide. In addition to the high prevalence of infectious diseases, there are some microorganisms resistant to antibiotic therapy, which lead to the increase of death rate due their ability to acquire and transmit drug resistance [2]. Nowadays, antibiotic resistance is known as one the most important and challenging health problems in the global health programs. Therefore, during the past two decades, much attention has been paid to the discovery and development of natural multi-target antimicrobial agents with high efficacy and low adverse effects [3]. Natural products are known as one of the most important and effective drugs for human disease treatment [4][5][6]. In addition to their efficacy, natural products are mostly non-toxic and therefore, they can be used as safe therapeutic strategies [7][8][9]. A plethora of scientific evidence reported that edible and medicinal plants have significant potential to synthesize antimicrobial agents as their defense mechanisms against biotic stresses such as microorganisms [10]. It has been reported that plant-derived antimicrobial compounds can be categorized into the different groups such as phenols, flavonoids, terpenoids, lectins, polypeptides, polyacetylenes as well as alkaloids [11]. In addition, in traditional medicine, many edible and medicinal plants have been widely used for the treatment of different infectious diseases [11,12].
The genus Eryngium contains more than 250 flowering species worldwide [13]. Genus Eryngium is the largest and most complex genus in Apiaceae family [13,14]. Some species in the genus Eryngium are endangered such as E. alpinum L., E. aristulatum Jeps., E. constancei M.Y. Sheikh, E. cuneifolium Small, and E. viviparum J. Gay [13,14]. According to the morphological studies, the genus Eryngium has been classified into five subgenera, including Eryngium subgenus Eryngium, E. subgenus Monocotyloidea, E. subgenus Fruticosa, E. subgenus Semiaquatica, and E. subgenus Foetida [15,16]. E. subgenus Eryngium is the most common one throughout Europe, Africa and Asia, while the other subgenera are widely distributed in Australia [15,16]. However, infrageneric analysis through sequence data of chloroplast DNA trnQ-trnK 5'-exon and nuclear ribosomal DNA ITS regions showed that there are two different subgenera including Eryngium and Monocotyloidea [17][18][19].
E. caucasicum is known as one the most common edible leafy vegetables in northern part of Iran and widely used in different foodstuff, pickles, etc. [28,29]. Besides, the fruits of E. foetidum are known as common edible food components in Nigeria [30]. Actually, some Eryngium species are also used as ornamental plants [31]. A mountain of scientific evidence has shown that different species of the genus Eryngium possess antimicrobial effects under in vitro and in vivo conditions [32][33][34][35][36].
Therefore, the present paper aims to review the scientific literature on the phytochemical composition and antimicrobial activities of essential oils and extracts obtained from genus Eryngium species. In addition, we discuss about traditional uses, cultivation and phytochemistry of Eryngium species to provide a complete picture of this genus.

Data sources and search strategy
Data were collected from Medline, Pubmed, Scopus, Web of Science (ISI Web of Knowledge), Science Direct, Embase, and BIOSIS Previews (from 1950 to July 20, 2015), via searching of these keywords: "Eryngium and phytochemistry or chemical compounds", "Eryngium and antibacterial", "Eryngium and antimicrobial", and "Eryngium and biological effect". We also scanned the reference list of each paper and searched Cochrane review library. Thereafter, the bibliographies of collected data were screened for further publications. Finally, collected data were analyzed and judged by second and third authors according to the scientific standard of conduct. However, some of the references may be published after the initial search date July 2015.

Traditional uses
In traditional medicine, some of Eryngium species have been used for treatment of several human diseases [24][25][26][27]. It has been reported that E. campestre L. is widely used as antitussive, stimulant, aphrodisiac, and diuretic agent in Turkish traditional medicine [37,38]. E. caucasicum is one of the most important edible plants in the northern part of Iran due to its multiple beneficial effects on human health [29,31,39,40]. Moreover, E. creticum has been used as hypoglycemic plant in the Jordanian traditional medicine [41], while E. elegans Cham. & Schltdl. has been reported to be utilized as diuretic agent in the Argentinian traditional medicine [42]. Most of Eryngium species are also known as medicinal plants worldwide for the treatment of several human diseases such as diarrhea, gastrointestinal problems, bladder and kidney dysfunctions, and venereal diseases [24][25][26][27][43][44][45]. In Chinese traditional medicine, E. foetidum is widely used for treatment of inflammation [24] and E. yuccifolium Michx. roots have been traditionally used to treat snakebites, toothache [27,46,47], digestive problems, diarrhea, headache, etc. [24][25][26][27][43][44][45]. A large scale of evidence has pointed out to multiple pharmacological effects of the genus Eryngium species including antioxidant, anti-inflammatory, antihemolytic, antinociceptive effects, and protective agent against neurodegenerative deseases [25,28,[48][49][50]. For instance, Eryngium planum, which is a rare medicinal plant, was studied to determine the effect of subchronic administration of a 70 % ethanol root extract (200 mg/kg, p.o.) on behavioral and cognitive responses in experimental animals (Wistar rats) linked with the expression levels of mRNA coding for enzymes such as acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), and beta-secretase (BACE-1). At the end of the experiment, after the last dose of the Eryngium extract, scopolamine (SC) was administered intraperitoneally to a group of animals (treated). In the animals treated with the Eryngium extract, an improvement in long-term memory produced by the EP extract in both scopolamine treated and control group was registered with decreased mRNA AChE, BuChE, and BACE-1 levels, especially in the frontal cortex, suggesting the potential efficacy of this extract in this kind of pathologies [25,28,[48][49][50]. This is only an example of the fact in the las decade some investigations have been carried out to demonstrate the potential pharmacological activity of Eryngium species extracts [51,52].

Cultivation
Several genus Eryngium species have been widely used as edible plants in many countries and, consequently, mainly cultivated as an economic crop in tropical areas of the world [24,28,29]. In fact, numerous reports refer to the ideal and effective conditions for cultivating, harvesting as well as post-harvesting conditions of Eryngium species [24,31,53]. In the northern parts of Iran, E. caucasicum is one of the most important garden vegetables, which is mostly used in preparation of foodstuff, pickles, etc. [28,29]. It has been reported that some Eryngium species are easily cultivated in dry, sandy, well-drained soils, and full sun [24,31,53]. Root cuttings are a common protocol for propagation of Eryngium species [24,31,[53][54][55], which can also be propagated using other plant cuttings [24,31,[53][54][55]. In addition, there is a close correlation between Eryngium species growth and fertilizer levels in the soil [24,31,[53][54][55]. However, Eryngium species are significantly endangered by plant diseases as well as insect attacks [24,31,[53][54][55].

Phytochemistry
Based on the review of the literature up to now, the aerial parts of Eryngium species have been reported to contain mainly saponins, flavonoids, and essential oil, while the underground parts contain triterpene saponins, monoterpene glycosides, phenolic compounds such as flavonoids and phenolic acids, coumarin derivatives, terpene aldehyde esters, acetylenes, essential oil, and oligosaccharides [56][57][58][59][60].
The first phytochemical studies performed on the genus Eryngium starting from early 1970s were focused on their saponin content. According to the survey of the literature data published between 1970-1978 and in 1985, the identification of the isolated compounds was limited to the absolute definition of the aglycons with possible positions of sugars and acyl substituents because of inadequate chemical facilities in structure elucidation techniques [61,[66][67][68]. In this period, E. planum, E. amethystinum L., E. giganteum M. Bieb., and E. bromeliifolium F. Delaroche were studied in more detail. As summarized in Table 1, A1-barrigenol and R1-barrigenol-type of aglycons containing dimethylacrylic, angelic, and/or tyglic acids as the acid moieties were isolated from the roots of the E. planum, named as eryngiumgenine A-D [69] of which only aglycon types were identified and classified according to the R f values. Later, compound (5) was isolated from the aerial parts of this species [70]. Further phytochemical studies on the same plant were continued on the leaves and

Essential oil
Essential oil compositions of numerous Eryngium species have been investigated by many researchers. As illustrated with their corresponding references in Table 3, a great interspecies variation could be easily observed. However, the most common monoterpenes analyzed in different plant parts of Eryngium species have been as follows; germacrene D, α-pinene, caryophyllene, muurolene, αand βselinene, limonene, αand β-bisabolol, etc., whereas many hydrocarbons and some aromatics such as trimethylbenzaldehyde and dodecenal have been detected.

Antimicrobial effects of the genus Eryngium plants
Plants are known to produce antimicrobial substances [10], which act as plant defense mechanisms and protect them against abiotic and biotic stresses. These antimicrobial agents, which are often characterized by low adverse effects and wide spectrum activities, belong to many chemical classes such as phenolics and polyphenolics, terpenoids, alkaloids, lectins, polypeptides, and polyacetylenes [11].
Among the plants belonging to the genus Eryngium, some species exhibit considerable antimicrobial activity against gram-positive and gram-negative bacteria, some species of fungi and yeasts and viruses. Within this genus, the most studied species has been E. foetidum, which is cultivated across South Asia and Europe, Tropical Africa, and Pacific islands. As regards the antibacterial activity of E. foetidum extracts, the literature data are conflicting. In fact, in 2003, Alzoreky and Nakahara reported that the acetone and buffered methanol extracts obtained from E. foetidum leaves did not show any antibacterial activity against Escherichia coli, Salmonella   More recently, Ndip et al. showed that the methanolic extract form E. foetidum leaves showed moderate antibacterial activity against 6 clinical strains of Helicobacter pylori out of 15 tested strains, using the disk diffusion technique as antibacterial susceptibility test [92]. Besides antibacterial activity, E. foetidum was tested for its antiplasmodial activity, using chloroquine as positive control to evaluate the sensitivity of susceptible Plasmodium falciparum strains. The leaf extracts were prepared using hexane, dichloromethane, and methanol to obtain three extracts. The dried extracts were then dissolved in DMSO to give a stock solution at 10 mg/mL, used for the biological tests. The results indicated that E. foetidum showed low in vitro antiplasmodial activity against P. falciparum, with an IC50 value of 25 μg/mL [93].
Another species belonging to genus Eryngium, is E. maritimum, which is a wild perennial species growing on sand beaches in West Europe, the Mediterranean basin, and the Black Sea and has been used for its diuretic, stimulant, cystotonic, stone inhibitor, aphrodisiac, expectorant, and anthelmintic properties (http://www.botanicals.com, http://www.crescentbloom.com). The essential oil, obtained from hydrodistillation of the aerial parts, was found to contain a known sesquiterpene (muurol-9-en-15-al) and three new oxygenated sesquiterpenes with a muurolane or cadinane skeleton (4βH-cadin-9-en-15-al, 4βHmuurol-9-en-15-ol, and 4βH-cadin-9-en-15-ol), The sesquiterpenoid-rich fraction was tested in vitro using the agar diffusion method and the minimum inhibitory concentration (MIC) in the liquid phase against L. monocytogenes and E.coli. The finding revealed that the antibacterial activity of the new oxygenated sesquiterpenes against the tested bacteria, with an inhibition diameter higher than 15 mm and a MIC value lower than 90 μg/mL [94].
Another investigation on E. maritimun showed that the leaf hydromethanolic extract fractionated into a polar (aqueous) and apolar (chloroformic) fraction and tested using the microdilution method against food-borne pathogens and clinical isolates, exhibited antimicrobial activity. The tested Gram-positive bacteria were S. aureus subsp. aureus, Micrococcus luteus, L. monocytogenes and B. cereus. The Gram-negative bacteria were two strains of Salmonella (S. enterica subsp. arizonae and S. enterica subsp. montevideo), three strains of Pseudomonas (P. aeruginosa, P. fluorescens, and P. marginalis), E. coli, and Erwinia carotovora subsp. carotovora, and a yeast (Candida albicans). According to the data obtained, the fractions resulted to be active against all bacteria with the exception of L. monocytogenes, while the most sensitive bacteria were P. aeruginosa and P. fluorescens, with MIC values of 1 and 2 μg/mL for the polar and apolar fractions, respectively [95].
The essential-oil composition and antimicrobial activity of three other species belonging to genus Eryngium (E. creticum, E. campestre, and E. thorifolium), whose infusions obtained from the aerial and root parts are commonly used in Turkish folk medicine as antitussive, diuretic, stimulant, and aphrodisiac, were studied [96]. The composition of E. thorifolium was found to be rich in α-pinene, a known antibacterial terpenic compound, which was present in fewer amounts in the other two species. Differently, E. creticum was found to be rich in hexanal, which was present in less amount in E. thorifolium and was not detected in E. campestre. Antibacterial activity of the essential oils was tested with the disc diffusion method against nine clinical strains of methicillin-resistant S. aureus (MRSA). The essential oil obtained from E. thorifolium, which caused an inhibition zone ranging from 13 to 19 mm (similar to that exhibited by vancomycin and oregano essential oil tested at 10 μL/disc and 5 μL/disc, respectively), was demonstrated to be the most active species [37]. As aforementioned, many Eryngium species, e.g. E. maritimum exerted antifungal activity against C. albicans and other strains. Especially, the essential oil obtained by water distillation of the aerial parts of E. duriaei Gay ex Boiss subsp. juresianum (M.Laínz) M. Laínz was tested for its antifungal activity against 13 fungi, among which there were seven dermatophyte species (Microsporum canis FF1, Fig. 11 Some phenolic acid derivatives found in E. alpinum Table 3 The major components in the essential oils of various Eryngium species Plant Name Plant part Major components Ref. Eryngium alpinum L. Aerial parts Caryophyllene oxide (21.6 %) [98] Bicyclogermacrene (11.8 %) Germacrene D (10.3 %) Eryngium amethystinum L. Aerial parts β-Caryophyllene (19.7 %) [98] α-Bisabolol (7.9 %) 2,3,6-Trimethylbenzaldehyde (7.9 %) Leafy parts of the shoots α-Pinene (11.8 %) [99] 2,3,6-Trimethylbenzaldehyde (  δ-3-Carene (13.0 %) Aerial parts Cyclobuta [1][2][3][4]dicycloocten Hexadecahydro (47.03 %) [35] n-Hexadecanoic acid (11.16 %)

Limonene (4.23 %)
Cis-α-bisabolene (2.14 %)  , caryophyllene oxide (7.6 %) and E-β-caryophyllene (6.3 %)] are probably the responsible for the antifungal activity [97]. Finally, in 2013, the antiphytoviral activity of the essential oils obtained by water distillation of the aerial parts of E. alpinum and E. amethystinum cultivated in Croatia was described. The antiviral activity was shown in Chenopodium quinoa treated with the essential oils (250 ppm) prior to the inoculation of cucumber mosaic virus associated with a satellite RNA. The number of leaf local lesions, registered in the presence of the essential oils, was strongly reduced from an average value of 14.9 ± 0.8 to 3.3 ± 0.2 and 2.9 ± 0.2, for E. alpinum and E. amethystinum essential oils, respectively. The authors ascribed the antiviral activity to the occurrence of caryophyllene oxide and β-caryophyllene, which were identified as the major components of E. alpinum and E. amethystinum, respectively. Moreover, other components such as germacrene D, α-bisabolol, and γ-eudesmol, which were detected as minor oil Table 3 The major components in the essential oils of various Eryngium species (Continued) Eryngium thorifolium Boiss.

Conclusion
The present paper shows that essential oils and extracts obtained from various Eryngium species have broad range antimicrobial activity against several strains of gram-positive and gram-negative bacteria, some species of fungi and yeasts, and viruses. The findings point out to the fact that these activities can be ascribed to the presence of different phytochemicals, especially apolar compounds. However, our search at https://clinicaltrials.gov/ with keyword "Eryngium" accessed on February 9, 2015 showed no clinical trial regarding the beneficial effects of the genus Eryngium plants against different types of infection available up to date. In addition, there are only a few papers on the toxicity of the active constituents belonging to the genus Eryngium plants, which resulted to be cytotoxic especially at high doses. It can be suggested that more toxicity studies should be carried out prior to the clinical trials.
According to this study, we conclude that the essential oils and extracts of those Eryngium species that have been submitted to in vitro investigation (Table 4) should be proceeded to toxicological studies and in vivo experiments as multi-target antimicrobial agents for the treatment of human infectious diseases, especially antibiotic-resistant bacterial infections. Thus, it is not quite possible to make a clear statement or comment about their clinical uses. Hence, we recommend that future studies should be performed on: -toxicity of the Eryngium plant extracts and essential oil resulted to be active in in vitro experiments.
-clinical studies of the safe extracts of Eryngium species with in vitro activity, -finding the exact mechanism underlying the antibacterial effects of the essential oils and extracts of members of the genus Eryngium and their antibacterial constituents, -separation, isolation, and structure identification of the most antibacterial constituents of the essential oils and extracts of different members of the genus Eryngium and their interactions with foods as well as common synthetic antibacterial compounds, -ascertaining the most effective and safe doses for clinical studies regarding the antibacterial effects of the essential oils and extracts of different genus Eryngium species against different infectious diseases.

Competing interests
The authors declare that they have no competing interests.