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Origanum is an important multipurpose herbaceous, aromatic and perennial herb of the family Lamiaceae and tribe Mentheae that mostly occurs in warm and mountainous areas. Ietswaart (Ietswaart, 1980) recognized 3 groups, 10 sections, 38 species and 17 hybrids within the genus; Since then, 5 more species (Skoula and Francis, 2002) and one more hybrid (Duman et al., 1998) have been recognized, raising the number of species to 43 and the number of hybrids to 18. Among them, O. vulgare L., the only one commonly known as “oregano”, is the most variable species of the genus. It occurs throughout the Mediterranean region, in the Irano-Turanian, and in most parts of the Euro-Siberian region (D’Antuono et al., 2000; Duman et al., 1998; Ietswaart, 1980; Vokou et al., 1993). The work of Ietswaart, distinguishes six subspecies of O. vulgare on the basis of morphological characters: gracile, glandulosum, hirtum, vulgare, virens and viride (Ietswaart, 1980; Kokkini, 1996).These species are well accepted in the Plant List (2013) (http://www. The plant list.org). The subspecies of O. vulgare that found in northern Mediterranean areas, are generally considered to be poor sources of essential oils, whereas those in the southern part are rich sources of volatiles (Kokkini, 1996). Those rich in essential oil are usually also rich in cymyl-compounds (with either carvacrol or thymol as the main compound) and possess the pungent oregano flavour and those with poor oil content are often characterised by complex mixtures of cymyl-compounds, bornane type compounds, acyclic compounds, sabinyl-compounds and larger amounts of sesquiterpenes (Figure 1) (Lukas et al., 2013; Skoula and Francis, 2002).
In the majority of O.vulgare essential oils, phenolic monoterpenoids constitute up to 70% of the total oil (Khan et al., 2018). In addition to essential oils, O. vulgare subspecies also contain several biologically active components, including phenolics, flavonoids, tannins, sterols and terpenoids (Morshedloo et al., 2017). Many species of O. vulgare are found growing only in the wild, but many others, used as medicinal, culinary herbs and garden plants, are also found as cultivated plants (Kokkini, 1996). Subspecies of O. vulgare are used as a traditional remedy to treat various ailments such as respiratory tract, gastrointestinal and urinary tract and dermatological disorders. (Baricevic and Bartol, 2002; Verma et al., 2010). The Essential oils (EOs) of species of O. vulgare generally recognized as safe (GRAS) for human consumption by the US Food and Drug Administration (FDA) (Pola et al., 2016). Therefore, species of O. vulgare can be considered as a natural food preservative, with high antibacterial activity, due to the presence of thymol and carvacrol as phenolic compounds. Furthermore, it was reported that Origanum species are used in many food products, cosmetics, perfumeries and toiletries, aromatherapy and pharmaceutical preparations (Asensio et al., 2015). Regarding to the importance of this species, this review focused on diversity of the volatile and non-volatile compounds of six subspecies of O. vulgare, their traditional uses and antimicrobial activities.
2. Traditional uses
There are four species that stand out in Origanum history and folklore: O. majorana, O. vulgare, O. dictamnus and O. syriacum. It is important to note, that the physical similarity of O. majorana and O. vulgare and difficulty with proper identification have been a historical problem since many authors have used the name marjoram to describe both plants. In most of the folklore, origanums have been steeped in religious tradition and myth (Meyers, 2005). The eighteenth-century herbalist K’Eogh described oregano as having “a hot dry nature” (Charles, 2012). For centuries, Origanum vulgare L. subspecies have traditionally been used for the favoring of traditional dishes and traditional medicine due to the high content of their essential oil (Lukas et al., 2010). The culinary history of the oregano back to the 7th century B.C. to flavor meat, vegetables, fish and wine (Padulosi, 1997). Aristotle claims O. vulgare could cure snakebites if drunk with wine (Meyers, 2005). Of all the subspecies of O. vulgare, only ssp. hirtum (Greek oregano), ssp. gracile (Russian oregano) and ssp. glandulosum (Algerian oregano) are useful in cooking, although ssp. vulgare is generally not recommended for cooking. Oregano is traditionally used in Italian, Greek and Mexican dishes. Fresh and dried leaves can be added to soups, casseroles, sauces, stew, stuffing, eggs, olives, teas and foods like chili and pizza (Meyers, 2005; Yan et al., 2016). The international market of oregano is expanding becoming by far the largest selling herb today, with Turkey predominating its production and trade (Aboukhalid et al., 2017). Subspecies of O. vulgare uses in folk medicine include respiratory problems, coughs, upset stomach, painful menstruation, rheumatoid arthritis, scrofulosis, to induce sweating, for urinary problems as a diuretic and antiurolithic. It is used internally for colds, flu, fever, painful periods and digestive disorders and externally for bronchitis, asthma, arthritis and muscle aches. In Chinese medicine, oregano is a remedy for colds, vomiting, fever, dysentery, jaundice and childhood malnutrition. The oil of oregano is reportedly used to kill lice, and in homeopathy, oregano is considered an aphrodisiac. Preparations include infusions, tea powders, gargles and baths (Grieve, 1970; Gruenwald, 2000; Khan et al., 2011; Klein et al., 1998; Lukas et al., 2010; Lukas et al., 2015; Martins et al., 2014). In form of teas or tinctures, it is used against cold, for digestive and respiratory problems as well as means for general well being of the body. Ointments derived from oregano are beneficial for infection and wound healing (Li?ina et al., 2013). As a decoction or infusion preparation, it is used for its digestive, expectorant, antiseptic and antispasmodic properties (Martins et al., 2014). The commonly used O. vulgare subspecies, their parts uses, methods of preparations and traditional uses are summarized in Table 1.
3. Phytochemistry
Oregano contains essential oil, phenolic acids, flavonoids, sterols, proteins, vitamins, acids and tannins. The most important phytochemicals found in oregano are grouped depending on their hydrophilic and hydrophobic properties into two categories: essential oil and phenolic compounds. Flavonoids and phenolic acids are the main types of phenolic compounds present in oregano. Flavones are the most abundant flavonoids present in oregano species followed by flavonols. On the other hand, hydroxycinnamic acids are part of the non-flavonoid phenolics and the main subgroup of phenolic acids distributed in the different oregano species. The secondary metabolites of this plant have been well studied. To date, more than one hundred volatile and nonvolatile compounds have already been identified with conventional phytochemistry methods.