With regard to the traditional uses of the Origanum spp. (specifically infectious diseases, cold and respiratory problems), many studies have been focused on the potential antimicrobial activity of oregano. Several studies have been conducted in order to determine and to evaluate the biological properties of O. vulgare EOs. Most of the studies are focused on the antimicrobial activity, such as antifungal, antibacterial and antiviral; which will be discussed in the next section of this review.
4.1. Antibacterial properties
Numerous in vitro and in vivo researches have been conducted to evaluate the potential antibacterial, antiviral and antifungal activities of EOs, leading to an increase in their use as antiseptics, food preservatives, and dietary supplements (Adame-Gallegos et al., 2016). This kind of studies are of great importance due to the emergence of antibiotic resistant strains, the increase in the population with lower immunity and increased incidences of drug resistant biofilm associated infections. It was reported that about two-thirds of clinically used antibacterial therapies are originated from natural products (Farha and Brown, 2016). Different species of oregano are among the most studied herbs due to the potential antibacterial activity. The intensity of antimicrobial activity varied depending on the species of microorganisms and on the type of plant extract. Previous studies reported that the EO of oregano include more antimicrobial substances than its extracts such as water, methanol, ethanol and hexane (?ahin et al., 2004). Also it has been demonstrated that EOs produced from herbs harvested during or immediately after flowering possess the strongest antimicrobial activity (Sankar et al., 2013). Some studies have shown that different activity exerted by the different EOs was mainly related to the synergistic or antagonistic effects of minor active constituents, such as ?-terpinene and p cymene. Results from different reports suggested that the EO, rather than individual components, had higher antimicrobial activity due to synergistic effects of its components (Adame-Gallegos et al., 2016; Boskovic et al., 2015; Marrelli et al.). The possible mechanism of action of the EOs and their compounds is based on their ability to disrupt the bacterial cell wall and alter both the permeability and the function of the membrane proteins. Particularly, the EOs which are rich in phenolics, can penetrate into the phospholipids layer of the bacterial cell wall, bind to proteins and block their normal functions. Moreover, because of their lipophilic nature, EOs and their compounds can influence the percentage of unsaturated fatty acids and their structure and interacting with intracellular sites that lead to leakage of intracellular ATP and potassium ions. Disturbances of the pH gradient and the electric potential of the proton motive force can also cause membrane malfunction and ultimately cell death (Bakkali et al., 2008; Burt and Reinders, 2003; Helander et al., 1998; Lambert et al., 2001; Trombetta et al., 2005). It is recognized that the components of oregano EO action result in the release of the lipopolysaccharides from Gram-negative bacteria with the consequent cell membrane permeability increase and ATP loss (Faleiro, 2011). Antibacterial effects have been reported for oregano against, Gram-negative bacteria, including Salmonella typhimurium, Clostridium perfringens, Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Yersinia enterocolitica and Enterobacter cloacae, as well as Gram-positive bacteria such as Staphylococcus epidermidis, Staphylococcus aureus, Listeria monocytogenes and Bacillus subtilis that are related to different human pathologies. Gram-negative P. aeruginosa is known to possess a high level of intrinsic resistance to most of the antimicrobial agents due to a very restrictive outer membrane barrier (Kacániová et al., 2012). Although carvacrol exerts its activity against a wide range of microbial species, many works report that its inhibitory effect is greater against Gram-positive than Gram-negative bacteria. An additional outer membrane, surrounding a thinner peptidoglycan layer, is the main feature distinguishing the more sophisticated Gram-negative bacteria’s wall. Since difficult to overcome, this extra coating acts as protective barrier which strongly influences the inhibitory profile of antimicrobial compounds. Furthermore, due to the presence of lipopolysaccharides into the outer membrane, Gram-negative bacteria can effectively restrict the passage of lipophilic molecules into the cytoplasm (Marinelli et al.). Antimicrobial activity of major compounds of EOs has the following order: phenols ; alcohols ; aldehydes ; ketones ; ethers ; hydrocarbons; may be due to the impairment of a variety of enzyme systems, including those involved in energy production and distort the lipid–protein interaction in the cytoplasmic membrane (Esen et al., 2007). The antibacterial properties of terpenoids are due to the presence of delocalized electrons in the molecule, which allows for easier deprotonation, playing an important role in their antibacterial activity (Ultee et al., 2002). Several authors have studied the antimicrobial activity of O. vulgare subspecies. In a study, potential antibacterial activity of the EO of various phenological stages of Tunisian O. vulgare subsp. glandulosum against two Gram-positive bacteria (S. aureus and B. subtilis) and three Gram-negative bacteria (E. coli, S. typhimurium, Ampicillin-resistant P. aeruginosa) was tested. Oils during the early and late vegetative stage showed a very high bactericidal effect. However, oils showed a high bactericidal activity against the Gram-negative bacteria, the lowest MBC values have been detected against the two Gram-positive bacteria (Béjaoui et al., 2013). EOs obtained from inflorescences of three chemotypes of Origanum vulgare L. ssp. hirtum, growing wild in different locations in Southern Italy, were evaluated for their antibacterial activity. The first, with a prevalence of carvacrol/thymol; the second, with the prevalence of thymol/?-terpineol and the third, with the prevalence of linalyl acetate/linalool. The results showed that EOs rich in phenolic compounds such as carvacrol and thymol, possess high levels of antimicrobial activity. Linalol and linalyl acetate proved to be a very active compounds and showed bacteriostatic activity. The EOs mainly showed action against the Gram-positive pathogens, among which S. epidermidis was the most sensitive (De Martino et al., 2009). ?ahin et al. have found that the EO of O. vulgare ssp. vulgare with main constituents of caryophyllene and germacrene-D had great potential of antibacterial activity against the agents of foodborne diseases and food spoilage such as E. coli, Enterobacter spp., Bacillus spp., Salmonella spp. and Staphylococcus aureus. On the other hand, the methanol extract from aerial parts of O. vulgare ssp. vulgare plants showed no antimicrobial activity (?ahin et al., 2004). Antimicrobial activity of five extracts (Water, ethanol, acetone, ethyl-acetate, and diethyl ether extracts) obtained from O. vulgare growing wild in southwest Serbia were determined by microdilution method. The major compounds were: sabinene, terpinen-4-ol, 1.8 cineole, ?-terpinene and caryophyllene oxide. Gram-positive bacteria such as Bacillus species and Staphylococcus aureus, were more sensitive to the EO. Significant antibacterial effect was also observed against species from the genus Bacillus, especially Bacillus pumilis for all extracts. The water extract also showed the great inhibitory effects against tested bacteria (Li?ina et al., 2013). Other studies also showed antibacterial activity of aqueous infusion, aqueous decoction and EO of oregano seeds. The antibacterial effect of the aqueous infusion of oregano was similar to the oil and exhibited significant inhibitory activity against Klebsiella pneumoniae (20.1 ± 6.1), Klebsiella ozaenae (19.5 ± 0.5) and Enterobacter aerogenes (18.0 ± 00). It is interesting to note that the aqueous infusion of oregano inhibited all type of tested bacterial strains. It was found that the aqueous decoction of oregano seeds did not posses any antibacterial effect against tested Gram-negative bacilli (Chaudhry et al., 2007). Probably, active components are decomposed during decoction that’s why aqueous infusion or extract shows significant antibacterial effect rather than aqueous decoction. In another research, high antimicrobial activity against ulcer-associated H. pylori was found due to high phenolic content and rosmarinic acid in O. vulgare. The ethanol extract exhibited higher activity against H. pylori. Differences in physico-chemical properties of phenolic acids consisting of C6-C1-COOH, and C6-C3-COOH structures were hypothesized to play a role in growth inhibition of H. pylori. (Chun et al., 2005). A moderate antimicrobial activity was found from O. vulgare subspecies extracts (cyclohexane, DCM and MeOH extracts) against a panel of microorganisms. Cyclohexane extract of O. vulgare did not show any activity against tested H. pylori, while all other tested extracts were active against bacteria, especially against Gram-positive bacteria (Br?anin et al., 2015). Antimicrobial activity of decoction, infusion and hydroalcoholic extract of O. vulgare was investigated against Gram-positive species and Gram-negative species. Decoction and infusion had weak to moderate potential against almost all the tested bacteria, whereas the hydroalcoholic extract showed relatively higher efficacy. This study confirms that hydroalcoholic extract can be incorporated in formulations for antimicrobial purposes. Moreover, the use of infusion/decoction, by internal or external use, can avoid the toxic effects showed by other oregano fractions such as EO (Martins et al., 2014). Saeed and Tariq (2009) found that the infusion was more effective than the EO of O. vulgare against gram-positive bacteria whereas no antibacterial activity was found for decoction (Saeed and Tariq, 2009). Érika et al (2015) evaluated the antimicrobial activity of the EO against the main bacteria such as Micrococous luteus and Proteus vulgaris responsible for bad perspiration odor. Seventeen constituents were also identified including that ?-terpinene and carvacrol as major components. The EO exhibited antimicrobial activity against all microorganisms tested. There are few reports about the effects of plant derivatives on physiological attributes of staphylococci including extracellular enzymes. de Barros et al. (2009) found that the EO of O. vulgare possessed an interesting inhibitory effect on the cell viability of S. aureus and strongly interfered with some of its physiological characteristics (de Barros et al., 2009). Anti H. pylori potential of a mixture of Satureja hortensis and O. vulgare ssp. hirtum EOs (2:1, 2MIX) was investigated in vivo. With oral administration of this mixture, 70% of the animals group had been cured without any side effect or immune response, which make this combination as a new, effective, and safe therapeutic agent against H. pylori (Harmati et al., 2017). Stefanakis et al. have studied the antibacterial activities of the EOs of O. vulgare ssp. hirtum, O. onites L., and O. marjorana L. collected from different parts of Greece. Potent and broad spectrum activity of O. vulgare ssp. hirtum against bacterial strains of E. coli, S. cerevisiae, Listonella anguillarum (CECT 522), Vibrio splendidus DMC-1 as well as Vibrio alginolyticus was shown (Stefanakis et al., 2013).