## 5 EOs Mechanism of Action in Inhibiting Growth of Microorganisms  
Thus far, many studies have been conducted to investigate the mechanisms of EOs in the growth inhibition of microorganisms. Since terpen have been recognized as primary antimicrobial agents, a practical mechanism of these compounds has been attributed to the EOs [21]. The high activity of terpen, such as thymol, carvacrol, and eugenol, is most probably correlated to their phenolic structures. The action mode of phenolic compounds is usually in the form of interference with the action of the cytoplasmic membrane, including protein stimulation and active transportation [17]. The EO inhibition capacity can be attributed to the presence of an aromatic ring attached to the polar group [44]. The widespread use of phenols and related compounds as disinfectants can be considered as evidence. For example, due to the hydroxy phenolic group in the structure of thymol, hydrogen bonding can easily be performed with active sites of enzymes; consequently, a high activity is dedicated to this substance. Borneol and geraniol have a lower inhibitory effect compared to thymol due to the lack of an aromatic ring [21]. Cinnamon (_Cinnamom verum_) essential oil, possessing a high content of cinnamaldehyde, disrupts the function of proteins and inhibits the amino decarboxylase enzyme, thus the lipophilic part of the EO combines directly with the hydrophilic parts of the protein [42]. In the case of volatile aldehyde constituents, the growth inhibition of microorganisms is mainly due to the reactions of aldehydes with sulfhydryl groups that are effective in fungal growth, and in other cases sulfhydryl compounds are not effective in inhibiting aldehydes [21]. By calculating the molecular orbital energy of aldehydes, it was found that the antifungal activity of these compounds is related to the energy of the lowest molecular orbital, which is empty, thus the lower energy level leads to a higher antifungal activity [21]. These results indicated that the antifungal actions and activities of aldehydes, in addition to their reaction with the sulfhydryl group, are probably due to their ability to form charge transfer complexes with electron donors [45]. The examination of the fungicidal mechanism of _Melaleuca alternifolia_ EOs showed that it disrupts the mitochondrial and tricarboxylic acid cycles, therefore, accumulating reactive oxygen species and disrupting the _Botrytis cinerea_ fungal activity [46]. Lipids are one of the most important components in cell membranes and responsible for membrane stability and the regulation of membrane permeability to water-soluble substances [3]. [PERSON] et al. (2003) reported that the fat structures of cells are broken down by terpen, since fat saturation is increased [47]. The destruction of the _Shigella dysenteriae_ cell membrane by _Foeniculum vulgare_ EO was confirmed [48]. Due to the hydrophobic properties of EOs, they are dispersed in the lipid part of the mitochondrial and cell membrane of microorganisms; subsequently to the increase in permeability, they lead to a leak of intracellular materials and the disruption of many biological activities [49]. Moreover, [PERSON] et al. (2003) believes that the lipid structures of cells are broken down by terpen via their increasing saturation [47]. Damage to the cell wall and membrane can lead to macromolecular leakage and cell deterioration. Gram-positive bacteria such as _Staphylococcus aureus_, _Listeria monocytogenes_, and _Bacillus cereus_ are more sensitive to EOs than Gram-negative bacteria such as _Escherichia coli_ and _Salmonella enteritidis_[50]. In general, it is supposed that EOs possess more antimicrobial activity against Gram-positive bacteria due to the direct interaction of water-repellent chemical components with their cell membrane [51]. Gram-negative bacteria are more resistant to EOs due to their hydrophilic cell walls. This membrane prevents the penetration of hydrophobic compounds (Figure 2) [52].