**4. Mechanisms of essential oils on microorganisms**

As above indicated, the mechanisms which allow EOs to damage bacteria are largely dependent on their composition. Usually, antimicrobial activity can originate from a flow of reactions implicating the total bacterial cell; this is essentially due to the fact that, since the EOs are composed of many groups of chemical compounds, these last act in different ways [30]. Generally, Gram-positive bacteria and Gram-negative bacteria are differently susceptible to the action of EOs, due to the structural differences of their cell wall of these two groups of bacteria. The higher susceptibility of Gram-positive bacteria is caused mainly by the presence of peptidoglycan within their cell wall, which allows more easily the hydrophobic molecules to have access within the cell, acting therein with cytoplasm [30]. The cell wall of Gram-negative bacteria shows an outer membrane, composed of a double layer of phospholipids linked to the peptidoglycan layer by lipopolysaccharides. This allows these bacteria to exhibit greater resistance to the penetration of essential oils and/or their components; in fact, some hydrophobic molecules can be capable to enter into the cell, only through the access given by the porins, proteins that form water-filled channels distributed all over the cell wall. The different compositions of cell wall let it that Gram-negative bacteria are even more resistant to hydrophobic antibiotics [30, 67]. The mechanism through which the EOs or their components act on microbial cell is well known: it includes one or more simultaneous actions, ranging from cell wall degradation, to the damage caused to the cytoplasmic membrane and membrane proteins, as well as to a reduction of the proton-motive force until to damage to the ATP synthesis mechanism. Lipophilic character of EO compounds allows them to penetrate the cell membrane and remain between the phospholipids and/or affect the synthesis of membrane lipids, with a consequent change of membrane structure and with an alteration of its permeability. In addition, EOs can affect directly also the morphology of bacterial cell, altering it even irreversibly, to cause the complete destruction of the entire microbial cell scaffolding [30, 68] (**Figure 2**).

**Figure 2.** *Effect of essential oils on microbial cells (modified from [65]).*
