Most plants contain several compounds with antimicrobial properties for protection against aggressor agents, especially microorganisms. The chemical structures of some antimicrobial compounds, according to Cowan (2), obtained from plants are shown in Figure 1.
Active compounds found in some plants have antiseptic action; for example, thyme has thymol and carvacrol, clove has eugenol and isoeugenol, and oregano has carvacrol and terpinenol-4. In some cases, terpenes from essences that are soluble in water have higher antibacterial power than others (47).
The sites or structures of the bacterial cell that are considered targets for action by the components of natural products are illustratedin Figure 2. The action mechanisms of natural compounds are related to disintegration of cytoplasmic membrane, destabilization of the proton motive force (PMF), electron flow, active transport and coagulation of the cell content. Not all action mechanisms work on specific targets, and some sites may be affected due to other mechanisms (48).
Important characteristics responsible for the antimicrobial action of essential oils include hydrophobic components that allow the participation of lipids from the bacterial cell membrane, which disturbs cell structures and make them more permeable (49).
Chemical compounds from essential oils also act on cytoplasmic membrane proteins (47). Cyclic hydrocarbons act on ATPases, enzymes known to be located at the cytoplasmic membrane and surrounded by lipid molecules. In addition, lipid hydrocarbons may distort the lipid-protein interaction, and the direct interaction of lipophilic compounds with hydrophobic parts of the protein is also possible (50). Some essential oils stimulate the growth of pseudo-mycelia, evidencing that they may act on enzymes involved in the synthesis of bacterium structural components (51).
Several compounds and their mechanisms of action on microorganisms are listed below.