WHY ARE ESSENTIAL OILS EFFECTIVE AGAINST PATHOGENIC BACTERIA?

Many efforts have been spent already in order to learn the mechanisms of action that essential oils have against pathogenic bacteria. Although the entire process is not exactly known, much information has been gathered already. Essential oils have different mechanisms but most of them target the bacterial wall. That explains why, in general, essential oils are more effective against Gram+ than Gram- bacteria as they differ in bacterial wall composition (Seow et al., 2014). The cell wall of G+ bacteria is 90-95% peptidoglycan what allows hydrophobic molecules (like most essential oils) to easily penetrate the cells being able to act against the wall and the cytoplasm (Nazzaro et al., 2013). Like G+, the G- bacterial cell wall is composed of peptidoglycan but only a couple of layers, representing a 5-10%. However, they have a plasma membrane located outside of the peptidoglycan layers: the outer membrane (OM). The OM coats the cell wall making it more difficult for essential oils to act against G- bacteria (Vaara, 1992).

The effectiveness of the essential oils is determined by their compounds, which can be classified into 2 major classes: terpenes (like carvacrol or thymol) and phenylpropenes (like cinnamaldehyde or eugenol).

Carvacrol and thymol have a lipophilic structure that allows them to get into the G+ bacterial membranes between the fatty acid chains, changing its permeability and allowing cell contents to leak (Brenes and Roura, 2010). It has been proven that thymol and carvacrol were also able to affect the OM of S. typhimurium and E. coli (both G-) by releasing lipopolysaccharides and sensitizing the membrane (Helander et al., 1998).

Interestingly, eugenol is more effective against G- than G+. It is able to act on the OM and affect the transport of ions and ATP which does not allow the bacteria to function normally (Nazzaro et al., 2013).

In conclusion, it is very important to use different essential oils to make sure they are acting against the widest range of bacteria possible.