A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol

Aims: The minimum inhibitory concentration (MIC) of oregano essential oil ( OEO) and two of its principle components, i.e. thymol and carvacrol, agains t Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated. Methods and Results: The applied technique uses 100-well microtitre plate a nd collects turbidimetric growth data. To produce the inhibition profiles, a wide range of concentrations were tested for each of the three compounds, as well as for carvacrol-thymol mixtures. Following a specific mathematica l analysis of the observed inhibition profiles from all compounds, it was s uggested that mixtures of carvacrol and thymol gave an additive effect and that the overall inhibition by OEO can be attributed mainly to the additive antimicrobial action of these two compounds. Addition of low amounts of ea ch additive: (a) increased permeability of cells to the nuclear stain EB, ( b) dissipated pH gradients as indicated by the CFDA-SE fluorescent probe ir respective of glucose availability and (c) caused leakage of inorganic ions . Conclusions: Mixing carvacrol and thymol at proper amounts may exert the to tal inhibition that is evident by oregano essential oil. Such inhibition is due to damage in membrane integrity, which further affects pH homeostasis and equilibrium of inorganic ions. Significance and Impact of the Study: The knowledge of extent and mode of i nhibition of specific compounds, which are present in plant extracts, may c ontribute to the successful application of such natural preservatives in fo ods, since certain combinations of carvacrol-thymol provide as high inhibit ion as oregano essential oil with a smaller flavour impact.