The antimicrobial activity of essential oil compounds against escherichia coli and staphylococcus aureus

Essential oils (EOs) are aromatic and volatile liquids with antimicrobial properties. EOs or their compounds have been shown antibacterial, antiparasitic, insecticidal, antiviral, antifungal, anticancer and antioxidant activities. In the food industry, they play an important role in food preservation as effective alternatives or complements to synthetic preservatives which have a negative consumer perception. However, new applications require more knowledge about the solubility and the minimum inhibitory concentration (MIC) of EOs and their compounds, the effect of the individual compounds on target microorganisms as well as synergistic/antagonistic interactions between compounds. In order to fill this knowledge gap, this thesis focused on seventeen pure compounds of essential oil (EO): ocimene, geraniol, citral, geranyl asetat, menthol, menthone, menthyl asetat, p-cymene, thymol, eugenol, estragole (4-allylanisole), cinnamyl alkol, cinnamaldehyde, α-pinene, camphor, eucalyptol (1,8-cineole) and borneol. More specifically, maximum soluble concentrations of pure EO compounds supplemented with surfactant and/or solvent in a water based broth was first tested. Furthermore, their antimicrobial activity was investigated in vitro using a growth inhibition assay against Escherichia coli (3 strains), as model organism for the Gram (-) bacteria and against Staphylococcus aureus (3 strains) as model organism for the Gram (+) bacteria. Some of these compounds such as geraniol (11,25 mM), citral (13,5 mM) and cinnamaldehyde (14,4 mM) expressed antibacterial activity against all tested strains of S. aureus and E. coli when they are applied as single compounds. Geraniol was a more effective EO compound against S. aureus than E. coli. The combination of cinnamaldehyde and p-cymene did not show antagonistic effect towards all tested bacteria. The interaction between cinnamaldehyde and eugenol could be an additive or synergistic effect against S. aureus LMG8224, which was the most susceptible bacterium to this combination, and E. coli JG33. The interaction of the same combination against other tested strains of E. coli and S. aureus was nonantagonism.