Combination of Carvacrol and Thymol: Antimicrobial Activity Against Staphylococcus aureus and Antioxidant Activity.

Plant and essential oil extracts have been used for some time as antimicrobials and antioxidants, although little is known about the interactions between the main components of these plant materials. This knowledge could help to design more potent antimicrobial and antioxidant mixtures. Carvacrol and thymol, the main components of the essential oils of the Lamiaceae family of plants, were assessed in combination to evaluate their antioxidant activity and antimicrobial effect against 19 strains of Staphylococcus aureus (S. aureus) of different origins (clinical, meat, milk, and other) and mostly (12) enterotoxin producers. The microdilution test assay was used to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the two phenolics alone and in combination. Based on the fractional inhibitory concentration index (FICI), no antimicrobial interaction (0.5 < FICI <4) between carvacrol and thymol was observed against 42% of the S. aureus strains and an antagonistic interaction (FICI >4) was observed in the rest, which indicates different behavior among strains in relation to this antimicrobial combination. Particularly, an antagonistic effect was observed in 29% of the meat origin strains and 57% of the dairy origin strains. Combinations of carvacrol and thymol were bactericidal (differences in MIC and MBC values not more than twofold) for 60% of the tested strains. At low concentrations of both components, the antioxidant effect is additive. However, at high concentrations (2.50 or 2.66 mM) of at least one of the components of the combination, it is antagonistic. The different types of interactions of the components in the combination can depend on many factors (ratio, structural characteristics, and the establishment of intermolecular complexes). The results could be used as reference to apply this combination in foods to control S. aureus, to maintain the organoleptic properties and to extend the shelf-life of them.