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.

Antibacterial activity against foodborne Staphylococcus aureus and antioxidant capacity of various pure phenolic compounds.

Six pure phenolic compounds (hydroquinone, thymol, carvacrol, butylated hydroxyanisole, gallic acid, and octyl gallate) were tested for their minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against several strains of Staphylococcus aureus isolated from dairy and meat products. In addition, S. aureus reference strains (American Type Culture Collection) for antimicrobial studies and/or isolated from human infections and outbreaks of food poisoning were included in the study. Of the compounds tested, octyl gallate and hydroquinone were the most effective against S. aureus (mean MIC values of 20.89 and 103.05 µg/mL, respectively) and carvacrol and thymol the least (mean MIC values of about 413 µg/mL). The mean MBC values were 40.84, 194.37, 417.46, and 581.90 µg/mL for octyl gallate, hydroquinone, carvacrol, and thymol, respectively. Meat isolates were more resistant than those of dairy origin to hydroquinone, gallic acid, and octyl gallate, as well as to penicillin G (used as a control of the methodology used); gallic acid and penicillin G showed the highest differences in MIC values between the groups of strains (about 10 and 200 times, respectively). On the other hand, when we tested the isolates included in each group of strains (dairy, meat, and other/mixed sources) we only detected significant differences (p < 0.05) among dairy and isolates from other/mixed sources for hydroquinone and thymol, respectively. However, strains of meat origin exhibited significant differences among each other (p < 0.05) to most of the phenolic compounds tested (hydroquinone, carvacrol, gallic acid, and octyl gallate). The relationship between MICs and MBCs for each of the phenolic compounds tested suggested a bactericidal mechanism of action against S. aureus. Gallic acid and octyl gallate exhibited the highest antioxidant capacity and thymol and carvacrol the lowest. So, octyl gallate is an agent with both antimicrobial and antioxidant properties, which would be of interest to use in the food industry.

Screening of phenolic antioxidants for their inhibitory activity against foodborne Staphylococcus aureus strains.

Seventeen phenolic compounds that are allowed to be used in the European food industry as aromatizants or antioxidants or that are naturally present in plants were tested for their ability to inhibit 19 strains of Staphylococcus aureus by using a standardized paper disc assay. Most of the strains assayed were foodborne (dairy and meat products). Human isolates and/or strains recommended for testing antimicrobial agents were also included in the study, and some of the test strains were enterotoxin producers. When the content was 200 microg/disc, various phenolic compounds had shown antimicrobial activity against all (hydroquinone, thymol, carvacrol, butylated hydroxyanisole, octyl gallate, and tannic acid) or most (gallic acid, propyl gallate, and ellagic acid) of the S. aureus strains tested. Significant differences in the inhibition zones (p < 0.05) among strains of the same, or similar, origin and among the different origins were observed for most of the phenolic compounds that showed antimicrobial activity for all or most of the strains tested.