In Vitro Evaluation of the Combination Activity of Carvacrol and Oxacillin against Methicillin-Resistant Staphylococcus aureus Strains

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Aim: Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most resistant bacteria to antibiotics. Many antibiotics are known to be insufficient to treat infections caused by these bacteria. Plant-derived antibacterials have drawn more attention as a source of new therapeutics. Carvacrol is a monoterpenic phenol compound found in various essential oils and shows antimicrobial activity against many pathogens. In this study, the combination activity of carvacrol and oxacillin against ten MRSA clinical strains was evaluated. Material and Methods: To determine Minimum Inhibitory Concentrations (MIC) of carvacrol and oxacillin against ten MRSA clinical strains, broth microdilution method was performed. The combination activity of carvacrol and oxacillin was determined with checkerboard synergy test. Whether both antimicrobials and their combinations caused membrane damage in MRSA-6 strain was detected by measuring the amount of nucleic acid leaking out of the cell across membrane with UV spectrophotometer. Results: Carvacrol showed antibacterial activity against all MRSA strains with MIC values in the range of 64-256 µg/ml. The synergistic effect (FICI≤0.5) between carvacrol and oxacillin was determined against seven strains. Carvacrol caused a membrane damage on MRSA-6 strain. As a result of combination with oxacillin, the increase in the membrane damage of MRSA-6 strain was found to be statistically significant (p<0.001). Conclusion: According to the results of this study, carvacrol increased the antibacterial effect of oxacillin against MRSA strains. Thus, carvacrol can be used in combination with oxacillin against MRSA as a novel antimicrobial agent. However, the results of this study should be supported by further studies.