Effect of sub-lethal treatment of carvacrol and thymol on virulence potential and resistance to several bactericidal treatments of Staphylococcus aureus

This study examined the changes in biofilm-formation ability, hemolytic/lipase/nuclease/protease activities, and resistance to various bactericidal treatments of Staphylococcus aureus after sublethal treatment with carvacrol and thymol. The minimum inhibitory concentrations (MICs)of carvacrol and thymol for S. aureus were 0.4 and 0.3 mg/ml, respectively, and sublethal concentrations (1/2 and 1/4 MIC)were determined based on these concentrations. Sublethal treatment with carvacrol and thymol did not change the protease and lipase activities but reduced the hemolytic and nuclease activities of S. aureus. After sublethal treatment with carvacrol and thymol, the biofilm-formation ability of S. aureus was decreased, which was found to be due to the decrease in cell surface hydrophobicity. In addition, after sublethal treatment with carvacrol and thymol, the resistance of S. aureus to heating and malic acid treatments increased, whereas resistance to ultraviolet and hypochlorous acid (HOCl)decreased, and resistance to hydrogen peroxide (H2O2)did not change. The results of this study show that sublethal treatment with carvacrol and thymol can effectively suppress some virulence factors of S. aureus and that applying HOCl or ultraviolet radiation would be an effective subsequent treatment to inactivate S. aureus after sublethal treatment.