Staphylococcus aureus can exhibit resistance to various antibiotics. Among its resistance mechanisms, the active efflux of antibiotics can be seen as relevant. This study aimed to evaluate the ability of resveratrol to modulate norfloxacin resistance in S. aureus. The antimicrobial activity of resveratrol was assessed using the broth microdilution method to determine the minimum inhibitory concentration (MIC). Then, the modulatory effect of resveratrol was evaluated using the MIC determination for the antibiotic or ethidium bromide in the presence and absence of resveratrol at a sub-MIC level. The MIC of norfloxacin against S. aureus SA1199B (NorA-overexpressing strain) decreased 16-fold when in the presence of resveratrol, with a similar behavior being observed for ethidium bromide. An evaluation of the ethidium bromide accumulation was also performed, showing that in the presence of resveratrol, the SA1199B strain had augmented fluorescence due to the accumulation of ethidium bromide. Altogether, the results suggested that resveratrol may act by inhibiting NorA. These in vitro data were supported by docking results, with interactions between resveratrol and the NorA efflux pump predicted to be favorable. Our findings demonstrated that resveratrol may modulate norfloxacin resistance through the inhibition of NorA, increasing the effectiveness of this antibiotic against S. aureus.

Among the bacterial resistance mechanisms, active efflux pumps play a role in the extrusion of different molecules, and, thus, contribute to antimicrobial resistance. Staphylococcus aureus is a Gram-positive bacterium that can present resistance to various antibiotics, for which NorA, a predominant efflux pump of these strains, is known to promote resistant to fluoroquinolones. Thus, the inhibition of this efflux pump may modulate resistance in S. aureus, namely, to fluoroquinolones. Therefore, this study aimed to investigate the ability of a natural compound, resveratrol, to modulate fluoroquinolone resistance in S. aureus. The antimicrobial activity of resveratrol, ethidium bromide (EtBr) and norfloxacin was determined through the minimum inhibitory concentration (MIC). Then, the modulatory effect of resveratrol was evaluated by the determination of the MIC of the antibiotic or EtBr in the presence and absence of resveratrol at a sub-MIC level. The results showed that the MIC of norfloxacin against a wildtype S. aureus strain decreased by 2-fold, and for a NorA-overexpressing (norA++) strain decreased by 16-fold when in the presence of resveratrol. A similar behavior was observed for EtBr. Furthermore, an EtBr accumulation assay was also performed, showing that, in the presence of resveratrol, the norA++ strain had an augmented fluorescence as a consequence of the accumulation of EtBr. Altogether, the results suggested that resveratrol may act by inhibiting NorA. The postantibiotic effect (PAE) of norfloxacin alone and in combination with resveratrol was also determined, showing that the most extended PAE was observed with norfloxacin at 32 mg/L when tested in combination with resveratrol. Furthermore, there was a decrease in the mutation prevention concentration of norfloxacin when combined with resveratrol. Our findings demonstrated that resveratrol could modulate norfloxacin resistance by the inhibition of NorA, increasing the effectiveness of this antibiotic against S. aureus.