Evaluation of nootkatone in the antibacterial activity and inhibition of the efflux pumps NorA and MepA in Staphylococcus aureus strains

Microorganisms naturally exist in the environment and live on humans as part of the normal microbiota. However, multidrug-resistant bacteria, which cause numerous infections and deaths annually, are a significant concern. Nootkatone, a compound known for its antibacterial, anti-inflammatory, anticancer, and antioxidant properties, is used in various industries. This study aimed to evaluate the antibacterial effects of nootkatone on Staphylococcus aureus strains, including those with efflux pump proteins NorA and MepA. Using the broth microdilution method, the minimum inhibitory concentration (MIC) of nootkatone was determined both alone and in combination with antibiotics norfloxacin, ciprofloxacin and with standard efflux pump substrate the ethidium bromide (EtBr). Nootkatone demonstrated intrinsic antibacterial action against the S. aureus K2068 strain with an MIC of 203 µg/mL, but no activity against S. aureus ATCC 29213 and S. aureus 1199. When combined with norfloxacin, it significantly reduced the MIC of the antibiotic against S. aureus ATCC 29213 and S. aureus 1199. With a significant increase in fluorescence against S. aureus 1199, nootkatone also showed potential as an inhibitor of the NorA efflux pump. The results suggest that nootkatone may inhibit efflux pumps, thereby enhancing the effectiveness of antibiotics. Further molecular studies are needed to fully understand its antibacterial mechanisms. • The sesquiterpene nootkatone demonstrated intrinsic antibacterial activity against the S. aureus strain K2068. • Nootkatone has been shown to be a potential and putative inhibitor of the efflux pumps NorA and MepA. • Nootkatone also enhanced the antibiotic activity of fluoroquinolones.