Inhibition of the NorA multi-drug transporter by oxygenated monoterpenes

• Oxygenated monoterpenes nerol, 3,7-dimethyl-octan-1-ol and estragol were tested for their intrinsic antibacterial activity and for their ability to act as efflux pumps inhibitor of NorA multi-drug transporter of Staphylococcus aureus.
• Nerol showed intrinsic activity against Escherichia coli and C. albicans.
• Open chain monoterpenes nerol and 3,7-dimethyl-octan-1-ol were able to reduce both norfloxacin and ethidium bromide minimal inhibitory concentrations against S. aureus SA1199-B, indicating that these compounds are NorA inhibitors.
• Combination of norfloxacin with nerol or 3,7-dimethyl-octan-1-ol efflux pump could be a promise strategy to reverse bacterial resistance mediated by efflux pumps.

The aim of this study was to investigate intrinsic antimicrobial activity of three monoterpenes nerol, dimethyl octanol and estragole, against bacteria and yeast strains, as well as, investigate if these compounds are able to inhibit the NorA efflux pump related to fluoroquinolone resistance in Staphylococcus aureus. Minimal inhibitory concentrations (MICs) of the monoterpenes against Staphylococcus aureus, Escherichia coli and Candida albicans strains were determined by micro-dilution assay. MICs of the norfloxacin against a S. aureus strain overexpressing the NorA protein were determined in the absence or in the presence of the monoterpenes at subinhibitory concentrations, aiming to verify the ability of this compounds act as efflux pump inhibitors. The monoterpenes were inactive against S. aureus however the nerol was active against E. coli and C. albicans. The addition of the compounds to growth media at sub-inhibitory concentrations enhanced the activity of norfloxacin against S. aureus SA1199-B. This result shows that bioactives tested, especially the nerol, are able to inhibit NorA efflux pump indicating a potential use as adjuvants of norfloxacin for therapy of infections caused by multi-drug resistant S. aureus strains.

MIC of antibiotics norfloxacin (NOR) in absence and presence of nerol against SA1199-B. Each result is the geometric mean of three simultaneous experiments. (***) Statistically significant values (p < 0.001).Figure optionsDownload as PowerPoint slide