Structural characterization, antibacterial activity and NorA efflux pump inhibition of flavonoid fisetinidol

•Structural characterization by nuclear magnetic resonance and FT-IR of fisetinidol, a flavonoid isolated from B. pentandra stems.•Antibacterial and modulatory activities of fisetinidol against Escherichia coli and S. aureus.•Inhibition of the NorA efflux pump of S. aureus SA-1199B. Flavonoids belong to the extensive group of polyphenols and have several biological activities such as anti-inflammatory, anti-ulcer, anticancer, antiviral, antibacterial, and antispasmodic. These compounds can thus be explored as a tool against multidrug resistant bacteria, as nosocomial infections now constitute a major public health problem worldwide. We have carried out a structural characterization by nuclear magnetic resonance and FT-IR of fisetinidol, a flavonoid isolated from B. pentandra stems. In addition, this flavonoid was tested for antibacterial and modulatory activities, as well as inhibition of the NorA efflux pump. Fisetinidol has no intrinsic antibacterial activity. However, there is synergism in association with norfloxacin, with a two-fold decrease in MIC. When compared to chlorpromazine, a standard efflux pump inhibitor, a two-fold decrease in MIC was also verified when used against S. aureus SA-1199B, which over expresses the NorA gene encoding the NorA efflux protein. As such, it could become an alternative source to reverse the bacterial resistance if administered in association with antibiotics.