Inhibition of NorA efflux pump of Staphylococcus aureus by anacardic acids isolated from the cashew nutshell liquid of Anacardium occidentale L

The rising of diseases caused by multidrug-resistant bacteria has encouraged researchers to explore more antimicrobial substances, as well as chemicals capable of potentiating the action of existing ones against multidrug-resistant bacteria. Anacardium occidentale produces a fruit known as cashew nut, filled with a dark, almost black, caustic, and flammable liquid called cashew nutshell liquid (CNSL). The goal of the study was to evaluate the intrinsic antimicrobial activity of the major compounds present in CNSL, called anacardic acids (AA), as well as their possible modulatory action as an adjuvant of Norfloxacin against a Staphylococcus aureus strain overproducing the NorA efflux pump (SA1199B). Microdilution assays were performed to determine the minimum inhibitory concentration (MIC) of AA against different microbial species. Norfloxacin and Ethidium Bromide (EtBr) resistance modulation assays were performed in the presence or absence of AA against SA1199-B. AA showed antimicrobial activity against Gram-positive bacterial strains tested but no activity against Gram-negative bacteria or yeast strains. At subinhibitory concentration, AA reduced the MIC values for Norfloxacin and EtBr against the SA1199-B strain. Furthermore, AA increased the intracellular accumulation of EtBr in this NorA overproducer strain, indicating that AA are NorA inhibitors. Docking analysis showed that AA probably modulates Norfloxacin efflux by spatial impediment at the same binding site of NorA.