This study elucidated the role of
boeravinone B, a NorA multidrug efflux pump inhibitor, in biofilm inhibition. The effects of
boeravinone B plus
ciprofloxacin, a NorA substrate, were evaluated in NorA-overexpressing, wild-type, and knocked-out Staphylococcus aureus (SA-1199B, SA-1199, and SA-K1758, respectively). The mechanism of action was confirmed using the
ethidium bromide accumulation and efflux assay. The role of
boeravinone B as a human
P-glycoprotein (P-gp) inhibitor was examined in the LS-180 (
colon cancer) cell line. Moreover, its role in the inhibition of biofilm formation and intracellular invasion of S. aureus in macrophages was studied.
Boeravinone B reduced the minimum inhibitory concentration (MIC) of
ciprofloxacin against S. aureus and its methicillin-resistant strains; the effect was stronger in SA-1199B. Furthermore, time-kill kinetics revealed that
boeravinone B plus
ciprofloxacin, at subinhibitory concentration (0.25 × MIC), is as equipotent as that at the MIC level. This combination also had a reduced mutation prevention concentration.
Boeravinone B reduced the efflux of
ethidium bromide and increased the accumulation, thus strengthening the role as a NorA inhibitor. Biofilm formation was reduced by four-eightfold of the minimal biofilm inhibitory concentration of
ciprofloxacin, effectively preventing bacterial entry into macrophages.
Boeravinone B effectively inhibited P-gp with half maximal inhibitory concentration (IC50) of 64.85 μM. The study concluded that
boeravinone B not only inhibits the NorA-mediated efflux of
fluoroquinolones but also considerably inhibits the biofilm formation of S. aureus. Its P-gp inhibition activity demonstrates its potential as a bioavailability and bioefficacy enhancer.