Baicalein Inhibits Energy-Dependent Efflux Pump Activity in Candida Species

Methods: Minimum inhibitory concentration (MIC) of baicalein was determined by broth microdilution assay (CLSI guidelines). MIC was defined as the lowest concentration of baicalein for 80% inhibition of visible growth when compared with control. Rhodamine 6G (R6G) efflux was evaluated to investigate the effect of baicalein on energy-dependent efflux pumps. Briefly, R6G (10 µM) was added to stared fungal cells for 1 h at 35°C, followed by addition of glucose (1 mM) and baicalein (1×, 2× or 4× MIC). After 2 h incubation, intensity of R6G fluorescence was determined by fluorescence spectrophotometer at excitation wavelength 485 nm and emission wavelength 535 nm.

Results: Baicalein exhibited potent in vitro anti-Candidal activity (MIC range = 2.7-5.4 µg/mL) and strongly inhibited the energy-dependent efflux pump activity in a dose-dependent manner. The efflux pump activity of C. albicans was reduced by ~10-30% at 1× MIC of baicalein and by ~53-73% at 4× MIC of baicalein. A similar trend was observed in C. glabrata (~21-34% at 1× MIC; ~53-57% at 4× MIC), C. krusei (~31-38% at 1× MIC; ~68-75% at 4× MIC) and C. parapsilosis (~10-45% at 1× MIC; ~39-75% at 4× MIC).

Conclusions: Baicalein possesses wide spectrum of anti-Candidal activity. Furthermore, our findings not only demonstrated that baicalein strongly inhibited the efflux pump activity, but also suggested the existence of similar efflux pump activity across Candida species.