Methoxylated flavones modulate drug activity by reducing the concentration needed to inhibit the growth of the drug-resistant (effluxing) bacteria. This activity may be related to flavanoid lipophilicity due to the presence of methoxyl groups. Lipophilicity is a common feature of several efflux pump inhibitors and may be a key factor for inhibition in Gram-positive bacteria [28].
Ethidium bromide is a well-known substrate for the NorA efflux protein, and active efflux is the only known mechanism of resistance to this DNA-intercalating dye [35]. Therefore, the use of ethidium bromide against the strain SA-1199B was used to demonstrate that the methoxylated flavones evaluated here modulated norfloxacin resistance by efflux pump inhibition.
Pefloxacin, a hydrophobic quinolone, is a poor substrate of the NorA efflux pump, and it was used as a negative control [25]. Reductions in MICs of norfloxacin and ethidium bromide when combined with chlorpromazine or trifluoperazine were also observed (data not shown), and the results were consistent with those reported by Kaatz et al. [24] and by Falcão-Silva et al. [32]; both phenothiazines were used as positive (internal) control.