Efflux inhibitory activity of flavonoids from Chromolaena odorata against selected methicillin-resistant Staphylococcus aureus (MRSA) isolates

Ten flavonoid compounds isolated from the methanolic extract of a tropical weed plant, Chromolaena odorata leaves were tested against three multidrug-resistant (MDR) methicillin-resistant Staphylococcus aureus (MRSA) isolates with active efflux properties. Only low inhibitory potential were detected through the preliminary evaluation as indicated by their respective minimum inhibitory concentration (MIC) value. These results justify the need to conduct subsequent evaluation namely the efflux inhibitory potential assay. Ethidium bromide (EtBr) and reserpine were used as the standard efflux substrate and efflux inhibitor, respectively while S. aureus ATCC 25923 was used as the reference strain. Three flavonoid compounds namely eriodictyol-7,4'-dimethyl ether, naringenin-4'-methyl ether and 2'4-dihydroxy-4'5'6'-trimethoxychalcone showed high efflux inhibitory activity with four to eight fold reduction of EtBr MIC value, hence suggesting their potential to be develop into efflux inhibitors for MRSA.