Antibacterial activity of isoflavonoids isolated from Erythrina variegata against methicillin-resistant Staphylococcus aureus

Aims: To screen 16 isoflavonoids isolated from Erythrina variegata ( Leguminosae) for their antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA). Methods and Results: The roots of E. variegata were macerated with acetone. The chloroform-soluble fraction of the residue was subjected to repeated silica gel column chromatography followed by elution with various solvents. Structures of the isolated compounds were determined by extensive spectroscopic studies. Each compound was dissolved in dimethyl sulphoxide and added to agar plates (final concentration 1.56-100 mug ml(-1)) and suspensions of MRSA spotted onto the agar plates to determine the minimum inhibitory concentration ( MIC). Repeated silica gel chromatography yielded 16 compounds and spectroscopic studies revealed that all were isoflavonoids. Whilst 14 compounds showed antibacterial activity in this concentration range, the MIC values varied significantly among them. Of the active compounds, 3,9-dihydroxy-2,10-di(gamma,gamma-dimethylallyl)-6a,11a-dehydropterocarpan (erycristagallin) and 9-hydroxy-3-methoxy-2-gamma,gamma-dimethylallylpterocarpan (orientanol B) exhibited the highest activity with MIC values of 3.13-6.25 mug ml(-1). Conclusions: Erycristagallin and orientanol B showed the highest anti-MRSA activity (3.13-6.25 mug ml(-1)). Significance and Impact of the Study: Erycristagallin and orientanol B could be leading compounds for phytotherapeutic agents against MRSA infections.