Antibacterial activity of phytochemicals isolated from Erythrina zeyheri against vancomycin-resistant enterococci and their combinations with vancomycin

Six phytochemicals were isolated from the roots of Erythrina zeyheri (Leguminosae) by repeated silica gel column chromatography using various eluting solvents. Extensive spectroscopic studies revealed that all were isoflavonoids. The antibacterial activity of the six compounds against vancomycin-resistant enterococci (VRE) was estimated by determining the minimum inhibitory concentration (MIC). Of the six isoflavonoids, erybraedin A ((6aR, 11aR)-3,9-dihydroxy-4,10-di(gamma,gamma-dimethylallyl)pterocarpan) exhibited the highest growth inhibitory potency against VRE with an MIC value of 1.56-3.13 microg/mL, followed by eryzerin C ((3R)-7,2',4'-trihydroxy-6,8-di(gamma,gamma-dimethylallyl)isoflavan) (MIC 6.25 microg/mL). These compounds also inhibited the growth of methicillin-resistant Staphylococcus aureus (MRSA) at 3.13-6.25 microg/mL. The antibacterial effects of the two compounds against VRE and MRSA were based on bacteriostatic action. When erybraedin A or eryzerin C was combined with vancomycin, the fractional inhibitory concentration (FIC) index against VRE ranged from 0.5306 to 1.0 and from 0.5153 to 0.75, respectively. The combinations also showed FIC indices of 0.6125-1.0 against MRSA. The results indicate that, depending on the case, both compounds act either synergistically or additively with vancomycin against VRE and MRSA. Erybraedin A and eryzerin C show evidence of being potent phytotherapeutic agents against infections caused by VRE and MRSA.