Different antibacterial actions of isoflavones isolated from Erythrina poeppigiana against methicillin-resistant Staphylococcus aureus

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AimsTo screen six isoflavones isolated from Erythrina poeppigiana (Leguminosae) for their antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA).Methods and ResultsStem bark of E. poeppigiana was macerated with acetone and the methylene chloride-soluble fraction of the residue was applied to repeated silica gel column chromatography and eluted. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined by a broth dilution method. Inactive compounds that failed inhibiting bacterial growth at 25 μg ml−1 were further investigated for their combination effects with methicillin and oxacillin. Of the isolated isoflavones, 5,7,4′-trihydroxy-8,3′-di(γ, γ-dimethylallyl)isoflavone (isolupalbigenin) exhibited the highest anti-MRSA activity (MICs: 1·56–3·13 μg ml−1; MBCs: 6·25–12·5 μg ml−1), followed by 5,7,4′-trihydroxy-6-γ,γ-dimethylallylisoflavone (erythrinin B). Inactive compounds were combined with methicillin or oxacillin, 5,4′-dihydroxy-(3′′,4′′-dihydro-3′′-hydroxy)-2′′,2′′-dimethylpyrano[5′′,6′′:6,7]isoflavone (M-Wi-2) intensifying the susceptibility of MRSA strains to these antibiotics. In all but one strain, the MIC values of methicillin were reduced from ≥100 to 6·25–12·5 μg ml−1 in the presence of M-Wi-2 (25 μg ml−1).ConclusionsIsoflavones from E. poeppigiana showed two different antibacterial activities against MRSA: direct growth inhibition and intensification of methicillin sensitivity.Significance and Impact of the StudyIsolupalbigenin and M-Wi-2 could lead to the development of compounds for new approaches against MRSA infection.

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