Antibacterial activities of the methanol extracts and compounds from Erythrina sigmoidea against Gram-negative multi-drug resistant phenotypes

Background: In the present study, the methanol extracts from the leaves, as well as compounds namely sigmoidin I (1), atalantoflavone (2), bidwillon A (3), neocyclomorusin (4), 6a-hydroxyphaseollidin (5) and neobavaisoflavone (6) (from the bark extract) were tested for their activities against a panel of Gram-negative bacteria including multi-drug resistant (MDR) phenotypes. Methods: Broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) and the minimum bactericidal concentrations (MBCs) of the extracts as well as compounds 1-6. Results: The MIC results indicated that the crude extracts from the leaves and bark of this plant were able to inhibit the growth of 96.3 % of the 27 tested bacteria. Compounds 2-6 displayed selective activities, their inhibitory effects being obtained on 8.3 %, 41.7 %, 58.3 %, 58.3 % and 66.7 % of tested bacteria respectively for 2, 3, 5, 6 and 4. The lowest MIC value of 8 mu g/mL was obtained with 6 against Escherichia coli ATCC8739, Enterobacter cloacae ECCI69, Klebsiella pneumoniae KP55, Providencia stuartii NAE16 and Pseudomonas aeruginosa PA01. Conclusion: The present study demonstrates that Erythrina sigmoidea is a potential source of antibacterial drugs to fight against MDR bacteria. Neobavaisoflavone (6) is the main antibacterial consituents of the bark crude extract.