Isolated compounds from Alternanthera dentate (Moench) stuchlik leaf extracts and their antimicrobial effects

Abstract

Alternanthera dentata leaf extracts are used to treat infectious diseases such as skin diseases in folk medicine. This study isolated and evaluated activities of its bioactive compounds. A. dentata crude leaf extract obtained from 80% methanol was partitioned with n-hexane,  dichloromethane, ethyl acetate and n-butanol respectively. The four fractions obtained were assayed for their antimicrobial  activities against reference and clinical bacteria and fungi strains using agar dilution and micro-well dilution methods. Activity directed  fractionation of the ethyl acetate fraction afforded two compounds and their structures elucidated using NMR (1D and 2D) spectroscopic  data. The results of the antimicrobial assays showed that the ethyl acetate fraction had the best activities. This fraction afforded two  compounds identified as 7-O-flavone glycosides: chrysoeriol-7-O--glucoside (1) and apigenin-7-O--glucoside (2) from the spectroscopic  data. The compounds are reported from A. dentata extracts for the first time. Compound 1 showed moderate to weak antimicrobial activities against some of the selected organisms though better than compound 2. It had MIC of 0.63 mg/mL against Bacillus subtilis  (ATCC 1263), Escherichia coli (ATCC 700727) and clinical strain of Methicillin Resistant Staphylococcus aureus (MRSA) and 2.50 mg/mL  against Candida pseudotropicalis. Structure Activity Relationship Studies (SARS) of the antimicrobial properties of compounds 1 and 2  indicated that the relative potency of compound 1 when compared with compound 2 is due mainly to the presence of 3' methoxylation of  its ring B. This study concluded that, of the two compounds isolated, chrysoeriol-7-O--glucoside (1) contributes more to the  antimicrobial activities of the plant.