Bio-Guided Study on Melastoma malabathricum Linn Leaves and Elucidation of its Biological Activities

The present study aimed to isolate and identify constituents with antibacterial activity from the Methanol extract of Melastoma malabathricum leaves (MMML) through bioassay guided fractionation. Furthermore, the study scoped to evaluate the cytotoxic effects of M. malabathricum leaves fractions for possible protective effect of antoxidative constituents present in the fractions. The MMML extract was fractionated by Vacuum Liquid Chromatography (VLC) to afford M. malabathricum leaves fractions (ML1-ML6) of increasing polarities. Antibacterial activity of plant extract, six fractions and two bioactive constituents against Staphylococcus aureus reference strain, methicillin-resistant Staphylococcus aureus reference strain (MRSA), eleven clinical MRSA isolates, three clinical Pseudomonas aeruginosa isolates and P. aeruginosa reference strain was evaluated using Minimum Inhibitory Concentration (MIC). The bioactive constituents responsible for antibacterial activity of MMML, fractions were detected by direct TLC-bioautography. The data obtained from MIC assay showed that ML5 effectively inhibited growth of all test bacterial pathogens. Direct TLC-bioautography revealed that ML5 had the highest number of antibacterial compounds. Following bioassay-guided fractionation, Kaempferol-3-O-(2", 6"-di-O-p-trans-coumaroyl)-β-glucopyranoside and Kaempferol were isolated from ML5. The structures of bioactive compounds were elucidated using Nuclear Magnetic Resonance (NMR) spectroscopy from ¹HNMR and ¹³CNMR. In addition, the cytotoxic effect of M. malabathricum leaves fractions (ML1-ML6) revealed that ML5 had the highest Cytotoxicity Concentration (CC₅₀) at 0.75 mg mL^-1, an observation traceable to the presence of flavonoids constituents with antioxidant properties. Therefore, the results clearly indicate that MMML, fractions and bioactive constituents have profound antibacterial activity. Furthermore, antioxidative compounds from ML5 fraction exert their effects by enhancing the level of CC₅₀.