Identification and Mode of Action of Antibacterial Components from Melastoma malabathricum Linn Leaves

The present study aimed to assess the antibacterial activity of the methanol extract of Melastoma Malabathricum Leaves (MMML), active fraction of M. Malabathricum Leaves (ML5) and isolated kaempferol-3-O-(2",6"-di-O-p-trans-coumaroyl)-β-glucopyranoside (Kf) and to describe the preliminary mode of action against Gram-positive and Gram-negative bacteria. The MMML extract was fractionated by Vacuum Liquid Chromatography (VLC) to afford M. malabathricum Leaves fractions (ML1-ML6) of increasing polarities. Bioassay-guided fractionation (direct-TLC bioautography) revealed that fraction of M. Malabathricum Leaves (ML5) had the highest antibacterial activity. Therefore, the ML5 was further subjected to Column Chromatography (CC) to afford Kaempferol-3-O-(2",6"-di-O-p-trans-coumaroyl)-β-glucopyranoside (Kf). The compound structure was elucidated using data obtained from ¹H NMR and ¹³C NMR spectra. Antibacterial activity of MMML, ML5 and bioactive constituents Kf against Staphylococcus aureus reference strain, Methicillin-Resistant S. aureus reference strain (MRSA), 11 clinical MRSA isolates, 3 clinical Pseudomonas aeruginosa isolates and P. aeruginosa reference strain was evaluated through disk diffusion method and Minimum Inhibitory Concentration (MIC). The time kill assay and field emission scanning electron microscopy was used to determine the preliminary mode of action. The data obtained from disk diffusion method and MIC showed that the MMML, ML5 and Kf showed antibacterial activity against all test bacteria with various values. Generally, the ML5 and Kf exhibited the highest antibacterial activity with the largest zones of inhibition and lowest MIC values against S. aureus and all tested MRSA. The MMML exerted the strongest killing effect against all tested bacteria while the overall performance of ML5 and Kf were significantly more effective against S. aureus and MRSA than P. aeruginosa. Field emission scanning electron microscopy revealed irregular bacterial surface indicating that the MMML, ML5 and Kf disrupted the integrity of membrane. The results obtained showed that MMML, ML5 and Kf have potential alternative to antibiotics.