A new flavonol derivative and other compounds from the leaves of <i>Bauhinia thonningii</i> Schum with activity against multidrug-resistant bacteria

Abstract

Investigation of the leaves of <i>Bauhinia thonningii</i> Schum led to the isolation and identification of a new flavonol derivative, 6-C-methylquercetin-3,4'-dimethyl ether (<b>1</b>) together with eleven known compounds (<b>2</b>-<b>13</b>), with two of them (<b>10</b> and <b>11</b>) obtained as a mixture. Their structures were established by extensive spectroscopic analyses. Antibacterial activity of compound <b>1</b> as well as the reference antibiotic, ciprofloxacin was tested on Gram-negative multidrug-resistant bacteria overexpressing active efflux pumps, and against methicillin-resistant strains of <i>Staphylococcus aureus</i> (MRSA). Samples were tested alone and in combination with an efflux pump inhibitor (EPI), phenylalanine-arginine-<i>β</i>-naphthylamide (PAßN). Results show that when compound <b>1</b> was tested alone, its inhibitory effects were obtained on 7/10 tested bacteria with the highest MIC value of 128 µg/mL whilst in the presence of EPI, this activity significantly increase in all the 10 bacteria. An interesting antibacterial activity was obtained with compound <b>1</b> against <i>Klebsiella pneumoniae</i> ATCC11296 (MIC of 4 µg/mL), KP55 and <i>Staphylococcus aureus</i> MRSA6 in the presence of the PaßN.