Background: Multidrug-resistant (MDR) bacteria remain a major cause of morbidity and mortality globally. The present study was designed to investigate the in vitro antibacterial activities of crude methanol extract and constituents isolated by Column Chromatography (CC) fromCassia sieberianabark (CSB) against ten MDR Gram-negative bacteria, as well as the mechanisms of action of the most active sample.Methods: The antibacterial activity of the tested samples (extract, the fractions and their compounds isolated by CC and the structures obtained by exploiting¹H and¹³CNuclear magnetic resonance(NMR) spectra) in the presence and absence of an efflux pumps inhibitor, phenylalanine-arginine β-naphthylamide (PAβN), was evaluated using the micro-dilution method. The effects of the most active sample were evaluated on the cell growth kinetic and on the bacterial H⁺-ATPase proton pumps.Results: Phytochemical composition of the crude extract showed a rather selective distribution of secondary metabolites (presence of polyphenols, tannins, steroids, triterpenes, flavonoids, alkaloids, saponins and absence of anthocyanins, anthraquinones). The tested samples displayed different antibacterial activities with minimal inhibitory concentrations (MICs) ranging from 64 to 512 μg/mL. Crude extract (CS) and fraction CSc showed the highest inhibitory spectra, both inhibiting all of the studied bacteria exceptEnterobacter aerogenesEA27 strain. Fraction CSc exerted bactericidal effects on most bacteria meanwhile, crude extract (CS) and sub-fraction CSc2 exerted bacteriostatic effects. Compounds 1 (spectaline) and 2 (iso-6-cassine) inhibited the growth of 70% (Escherichia coliATCC8739 and AG102,Klebsiella pneumoniaeATCC11296,Enterobacter aerogenesATCC13048 and EA27,Providencia stuartiiATCC29916,Pseudomonas aeruginosaPA01) and 60% (Escherichia coliATCC8739,Klebsiella pneumoniaeATCC11296 and KP55,Providencia stuartiiATCC29916,Pseudomonas aeruginosaPA01 and PA124) of bacteria respectively with MICs ranging from 128 to 512 μg/mL. In the presence of PAβN, the activities of crude extract CS, fraction CAc and sub-fraction CSc2 strongly increased on most bacteria strains as their MICs significantly decreased. Sub-fraction CSc2 inhibited the H⁺-ATPase proton pumps and altered growth kinetic ofEscherichia coliATCC8739.Conclusion: The overall results justify the traditional use ofC. sieberianafor the treatment of bacterial infections.