Antibacterial activity of phytochemicals from Bauhinia thonningii significantly increased in the presence of the efflux pump inhibitor, phenylalanine-arginine-β-naphthylamide towards multidrug-resistant phenotypes

Background: View the rapid spread of multidrug-resistant bacteria worldwide; alternatives to antibiotics are necessary for handling various bacterial infections. This study aimed to investigate the antibacterial activity of eight flavonoids isolated from the leaves methanol extract of Bauhinia thonningii against multi-drug resistant (MDR) bacteria expressing efflux pumps. Methods: The antibacterial activity of the phytochemicals alone and in the presence of phenylalanine-arginine-ß-naphthylamide (PAβN) was carried out by the broth micro-dilution method. Spectrophotometric methods were used to evaluate the effects of quercetin-3-O-L-rhamnopyranoside (5) on bacterial growth curve and bacteriolysis, whereas its effect on the bacteria H+-ATPase pumps was monitored by pH measurement. Methods: The antibacterial activity of the phytochemicals alone and in the presence of phenylalanine-arginine-ß-naphthylamide (PAβN) was carried out by the broth micro-dilution method. Spectrophotometric methods were used to evaluate the effects of quercetin-3-O-L-rhamnopyranoside (5) on bacterial growth curve and bacteriolysis, whereas its effect on the bacteria H+-ATPase pumps was monitored by pH measurement. Results: The tested phytochemicals had selective antibacterial activity with minimal inhibitory concentration (MIC) ranging from 8 to 128 μg/mL. Compounds 5, 6, and 8 were active against all the tested bacteria, and compound 8 showed the lowest MIC value (8 µg/mL), especially against E. coli AG102, S. aureus MRSA3, and P. stuartii NEA16. Compound 5 presented good activity (8 < MIC ≤ 32 µg/mL) against 50% of the tested bacteria. Moreover, compounds 5 and 6 have displayed excellent activity (4 < MIC ≤ 32 µg/mL) against P. aeruginosa PA124. In the presence of PAβN, the antibacterial activity of all the tested compounds increased 2 to >128 folds on at least one of the tested bacteria. Quercetin-3-O-L-rhamnopyranoside (5) inhibited bacterial growth in the exponential phase, induced bacteriolysis, and inhibited the H+-ATPase pumps in P. stuartii NEA16. Conclusion: Compounds 5, 6, and 8 isolated from the leaves of Bauhinia thonningii deserve further investigations as potential drugs to fight infections involving MDR bacteria expressing efflux pumps.