Thespesia populnea L.: Bioassay-guided approach and molecular docking analysis on pseudomonas aeruginosa protein targets

Abstract

Plant extracts have attracted scientists´ interest as natural source of safe anti-bacterials to offer alternatives for conventional antibiotics. Ethanolic extracts of bark, wood, leaves of Thespesia populnea L., and fractions of the most bioactive extract were screened for anti-bacterial activity. This was tested against Gram-positive( Staphylococcus aureus & Bacillus subtilis ) and Gram-negative( Escherichia coli & Pseudomonas aeruginosa ) bacteria by agar well diffusion method using ciprofloxacin(standard drug). Furthermore, minimum inhibitory concentration(MIC) and minimum bactericidal concentration(MBC) were determined using resazurin-based 96-well plate microdilution method. Ethanolic bark extract(EBE) exhibited the strongest activity against all tested bacteria with MIC range of 2.5-5mg/ml and MBC range of 5-10mg/ml. Methylene chloride fraction(Met-B) of EBE showed potent bacteriostatic and bactericidal actions on all tested bacteria, particularly against P. aeruginosa with high potency (MIC and MBC; 0.63 & 2.5mg/ml, respectively). Consequently, further investigation was conducted to identify and isolate bioactive constituents from Met-B. This led to isolation of six compounds(cryptolepine, quinodolinone, quinodoline, cryptoheptine, scopoletin and aescultin) using flash chromatography. Moreover, five compounds(quercetin, kaempferol, daidzein, caffeic and tannic acids) were identified using TLC/MS technique. Furthermore, molecular docking study was implemented on proteins that play crucial roles in virulence and pathogenicity of P. aeruginosa (DNA gyrase, Fab A, QS-LasR and QS-PqsR). The results demonstrated that cryptolepine, quinodolinone, quinodoline, cryptoheptine showed the strongest binding affinity to all targets confirming their role in inhibiting P. aeruginosa pathogenesis. These findings highlight T. populnea L. as a potential anti-bacterial agent against P. aeruginosa in traditional medicine and as a source for developing therapeutics against drug-resistant bacterial infections.