Bioguided isolation, identification and activity evaluation of antibacterial compounds from Combretum rhodanthum Engl. & Diels

Abstract

Infectious diseases cause over 13 million deaths worldwide annually as a result of the spread of microbial resistance. The use of medicinal plants in the treatment of infectious diseases is well documented. One such plant includes Combretum rhodanthum Engl.& Diels. (C. rhodanthum), which is traditionally used to treat skin and diarrheal infections, and gastrointestinal disorders, among others. Thus, the scientific validation of the use of C. rhodanthum in ethnomedicine is valuable. This study sought to investigate the activity-guided isolation of antibacterial compounds of C. rhodanthum. The plant was extracted with ethanol, and the resulting extract was partitioned using solvents of increasing polarity. After screening the as-prepared extract and fractions for antibacterial activity, the most active fraction (n-butanol fraction) was subjected to flash chromatography to afford eight subfractions, which were subsequently purified using column chromatography. Next, extract, fractions and compounds of C. rhodanthum were screened for antibacterial activity by microdilution method. The cytotoxicity assay was conducted on murine macrophage RAW 264.7 cells. Molecular docking studies were performed using AutoDock Vina. Herein, the ethanol extract inhibited the growth of the eleven bacterial strains tested (minimum inhibitory concentrations (MICs): 0.62 to 10 mg/mL). The fractions and subfractions showed more potent antibacterial activity with MICs and MBCs (minimum bactericidal concentrations) ranging from 1.25 to 10 mg/mL and from 0.62 to 5 mg/mL, respectively. Of the six compounds (CR₁CR₆) isolated from C. rhodanthum, only CR₆ showed moderate bactericidal activity against a multi-resistant strain of Staphylococcus aureus. Docking studies of CR₆ at the bioactive cascade of dihydropteroate synthase and UDP-N-acetylenolpyruvylglucosamine reductase have established that hydrogen bonding and optimized hydrophobic interactions stabilized the ligand at the target site. This novel contribution demonstrates the antibacterial activity of extracts and a pentacyclic triterpenoid (CR₆) from C. rhodanthum, which can serve as starting points for the discovery of effective antimicrobial drugs. Nevertheless, in-depth mechanisms of action, toxicity and pharmacokinetic studies, and in vivo experiments are warranted for the successful utilization of C. rhodanthum in ethnomedicine.