Isolation and identification of antimicrobial metabolites from rhus natalensis and rhus ruspolii

Abstract

Antimicrobial resistance has been reported to be on the increase. It is believed that, new antibiotics with activities and structures widely different from those in current use could be found in medicinal plants. This requires a follow up of promising leads with attempts to isolate and identify the active principles from these plants. Two Kenyan plants Rhus natalensis and R. ruspolii were selected for this study because of their reputation in folklore medicine as antimicrobial agents and the use of R. natalensis twigs as chewing sticks. Crude extracts of the roots root bark of these plants were bioassayed for antibacterial and antifungal activities. Based on the preliminary antimicrobial activity results, roots of the two plants were selected for isolation and chemical characterization of compounds from their organic extracts. Vacuum liquid chromatography (VLC) column chromatography (CC) and thin layer chromatography (TLC) were used for separation. isolation and purification of compounds from the extracts. Spectroscopic techniques were used to elucidate their structures. Six compounds were isolated which included: P-sitosterol (82), stigmasterol (83), lupeol (84), (3-sitosterol glucoside (85), epicatechin (86), and an urushiol (87) (partialy elucidated). The bacteria used to test for antibacterial activities included Bacillus subtilis, Staphylococcus aureus Escherichia coli and Pseudomonas aeruginosa while the antifungal test was carried using Candida albicans, Penicillium notatum and Aspergillus niger. The antibacterial test showed that all the crude extracts inhibited the growth of B. subtilis. The highest inhibition zone was shown by R. natalensis ethyl acetate extract which had an inhibition zone of 14 mm. All the crude extracts apart from R. natalensis hexane extract showed some moderate (11-15 mm) activity against S. aureus. Four extracts namely R. natalensis, ethyl acetate, methanol, R. ruspolii hexane and ethyl acetate showed some activity against E. coli which ranged from 9 to 12 mm. Two extracts R. natalensis, ethyl acetate and R. ruspolii ethyl acetate showed activities of 10 and 11 mm against P. aeruginosa, respectively. The antifungal activity was slightly lower but R. natalensis, ethyl acetate and methanol extracts had activities ranging between 9 and 11 mm against all the fungi used. Among the isolated compounds the urushiol and epicatechin had activity against all the four bacteria used in the test. The urishiol showed the highest activity of 16 mm against S. aureus. P-sitosterol (82), stigmasterol (83). lupeol (84) and (3sirosterol glucoside (85) showed activity against B. subtilis and S. aureus with inhibition zones between 9 and 12mm. Lupeol (84), P-sitosterol glucoside (85) had some moderate activity against E. coli.The compounds, lupeol (84), (3-sitosterol glucoside (85), epicatechin (86), and the urushiol (87) showed moderate activity ranging between 8 and 12 mm against the fungi used. The study showed that the studied plants contain many compounds which may be used to cure some illment caused by bacteria and fungi used in the study.