SCREENING FOR BIOACTIVE COMPOUNDS IN FIVE SELECTED NIGERIAN MEDICINAL PLANTS AGAINST SOME PATHOGENIC MICROORGANISMS

Abstract

The search for new drugs to combat infectious diseases has stimulated the interest of scientists globally due to emergence of Multi Drug-Resistant (MDR) microorganisms. Indigenous medicinal plants are potential reservoir of bioactive compounds from which new active drugs could be obtained, but are yet to be adequately exploited. This study was therefore designed to screen for new bioactive compounds from some indigenous medicinal plants against selected MDR microorganisms.

Harungana madagascariensis Lam. Ex Poir and Enantia chlorantha Oliv. barks, Senna alata Linn., Gossypium hirsutum Linn. and Alstonia bonnie De Wild leaves were collected from a farmland in Idi-Ayunre, Ibadan and authenticated at Forest Research Institute of Nigeria. Ethanol extracts from the leaves and barks were tested against reference strains of Escherichia coli, Bacillus subtilis, Salmonella typhi, Shigella flexneri, Pseudomonas aeruginosa and Staphylococcus aureus, Microsporum canis, Candida albicans, Candida glabrata, and Aspergillus flavus using Agar well diffusion method. Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of the extracts were determined using tube dilution method. Extract purification was carried out using column chromatographic techniques. Chemical structures of compounds obtained were determined using Mass spectroscopy, Ultraviolet spectroscopy, Infra-Red spectroscopy and (1D and 2D) Nuclear Magnetic Resonance. Purified compounds which showed the best antimicrobial activities were tested against MDR clinical strains of P. aeruginosa and S. aureus compared to standard antibiotics. The compounds’ antioxidant Radical Scavenging Activity (RSA), and in vitro alpha-glucoxidase enzyme inhibition activity was measured at 400 nm using Ultraviolet-Visible absorption spectrophotometer with arcabose as positive control. Data were analysed using ANOVA at p = 0.05.

Harungana madagascariensis, E. chlorantha and S. alata crude extracts showed broad spectrum antibacterial activity, inhibiting all the tested bacterial species with 24.3±0.3, 25.7±0.3 and 27.7±0.6 (mm) diameter zones of inhibition respectively. All the extracts except A. bonnie leave extract exhibited more than 90% inhibition against M. canis. Enantia chlorantha and H. madagascariensis bark extracts showed highest inhibition against C. glabrata (80%) and C. albicans (70%) respectively. The MIC of the extracts ranged from 5.0 to 20.0 mg/mL while the MBC ranged from 20.0 to 30.0 mg/mL. Minimum fungicidal concentration of 50.0 mg/mL of the extracts inhibited all the tested fungi. Five new prenylated anthranoids (Harundigin anthrone, kenganthranol D, E, F and G) from H. madagascariensis were identified. These purified compounds at 100 μg/mL inhibited all the tested microorganisms. The MDR S. aureus was susceptible to isolated kenganthranol G with MIC of 25 µg/mL, but was resistant to standard antibiotics used (oxacillin, amikacin, chloramphenicol, erythromycin, sulfamethozale and ciprofloxacin). Harungana madagascariensis exhibited most significant antioxidant activities against 1,1-Diphenyl-2-picrylhydrazyl radical with 92% RSA, IC50 (33.3±1.8) at 0.5 μg/mL. Harundigin anthrone and kenganthranol E were good inhibitors (97%) of alpha-glucooxidase at IC50 of 69.9±4.2 and 122.4±1.1 respectively as against arcabose with 59.1%.

The prenylated anthranoids obtained from the plants exhibited antimicrobial and anti-enzymatic activities against multidrug-resistant S. aureus. They could serve as alternative source of bioactive compounds against resistant strains of S. aureus.