Phytochemical Antibacterial And Antifungal Study Of Dombeya Torrida(J.F. Gmel) And Hydnora Abyssinica (A. Braun)
Ndwigah, Stanley Njagih
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Introduction Traditional medicines play an essential role in healthcare. The World Health Organisation (WHO) estimates that approximately 80 % of world inhabitants rely on traditional medicine for their primary healthcare. About 85 % of the traditional medicines involve use of plant extracts. At least 119 pure chemical substances derived from 90 plant species are important drugs currently in use in one or more countries. Of these 119 drugs, 74 % were discovered as a result of chemical studies directed at the isolation of active substances from plants used in traditional medicine. Thus natural products are the skeletal frameworks of about 60 % of modern drugs that are available today. Despite the investigation of terrestrial flora, it is estimated that only 5-15 % of approximately 250,000 species of higher plants have been systematically investigated, chemically and pharmacologically. The potential of large areas of tropical rain forests remains virtually untapped. Thus the interest in nature as a source of potential chemotherapeutic agents continues. In Kenya, many plants are used in management of various disease conditions but have hardly been studied. These plants include Dombeya torrida (J. F. Gmel.) and Hydnora abyssinica (A. Braun). The decoction of D. torrida bark is taken by the Maasai to treat indigestion after a large meal of meat. Its roots are used for treatment of chest pains and colds. The pseudo-rhizome of H. abyssinica is normally boiled and the infusion drunk as a cure for throat complaints, as an astringent in dysentery, for treatment of stomach trouble and for removing the placenta if it does not come out in time. It is also used in treatment of diarrhoea and amoebic dysentery and to treat typhoid, anthrax, cancer and East Coast Fever. In vitro bioassay of H. abyssinica aqueous, methanol and chloroform extracts, showed ability to inhibit growth of 6 human pathogenic fungi and 4 types of bacteria. Phytochemical analyses showed preponderance of condensed tannins and phenols in the H. abyssinica extracts. Bioassay-guided phytochemical investigation of H. abyssinica to isolate secondary metabolites yielded four compounds; tetra-decanoic acid, catechin, tyrosol and benzoic acid. The isolated compounds were demonstrated to have antioxidant and antiglycation potential. The present study was aimed at isolating compounds and investigating the antibacterial and antifungal activities of D. torrida and H. abyssinica. Methodology The D. torrida stem-bark and leaves; and H. abyssinica were collected from various parts of Kenya and identity established. Voucher specimens were kept at the School of Biological Sciences Herbarium, University of Nairobi. The D. torrida stem-bark leaves and flowers; and H. abyssinica pseudo rhizomes were air-dried, finely ground and the powder stored at room temperature in a dry place until used. Extraction of the plants was carried out using chloroform, methanol, acetone, ethyl acetate, petroleum ether and water. The water and methanol extracts were dissolved in distilled water while dimethyl sulfoxide was used to solubilise chloroform extract to a concentration of 100 mg/ml. These were then screened for antibacterial activity against Staphylococcus aureus, Staphylococcus epididermis, Bacillus pumilus and Escherichia coli; and for antifungal activity against Saccromyces cerevisiae and Candida albicans using agar diffusion method. Gentamicin and nystatin were used as positive controls. To isolate compounds, open column chromatography using normal phase silica gel and Sephadex LH20 were employed. Both isocratic and gradient fractionation procedures were carried out. Infra-red, mass spectrometry and NMR analysis were carried out to identify the compounds. Results Dombeya. torrida bark decoction had the highest activity against S. aureus with an inhibition zone diameter of 16.91 mm followed by D. torrida bark macerate with 16.73 mm. Hydnora abyssinica macerate had least activity against S. aureus with a zone diameter of 8.86 mm. Dombeya torrida bark decoction had the highest activity against S. epididermis with a diameter of 17.05 mm with Hydnora abyssinica macerate having the least activity. Activity against E. coli was highest for D. torrida bark decoction with zone diameter of 16.56 mm followed by H. abyssinica chloroform extract at 15.84 mm. Dombeya torrida bark macerate and methanol extracts were equipotent with zone diameters of 15.82 mm. Hydnora abyssinica chloroform extract had the highest activity against B. pumilus with a zone diameter of 17.04 mm with H. abyssinica macerate having the least activity at 8.51 mm. The highest activity observed against S. cerevisiae was with D. torrida chloroform extract with a zone diameter of 17.69 mm and H. abyssinica chloroform extract (17.38 mm) with H. abyssinica macerate having the least activity (7.70 mm). Dombeya torrida chloroform extract was the most active extract against C. albicans with a zone diameter of 20.09 mm. The autobioassay results showed H. abyssinica extracts to have appreciable antifungal and antibacterial activities. In this assay, dichloromethane: methanol extract of H. abyssinica had the highest activity against B. pumillus with 5 spots of inhibition. This extract had comparable effects to those of H. abyssinica chloroform extract against S. aureus. The D. torrida chloroform extract and H. abyssinica chloroform and dichloromethane: methanol extracts had a marked inhibition effects against C. albicans with big bands, an indication of the presence of antifungal constituents in these extracts. Methods of isolation of compounds from D. torrida and H. abyssinica using open column using normal phase silica gel and chloroform, dichloromethane, hexane, ethyl acetate and methanol were developed. Using these methods fifteen compounds were isolated from D. torrida and five from H. abyssinica. Five of these were identified as friedelin, friedelan-3β-ol, β-sitosterol, stigmasterol and taraxerol by use of spectroscopic techniques and also by comparison with spectroscopic data of compounds in literature. This is the first time these compounds were isolated from D. torrida. Five compounds were also isolated from H. abyssinica but could not be identified. Further spectroscopic work is needed to identify these compounds. Conclusion The plants under study, D. torrida and H. abyssinica were chosen on the basis of folklore. These antibacterial and antifungal results support the folklore that H. abyssinica is used as a cure for throat complaints, as an astringent in dysentery, treatment of diarrhoea and amoebic dysentery. Dombeya torrida extracts also had appreciable antifungal and antibacterial activities supporting its folklore use to treat chest pains and colds as many of these conditions are usually caused by bacterial infections. The study provides further knowledge regarding the phytochemistry of D. torrida with five compounds: friedelin, friedelan-3β-ol, β-sitosterol, taraxerol and stigmasterol being isolated and identified for the first time from this plant.