Phytochemical and antimicrobial investigation of ochna thomasiana engl. & gilg
Muema, Mbithi Justus
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Infectious diseases are the leading cause of death world-wide despite the vigorous campaigns that have been made to combat them. This has been occasioned by drastic growth of drug resistant pathogens. Phytomedicines derived from plants have shown the ability to overcome resistance in some organisms and great promise in the treatment of intractable infectious diseases. Plant based antimicrobials represent a vast untapped source of medicines and a further exploration of plant antimicrobials is called for. Plant extracts have led to the discovery of many clinically useful drugs such as emetine, berberines and quinine. There is a continuous and urgent need to discover new antimicrobial compounds with diverse chemical structures and novel mechanisms of action for new and re-emerging infectious diseases. Therefore, researchers are increasingly turning their attention to folk medicine, looking for new leads to develop better drugs against microbial infections. The plant species O. thomasiana has been reported be used as a herbal remedy by the Mijikenda community traditional medicinal practitioners. The study aimed at the determination and evaluation of the biological activities of the plant Ochna thomasiana. In this study, the plant extracts were screened for their antibacterial activity against selected strains of bacteria, including Gramnegative Salmonella typhi (clinical isolate) and Pseudomonas aeruginosa, and Grampositive Escherichia coli, Bacillus subtilis, Staphycoccus aureus. Various chromatographic techniques were utilized to separate and isolate the compounds. The purification of the extracts was done using silica gel, column chromatography (CC), Sephadex gel and preparative thin layer chromatography (PTLC). Structure characterization was carried out using standard spectroscopic methods: Infrared (IR), ultraviolet (UV) spectroscopy, mass spectroscopy (MS) and proton nuclear magnetic resonance (1H NMR), carbon-13 nuclear magnetic resonance (13C NMR), distortionless enhancement by polarization transfer (DEPT), coherence spectroscopy (COSY), heteronuclear multiple bond coherence (HMBC) and heteronuclear single quantam coherence (HSQC). Lophirone A (18), afzelone D dimethylether (20), calodenone (17), a mixture of stigmasterol (74) and β-sitosterol (23) and 3β-acetyl-24-ethylfriedelane (75) were identified. The stem and root bark methanol crude extracts showed high activity against the Gram-positive bacteria with zones of inhibition of 14, 15 and 20 mm against the same strains of bacteria. Lophirone A, afzelone D dimethylether and 3β-acetyl-24ethylfriedelane showed high activity against the Gram-positive Staphylococcus aureus with zones of inhibition of 14, 16 and 18, respectively. The results of the study showed the root of O. thomasiana contains biflavonoids, and some sterols as its constituents and their antimicrobial activity is significant and is a lead towards the development antimicrobial agents. This indicated that this plant contains important bioactive compounds and further, the antimicrobial activity of the crude extracts of this plant confirms its use in traditional medicine. However, there is need for in vivo and in vitro evaluation of the crude extracts and isolated compounds. The plant species should be propagated using good agricultural practices for medicinal plants for future evaluation of their activity against pathogens.