Pharmacological evaluation of South African medicinal plants used for treating tuberculosis and related symptoms.
Respiratory ailments are major human killers, especially in developing countries including South Africa. Tuberculosis is one of the most prevalent infectious respiratory tract disease posing a major threat to human healthcare worldwide. This disease is a highly contagious airborne bacterial disease that usually infects the lungs and sometimes other body parts. Tuberculosis spreads easily in overcrowded conditions from one person with an active respiratory disease to another via droplets that are emitted when they sneeze or cough. Approximately two million deaths that occur worldwide per annum are caused by tuberculosis and about 285,000 cases occur in South Africa. This is the seventh highest total number in the world. The emergence of drug-resistant tuberculosis and other pathogenic diseases over the past decades makes this disease a serious threat to human health worldwide. Emerging drug-resistant tuberculosis strains and the long duration of treatment has established an urgent need to search for new effective agents. According to a 2012 report by the World Health Organisation (WHO), South Africa, China, India and Russia are the countries with the highest prevalence of multi drug-resistant (MDR) tuberculosis. Most researchers in South Africa have focused on evaluating the antimycobacterial activity of medicinal plants against bacterial strains that cause tuberculosis, but there has not been sufficient focus on the related ailments. Therefore, one of the aims of the present study was the evaluation of the antimicrobial properties of the selected medicinal plants against Mycobacterium species and other bacterial strains related to respiratory infection. The floral diversity of South Africa has a potential for yielding new bioactive compounds, therefore pharmacological screening of plant extracts from this region is important. The aim of this study was the pharmacological evaluation of plants that are used traditionally in South Africa to treat tuberculosis and related symptoms against microorganisms that cause respiratory ailments, and the identification of compounds from antimicrobial active plant extracts. Ten plants: Abrus precatorius subsp. africanus (leaves and seeds), Asparagus africanus (leaves), Asparagus falcatus (leaves), Brunsvigia grandiflora (bulb), Ficus sur (bark and roots), Indigofera arrecta (leaves and roots), Leonotis intermedia (leaves and stem), Pentanisia prunelloides (leaves and roots), Polygala fruticosa (whole plant), and Terminalia phanerophlebia (leaves, roots and twigs) were selected based on a survey of available literature of medicinal plants used in South Africa for the treatment of tuberculosis and related symptoms. Ground plant material from different plant parts of the 10 plants were extracted sequentially with four solvents: petroleum ether (PE), dichloromethane (DCM), 80% ethanol (EtOH) as well as water, and a total of 68 extracts were produced. The plant extracts of the selected plants were evaluated for antibacterial activity against four microorganisms (Klebsiella pneumoniae, Staphylococcus aureus, Mycobacterium aurum A+ and Mycobacterium tuberculosis H37Ra) associated with respiratory infections using the microdilution assay. Cyclooxygenase-2 (COX-2) enzyme was used to evaluate the anti-inflammatory activity of the extracts. The Ames test and mitochondrial reduction (MTT) assays were used to establish toxicity of the extracts that showed noteworthy antimicrobial activity against the tested bacterial strains (Klebsiella pneumoniae, Staphylococcus aureus, Mycobacterium aurum A+ and Mycobacterium tuberculosis H37Ra). The extracts were tested for genotoxicity against Salmonella typhimurium (TA98 and TA100 strains) and cytotoxicity using monkey kidney Vero cells. Based on good antimicrobial activity observed, compounds were isolated from Terminalia phanerophlebia (leaves). Crude extracts obtained from 80% methanol (MeOH) of Terminalia phanerophlebia were successively extracted with hexane, DCM, ethyl acetate (EtOAc) and n-butanol. The fractions and isolated compounds obtained were tested for their antibacterial activity against Mycobacterium aurum A+, Mycobacterium tuberculosis H37Ra, Staphylococcus aureus and Klebsiella pneumoniae. Structure elucidation was carried out using NMR (1D and 2D) spectroscopic methods. This investigation revealed the pharmacological potential of the 10 plants used in South Africa for traditional treatment of tuberculosis and related symptoms: Abrus precatorius subsp. africanus (leaves and seeds), Asparagus africanus (leaves), Asparagus falcatus (leaves), Brunsvigia grandiflora (bulb), Ficus sur (bark and roots), Indigofera arrecta (leaves and roots), Leonotis intermedia (leaves and stem), Pentanisia prunelloides (leaves and roots), Polygala fruticosa (whole plant), and Terminalia phanerophlebia (leaves, roots and twigs). The minimum inhibitory concentration (MIC) values of the tested plant extracts ranged from 0.098 to 12.5 mg/ml. Out of 68 extracts tested from different plant parts of the 10 plant species, 18 showed good antimicrobial activity against at least one or more of the microbial strains tested with MIC values ranging from 0.098 to 0.78 mg/ml. For anti-inflammatory results, only three extracts showed high inhibition (˃ 70%) of the COX-2 enzyme. In the Ames test using Salmonella typhimurium (TA98 and TA100 tester strains), all the extracts tested were non-genotoxic. However, in the MTT assay nine extracts demonstrated cytotoxicity. Bioguided fractionation of the EtOAc fraction of Terminalia phanerophlebia (leaves) afforded two bioactive compounds: methyl gallate (methyl-3,4,5-trihydroxybenzoate) (1) and a phenylpropanoid glucoside; 1,6-di-O-coumaroyl glucopyranoside (2). These compounds are reported from Terminalia phanerophlebia for the first time. Both compounds showed good antimicrobial activity against all bacterial strains tested with MIC values ranging from 0.063 to 0.25 mg/ml. Inhibition of Mycobacterium tuberculosis by 1,6-di-O-coumaroyl glucopyranoside (2) at a MIC value of 0.063 mg/ml was noteworthy, as this bacterial strain is reported to be the leading cause of tuberculosis worldwide. The good antimicrobial property of Abrus precatorius subsps. africanus, Asparagus africanus, Asparagus falcatus, Terminalia phanerophlebia, Indigofera arrecta, Ficus sur, Leonotis intermedia and Pentanisia prunelloides partially authenticate their traditional use in the treatment of respiratory diseases. However, these plants must be used with caution as some of their extracts showed cytotoxicity against Vero cells. The results observed in this study indicate that Abrus precatorius subsp. africanus, Asparagus africanus, Asparagus falcatus, Ficus sur, Pentanisia prunelloides and Terminalia phanerophlebia could be investigated further against drug-resistant tuberculosis strains. Good antimicrobial activity exhibited by the compounds isolated from Terminalia phanerophlebia (leaves) authenticate the traditional use of this plant in treating tuberculosis and its related symptoms. Compound (2), 1,6-di-O-coumaroyl glucopyranoside showed noteworthy activity against a Mycobacterium tuberculosis strain H37Ra (0.063 mg/ml), therefore this compound could potentially serve as a lead in tuberculosis drug discovery.
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