Evaluation of anthelmintic, antiamoebic and antibacterial activity in traditional South African medicinal plants.

Abstract

Traditional medicine in southern Africa draws upon a vast selection of plants to treat gastrointestinal disorders such as diarrhoea and intestinal parasites. The evaluation of these plants for biological activity is necessary, both to substantiate the use of these plants by healers, and also a possible lead for new drugs or herbal preparations. After a survey of the existing ethnobotanical literature, plants used to treat stomach ailments such as diarrhoea, dysentery or intestinal worm infestations were selected and submitted to bioassays according to their traditional uses. Extracts of the chosen plants were made using the solvents hexane, ethanol and water, to ensure the extraction of compounds with a wide range of polarity. In total, 138 extracts were tested for antibacterial activity, 72 for anthelmintic activity, and 42 for antiamoebic activity. Antibacterial activity was evaluated using the disc-diffusion assay, and Minimal Inhibitory Concentration (MIC) values were determined using a microdilution assay. The extracts were tested against the Gram-positive bacteria Bacillus subtilis and Staphylococcus aureus, and the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae. Ethanolic extracts showed the greatest activity and Gram-positive bacteria were the most susceptible microorganisms. The free-living nematode Caenorhabditis elegans, which is morphologically similar to parasitic nematodes, was used in two different assays to evaluate anthelmintic activity. A microdilution technique was employed to investigate antiamoebic activity against the enteropathogenic Entamoeba histolytica, the causal organism of amoebic dysentery. These assays were suitable for the screening of a large number of extracts at one time. Several plants exhibited significant activity against these test organisms. Many species of plants belonging to the family Combretaceae are used in southern African traditional medicine against a variety of ailments, including abdominal complaints, bilharzia and diarrhoea. Extracts of powdered leaf material of 24 species belonging to the Combretaceae were prepared using the solvents ethyl acetate, acetone, methanol and water. These extracts were screened for anthelmintic activity. Significant activity was exhibited by C. apiculatum, C. hereroense and C. mossambicense. The most anthelmintic activity was shown by acetone extracts, followed by ethyl acetate, water and then methanol extracts. The aromatic rhizomes of Acarus calamus L. are used extensively in traditional medicine worldwide. They reportedly relieve stomach cramps and dysentery, and are used as anthelmintics. Rhizome extracts of A. calamus growing in KwaZulu-Natal, South Africa, exhibited anthelmintic and antibacterial activity in the initial general screening. Using bioassay-guided fractionation, the phenylpropanoid β-asarone was isolated from the rhizome. This compound possessed both anthelmintic and antibacterial activity. It has previously been isolated from A. calamus, and a related species, A. gramineus. Different varieties of A. calamus exhibit different levels of β-asarone, with the diploid variety containing none of the compound. Mammalian toxicity and carcinogenicity of asarones has been demonstrated by other researchers, supporting the discouragement of the medicinal use of Acarus calamus by traditional healers in South Africa. Schotia brachypetala was another plant to show good antibacterial activity in the initial screening. The roots and bark of S. brachypetala are used in South African traditional medicine as a remedy for dysentery and diarrhoea. The lack of pharmacological and chemical data on this plant prompted a further investigation into its antibacterial activity. The differences in activity of ethanol and water extracts with respect to plant part, season and geographical position were analysed. No extreme fluctuations in activity were noted. Two other Schotia species, S. afra and S. capitata, were included in the study, and both displayed good antibacterial activity. The storage of the plant, either as dried, ground plant material at room temperature, or as an extract residue at -15°C, had little effect on the antibacterial activity. Preparing the extracts from fresh or dry material also did not notably affect the activity. In general, the ethanolic extracts were more active than the aqueous extracts. The chemical profiles on TLC chromatograms were compared and found to be very similar in the case of ethanol extracts prepared in different months of the year, and from different trees. The extracts of the three species, and of the leaves stored under various conditions, as well as extracts prepared from fresh or dry material, also showed similar TLC fingerprints. However, various plant parts of S. brachypetala showed distinctly different chemical compositions. The leaves of S. brachypetala showed slightly higher antibacterial activity than the roots. Fractionation of the ethanol extract of the dried leaves using liquid-liquid partitioning and chromatographic techniques yielded 9,12,15-octadecatrienoic (linolenic) acid and methyl-5, 11,14,17-eicosatetraenoate. These fatty acids displayed antibacterial activity against the Gram-positive bacteria Bacillus subtilis and Staphylococcus aureus, and activity to a lesser extent against the Gram-negative Escherichia coli and Klebsiella pneumoniae. Linolenic acid is known to have antibacterial activity. The screening of plants for biological activity yielded valuable preliminary information about the plants used by traditional healers to treat gastrointestinal illnesses. The isolation of biologically active compounds from two highly active plants was achieved.