Bidens pilosa L. is a medicinal plant in many regions of the world. The antimicrobial activity of the methanol, acetone and water extracts from the root of the South African ecotype was evaluated using agar dilution method. The extracts showed significant activity against all the bacteria and some fungi species tested in this study. The methanol extract inhibited all Gram-positive and Gram-negative bacteria with minimum inhibitory concentrations ranging from 5 to 10 mg/ml. Although the water extract did not inhibit any of the bacteria, the acetone extract was able to suppress the growth of Staphylococcus aereus,Staphylococcus epidermidus, Escherichia coli and Klebsella pnuemoniae in the same concentration range as the methanol extract. All the extracts exhibited 100% inhibition against Penicilium notatum at 0.1 mg/ml. In the same pattern, all the extracts showed good activity against Aspergillus niger but none could inhibitAspergillus flavus. The best activity was observed in the methanol extract against most bacteria and fungal strains, which suggest the use of organic solvent extraction for good antimicrobial activity. Our results have shown that extracts from the root of this species could inhibit both Gram-positive and Gram-negative bacteria as well as some fungi species. This in vitro study on the root therefore support the folkloric use of the whole plant in the treatment of microbial infections but its medicinal use in infections associated with A. flavus is not recommended.
Key words: Bidens pilosa, root extract, antimicrobial, medicinal plant.
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