Studies on bioactivity and secondary metabolites of crude extracts of Bidens pilosa L. (Asteraceae): A medicinal plant used in the Transkei region of South Africa

Whole plant-parts of Bidens pilosa were powdered and extracted in concentrated hexane, acetone, ethanol, methanol and water. The extracts were tested for antimicrobial activity against Escherichia coli (25922), Bacillus subtilis (ATCC 6051), Enterococcus faecalis (51299) Staphylococcus aureus (ATCC 29213) and Pseudomonas aeruginosa (ATCC127853), using standard microbiological techniques. Active crude extracts were macerated in concentrated methanol and tested for secondary metabolites including tannins, saponins, alkaloids, cardiac glycosides, anthraquinones, steroids and flavonoids using standard phytochemical procedures. Hexane and methanol extracts demonstrated similar activity producing 8-17 mm and 11-18 mm inhibition zone-diameter ranges respectively. Further analysis for minimum inhibitory concentrations (MIC50) recorded 1.25-20mg/mL and 2.5-20mg/mL for hexane and methanol extracts respectively. The highest zones of inhibition diameters (22-36mm) and lowest MIC50 values (0.0002-0.0006mg/mL) were recorded for Gentamicin, the positive control. Minimum bactericidal concentration (MBC) ranges were between 10-80mg/mL and 0.001-0.005mg/mL for extracts and control antibiotic respectively. With the exception of anthraquinones, the plant crude extracts tested positive for all secondary metabolites analyzed. These results provide scientific basis for the use of B. pilosa in South African traditional medicine. The antibacterial activity reported herein may be attributed to one or more of the 6 secondary metabolites detected in the plant crude extracts.