Screening of some Traditional Medicinal Plants from Zimbabwe for Biological and Anti-microbial Activity

Abstract

In Zimbabwe medicinal plants play a critical role in the healthcare delivery system. Ethnobotanical surveys of five districts from the Matebeleland and Manicaland regions of Zimbabwe were done and most commonly used and endangered plant species used in traditional medical practice were selected. From twelve plants 20 extracts were prepared from the various plant parts and extracted by solvent extraction and freeze dried. The extracts were then screened for phytochemical constituents and biological activity using standard techniques.

Phytochemical screening was carried out using TLC (Thin Layer Chromatography) where UV detection was at 254 and 365nm followed by confirmatory tests. Three of the plant extracts contained alkaloids, five had anthraquinone derivatives, five had coumarins, eight had cardiac glycosides and flavonoids, fifteen had saponins and sixteen contained tannins. Saponins and tannins were the most abundant phytochemicals found. These phytochemical groups and isolated chemical compounds may be responsible for the various biological activities observed by traditional healers in ethnopharmacology.

Total phenolic contents were determined by the Folin-Ciocalteu method and the antioxidant activity was evaluated by DPPH (2, 2-diphenyl picrylhydrazyl) using â-carotene as reference. A high total phenolic content was observed for P. africanum bark 0.438±0.00424 and lowest for V. infausta leaves with 0.0048±0.00255TAE. Some extracts had high antioxidant activity: P. africanum leaves, bark, and roots had 97.6±0.354%, 96.3±0.354% and 96.40±0.00% respectively. The lowest antioxidant value was recorded in V. infausta roots with 39.7 ± 0.212%. Most plants showed great potential for use as antioxidants.

Brine shrimp lethality tests were used to predict potential cytotoxic activity of the plant extracts. Most of the extracts were non-toxic with LC50 values ranging from 1000 to 4000 µg/ml compared to the known toxic plant Nerium oleander used as a positive control with LC50 value of 142±68.2 µg/ml. V. infausta leaves (338±23.4 µg/ml), root (416±28.3 µg/ml) and P. angolensis bark (478±29.7 µg/ml) were moderately safe and should be used with caution. Plants showing significant toxicity to Artemia salina may have potential use as anti-tumour drugs.

Antimicrobial activity was carried out on selected microorganisms and fungal strains by the agar well diffusion method. Antibacterial activity was assessed against Staphylococcus aureus, Streptococcus Group A (Gram positive bacteria) and Escherichia coli, Pseudomonas aeruginosa (Gram negative bacteria). The fungi used were Candida albicans and Aspergillus niger. Activity was measured as a radius in mm to give a zone of inhibition. In general, the gram positive bacteria were more sensitive to the plant extracts compared to the gram negative bacteria. Out of the runs done for all the bacterial strains; 28 plant extracts showed no activity against the strains at the different concentrations used. The root extracts of X. caffra, P. angolensis and P.africanum were highly active (about 10mm radius) against E. coli. P. africanum root extract was also highly active against P. aeruginosa. A. stenophylla leaves were inactive against S. aureus, E. coli and P. aeruginosa. 65% of the extracts did show activity against Candida albicans and Aspergillus niger. The highest zone of inhibition was shown by D. anomala tuber against C. albicans; reading of 5.5±0.58mm and P. africanum bark 4.0±0.82mm.The lowest values were recorded from C. anisata leaves and L. edulis leaves against A. niger; readings of 1.00±0mm and 1.00±0.58mm respectively.

The results from these studies indicated that most of the commonly used traditional medicinal plants have merit for use in traditional medical practice as they have shown zones of inhibition on various microorganisms tested, meaning they are potential antimicrobial agents and so should be preserved and harvested with care.