Pharmacological evaluation of extracts from Buxus macowanii, Polygala myrtifolia, Scilla sp. and Xanthocercis zambesiaca

Abstract

The outbreak of drug resistant pathogens, the high cost of health care, limited accessibility of the conventional drugs and their side effects are problems that make the treatment of infectious diseases difficult all over the world. These challenges have led to the search for novel drugs and drug leads that can surpass the quality of the currently available antimicrobial agents. Medicinal plants are considered to be the best candidates for the discovery of new drugs because of their long history of use in the treatment of various ailments in communities. The current study was aimed at investigating the antimicrobial activity, cytotoxic activity and phytochemical composition of the methanol extracts from Buxus macowanii, Polygala myrtifolia, Scilla sp. and Xanthocercis zambesiaca. Staphylococcus aureus, Clostridium perfringens, Pseudomonas aeruginosa, Enterococcus faecalis, Escherichia coli, Staphylococcus epidermidis, and the fungal species Candida albicans and Candida tropicalis were used to evaluate the antimicrobial activity of the selected plant extracts using the broth Microdilution method. All the plants extracts tested showed no activity against all the bacterial and fungal species except Buxus macowanii. Buxus macowanii inhibited the growth of Staphylococcus aureus, Clostridium perfringens, Pseudomonas aeruginosa, Staphylococcus epidermidis, Candida albicans and Candida tropicalis at the MIC of 2.5 mg/ml while Enterococcus faecalis and Escherichia coli were inhibited at 1.2 mg/ml. Buxus macowanii was selected for further studies because it presented the best antimicrobial properties. Antimicrobial compounds were located using TLC bioautography. Four clear zones possibly flavonoids and alkaloids were detected on the TLC chromatogram. These findings suggest that the antimicrobial activity of Buxus macowanii was not attributed to a single compound but to a synergy of compounds. The effect of Buxus macowanii on the bacterial cell morphology was also evaluated. Morphological changes such as damage to the cell wall, loss of intracellular contents, incomplete cell division and shrinkage of the cells were observed using Scanning and Transmission Electron Microscopy. Bacterial cells were affected morphologically after treatment with the extracts of B. macowanii. In order to evaluate the safety of the extracts used in the study, the Sulforhodamine cytotoxicity assay was carried out using the WI-38 cell line (Normal human fetal lung fibroblast). P. myrtifolia was inactive against the WI-38 cell line whereas B. macowanii and X. zambesiaca were found to be moderately hazardous. Scilla extracts were found to be hazardous. These results indicate that caution should be exercised when employing plants like B. macowanii, X. zambesiaca and Scilla sp. for treatment of ailments. The phytochemical screening of B. macowanii, P. myrtifolia, Scilla and X. zambesiaca using standard methods, TLC and GCMS revealed compounds that have important health benefits. Bioactive compounds such as flavonoids, alkaloids, terpenes, cardiac glycosides, steroids, saponins and tannins were found in most of the extracts and their presence may explain the medicinal usage of the plants. GCMS also revealed compounds such as neophytadiene that was found in the extracts of Buxus macowanii, n-hexadecanoic was also found in the extracts of scilla sp and X. zambesiaca. 2-methoxy- 4-vinylphenol was found in the extracts of P. myrtifolia and X. zambesiaca. The results obtained in this study show that B. macowanii is a promising source of antimicrobial drugs. Further investigation into the isolation and identification of the bioactive compounds as well as in vivo screening is recommended.