Evaluation of the antibacterial activity of some African medicinal plants

Aiming to find new antibacterial compounds from African medicinal plants, forty extracts of different polarity from eight plant species were screened against Gram positive and Gram negative bacteria. The selected plants, Cleistochlamys kirkii, Colophospermum mopane, Cladostemon kirkii, Chrysophyllum viridifolium, Gardenia ternifolia, Grewia hexamita, Xeroderris stuhlmannii and Albizia adianthifolia, are used in Mozambique traditional medicine to treat infectious diseases. The best results were obtained for both apolar and polar extracts of C. kirkii root bark (Annonaceae) with minimum inhibitory concentration (MIC) of 7.5 – 62 µg/mL against Gram positive strains. Bioassay-guided fractionation of the methanol extract of C. kirkii allowed the isolation of several known compounds with different scaffolds, namely three flavanones, two α,β-unsaturated lactones, one mono-tetrahydrofuran derivative, one triterpene and one alkaloid. Their structures were assigned based on spectroscopic methods namely 1D-NMR and 2D-NMR experiments.

The evaluation of the antibacterial activity was performed by the microdilution method, against Gram positive Staphylococcus aureus (sensitive and resistant strains), vancomycin-resistant Enterococcus faecalis, Bacillus subtilis and Gram negative Salmonella typhimurium and Pseudomonas aeruginosa. MIC and MBC (minimum bactericidal concentration) values were determined for all compounds. The best results were obtained for some of the flavanones that displayed MIC values lower than 4 µg/mL against all the Gram positive strains tested. Furthermore, using the checkerboard method, all the compounds, excepting the alkaloid, were also evaluated for synergistic effects in combination with the β-lactam antibiotics amoxicillin and oxacillin against Staphylococcus aureus (methicillin-resistant and sensitive strains). The fractional inhibitory concentration index (FICI) values indicated synergistic activities for two of the compounds.