A Comparison of the Antimicrobial Activity and Toxicity of Six Combretum and Two Terminalia Species from Southern Africa

Background: Plants of the family Combretaceae are amongst the most widely used plants for traditional medicinal purposes in southern Africa. In particular, many species of Combretum and Terminalia are used for their antibacterial, antifungal, antiprotozoal, antiviral, antidiarrhoeal, analgesic, antimalarial, antioxidant, anti‑inflammatory and anticancer activities, yet their antimicrobial potential has not been rigorously studied and compared. Materials and Methods: A survey of antimicrobial activity was undertaken on selected South African Combretum and Terminalia species. Sixteen extracts from 6 Combretum and 2 Terminalia plant species with a history of medicinal usage were investigated by disc diffusion assay against a panel of bacteria and fungi and their MIC values were determined. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: All extracts tested displayed broad spectrum antibacterial activity, inhibiting the growth of 12-16 (75-100%) of the bacteria tested, with Gram‑positive and Gram‑negative bacteria being approximately equally susceptible. Potent antibacterial activities (generally in the range 200-5000 µg/ml) were evident for all Combretaceae extracts against both Gram‑positive and Gram‑negative bacteria. Similarly, the extracts also displayed good antifungal activity, inhibiting the growth of 2-3 (66.7-100%) of the fungal species tested, with fungal growth inhibition activities generally in the range 200–4000 µg/ml. In general, the Terminalia extracts had better efficacies than the Combretum extracts. Furthermore, the methanol extracts were generally better antimicrobial agents than the water extracts. All extracts were also shown to be non‑toxic in the Artemia nauplii bioassay. Conclusion: The lack of toxicity of these extracts and their inhibitory bioactivity against a panel of bacteria and fungi indicate their potential as medicinal agents and partially validate their usage in multiple South African traditional medicinal systems.