Tannins, flavonoids and stilbenes in extracts of African savanna woodland trees Terminalia brownii, Terminalia laxiflora and Anogeissus leiocarpus showing promising antibacterial potential

Terminalia laxiflora , Terminalia brownii and Anogeissus leiocarpus are used as decoctions, macerations, infusions and fumigations in East and West African traditional medicine for treatment of infectious diseases and their symptoms. Using this ethnopharmacological information as a guideline for our research and owing to the fact that these species have not been subjected to in depth antibacterial and phytochemical studies, thirty-nine extracts of various polarities of the stem bark, stem wood and roots were studied for growth inhibitory effects against the human pathogenic bacteria Staphylococcus epidermidis , Staphylococcus aureus , Micrococcus luteus and Pseudomonas aeruginosa . Our results indicate that the studied species contain antibacterial compounds of a wide range of polarities. All polar root extracts of T. laxiflora and various polar extracts of T. brownii roots, including hot water decoctions, gave broad-spectrum antibacterial effects and low MIC values of 39 μg/ml. The main ellagitannins in an ethyl acetate extract of the root of T. laxiflora were found to be corilagin and its derivative and punicalagin. A methanol extract of the roots of T. brownii contained methyl-(S)-flavogallonate and its derivative as the main identified ellagitannins. Moreover, both Terminalia species were found to contain ellagic acid xylopyranoside and methyl ellagic acid xyloside and pure ellagic acid was present in T. brownii . Pure punicalagin did not give as low MIC as an ethyl acetate extract of the roots of T. laxiflora , containing punicalagin as one of its main compounds, although this ellagitannin totally inhibited the growth of S. aureus at 125 μg/ml and P. aeruginosa at 500 μg/ml. Similarly, pure ellagic and gallic acid gave higher MIC values than the methanolic root extract of T. brownii against S. aureus and P. aeruginosa . Moreover, a Sephadex LH-20 fraction of the methanolic extract of the roots of T. brownii , enriched with methyl-(S)-flavogallonate and its isomer, gave higher MIC values than the crude methanolic extract. These results suggest that the polyphenols in the extracts might act synergistically with each other. A methanolic soxhlet extract of the roots of A. leiocarpus , containing ampelopsin, aromadendrin, taxifolin, pinosylvin and 4′-methylpinosylvin gave a low MIC value of 39 μg/ml against all bacterial strains used in this investigation. Our results demonstrate that the roots, stem bark and stem wood of T. brownii , T. laxiflora and A. leiocarpus are rich sources of (new) antimicrobial compounds and justify the uses of these plants for treatment of infections in African traditional medicine.