Preliminary evidences of the direct and indirect antimicrobial activity of 12 plants used in traditional medicine in Africa

Abstract

In a world of increasing resistance to current antibiotics, search of novel therapeutic options is urgently needed. The aim of this work was to screen plant crude extracts for direct or indirect (inhibition of resistance) antimicrobial activity. Four crude extracts from 12 plants traditionally used in Africa for the treatment of infections were obtained by successive extraction with hexane, dichloromethane, ethyl acetate, and methanol. All extracts were tested against Staphylococcus aureus MRSA ATCC33591 [resistant to β-lactams by production of β-lactamases and of a modified PBP target (PBP2a)]. Direct antimicrobial activity was tested by determination of Minimal Inhibitory Concentrations (MIC), and indirect activity, by determining interactions between antibiotics and extracts using checkerboard titration and calculation of Fractional Inhibitory Concentration Index (FICI; synergy: FICI ≤ 0.5; additivity: FICI ≤ 1). Combined antibiotics were ampicillin (sensitive to resistance mediated by β-lactamases and PBP2a) and oxacillin (sensitive to resistance mediated by PBP2a only). The dichloromethane extract of Vitellaria paradoxa leaves, the methanol extracts of Vitellaria paradoxa, Cola gigantea leaves and twigs, and of Tapinanthus bangwensis aerial parts showed direct antimicrobial activity (MIC 250–500 mg/L). The methanol extracts of Vitellaria paradoxa and Cola gigantea leaves and twigs showed additive or synergistic effects with oxacillin and ampicillin on MRSA ATCC33591 (FICI 0.28–1), suggesting a possible inhibition of PBP2a. The methanol extract of Tapinanthus bangwensis aerial parts and Anchomanes difformis roots improved the activity of ampicillin only (FICI 0.38–1), suggesting β-lactamase inhibition. Polyphenols and particularly tannins were shown to be responsible for these last effects, at least partially for Vitellaria paradoxa. These data need further research aiming at identifying the active compounds in these extracts.