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 b-lactams by production of b-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 B 0.5; additivity: FICI B 1). Combined antibiotics were ampicillin (sensitive to resistance mediated by b-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 b-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.

Screening and comparative study ofin vitroantioxidant and antimicrobial activities of ethanolic extracts of selected Vietnamese plants

“…[97][98][99] This could explain the fact that P. amarus displayed high antioxidant and antimicrobial activities. This is in agreement with Catteau et al [100] , reporting that tannins were at least in part responsible for the activities of several plant methanolic extract effects, but other compounds may also explain a part of the activity.…”

Synergy between Ursolic and Oleanolic Acids from Vitellaria paradoxa Leaf Extract and β-Lactams against Methicillin-Resistant Staphylococcus aureus: In Vitro and In Vivo Activity and Underlying Mechanisms

“…A synergistic effect (≥2 log 10 -fold reduction in inoculum as compared to the antibiotic alone) was obtained for concentrations of 4 and 8 mg/L of UA and OA (i.e., ¼ MIC), respectively with both antibiotics. At the highest triterpenic acid concentrations tested, a 1.5–2 log 10 decrease from initial inoculum was observed for both β-lactams, indicating that the presence of large concentrations of UA and OA in the VP leaf extract could explain the previously described direct and indirect antimicrobial activity against MRSA exposed to this extract alone or combined with β-lactams [ 10 ]. Conversely, the absence of activity for the twig extract could be attributed to its reduced content in these compounds.…”

“…Building on previous data [ 10 ], we began by identifying the active components of Vitellaria paradoxa crude extracts. LC/MS analysis of the crude dichloromethane leaf extract allowed identifying its two major components, UA and OA, by comparison of retention times, mass spectra and fragmentations with standard compounds.…”

“…Having identified the main components of VP crude extracts, we assessed the direct and indirect antimicrobial activity on MRSA (reference strain ATCC33591 and four clinical isolates) of the pure compounds to determine if they could account for the antimicrobial activity of the leaf extract (MIC 250 mg/L [ 10 ]). As shown in Table 1 , both triterpenic acids were active against MRSA strains with MICs ranging from 8–16 mg/L for UA and 32–128 mg/L for OA, respectively.…”

“…To date, the antimicrobial activity of VP stem bark and leaf extracts has only been reported on strains of S. aureus using an agar diffusion method and high extract concentrations (50 g/L) [ 2 , 9 ]. However, during a preliminary screening of the direct and indirect antimicrobial activity of plants used in traditional medicine in Benin, we found that a VP leaf dichloromethane extract showed the best direct and indirect antimicrobial activities with a MIC = 250 mg/L and fractional inhibitory concentration indices (FICI) = 0.14–1 and 0.51–1 in combination with ampicillin and oxacillin, respectively, against a strain of MRSA [ 10 ]. In the present study, we continued our work on the dichloromethane extracts and identified, for the first time in leaves and twigs, oleanolic (OA) and ursolic (UA) acids as bioactive compounds.…”