Identification of natural molecules as antimicrobials or β-lactam resistance modifying agents against Staphylococcus aureus

Staphylococcus aureus is a major cause of community- and hospital- acquired infections, with β-lactams often considered as the first line of treatment. However, since their emergence, methicillin-resistant S. aureus (MRSA) strains have evolved resistance to multiple classes of antibiotics, making the discovery of novel antimicrobials or treatment regimens urgently needed. Given the potential of natural molecules, we screened 12 plants used in traditional medicine in Africa for their direct (used alone) or indirect (combined with β-lactams) antistaphylococcal activities. Focusing on the most promising extract, the leaf dichloromethane extract from the shea butter tree (Vitellaria paradoxa), we identified 5 triterpenic acids together with 6 triterpenic esters. After having associated the antimicrobial activity of this crude extract to its high contents in ursolic (21%) and oleanolic (6%) acids, we showed that both compounds synergized with two β-lactams (ampicillin & oxacillin). Exploring their mechanism of action, we showed that both compounds caused a delocalization of PBP2 from the cell wall biosynthetic machinery site as well as an inhibition of β-lactamases in living bacteria. Moreover, we showed that local administration of UA provided a synergistic benefit to nafcillin in reducing lesion size and inflammatory cytokine production in a murine model of subcutaneous MRSA infection. Finally, all isolated/identified compounds were tested in vitro for their antiparasitic activity as well as for their cytotoxicity. One isolated triterpenic ester showed a promising antitrypanosomal activity (IC50 = 0.79 µg/mL) with a low cytotoxicity (IC50 = 21.04 µg/mL) and was shown to decrease the parasitemia and to improve significantly the survival of Trypanosoma brucei brucei infected mice. This work thus showed that the underexplored leaves of the well-known shea butter tree contain interesting compounds to fight against MRSA as well as Trypanosoma.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 201

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

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

Natural and hemi-synthetic pentacyclic triterpenes as antimicrobials and resistance modifying agents against Staphylococcus aureus: a review

Staphylococcus aureus infections are considered as seriously problematic for human health and necessitate the development of new medicines and innovative antimicrobial strategies. Plant secondary metabolites have already demonstrated their potential as antibacterials when used alone but also in combination with other antimicrobial agents to potentiate their activity. Triterpenoids are widely distributed in the plant kingdom and known to have many beneficial effects, including anti-inflammatory, immunomodulatory, anti-proliferative, antimycotic, or antimicrobial activity. The aim of this paper is to review the activity of pentacyclic triterpenoids belonging to the ursane, oleanane, or lupane groups against Staphylococcus aureus. We summarize their activity as anti-staphylococcal compounds but also as resistance modifying agents when combined with common antibiotics

Effect of Ursolic and Oleanolic Acids on Lipid Membranes: Studies on MRSA and Models of Membranes

Staphylococcus aureus is an opportunistic pathogen and the major causative agent of life-threatening hospital- and community-acquired infections. A combination of antibiotics could be an opportunity to address the widespread emergence of antibiotic-resistant strains, including Methicillin-Resistant S. aureus (MRSA). We here investigated the potential synergy between ampicillin and plant-derived antibiotics (pentacyclic triterpenes, ursolic acid (UA) and oleanolic acid (OA)) towards MRSA (ATCC33591 and COL) and the mechanisms involved. We calculated the Fractional Inhibitory Concentration Index (FICI) and demonstrated synergy. We monitored fluorescence of Bodipy-TR-Cadaverin, propidium iodide and membrane potential-sensitive probe for determining the ability of UA and OA to bind to lipoteichoic acids (LTA), and to induce membrane permeabilization and depolarization, respectively. Both pentacyclic triterpenes were able to bind to LTA and to induce membrane permeabilization and depolarization in a dose-dependent fashion. These effects were not accompanied by significant changes in cellular concentration of pentacyclic triterpenes and/or ampicillin, suggesting an effect mediated through lipid membranes. We therefore focused on membranous effects induced by UA and OA, and we investigated on models of membranes, the role of specific lipids including phosphatidylglycerol and cardiolipin. The effect induced on membrane fluidity, permeability and ability to fuse were studied by determining changes in fluorescence anisotropy of DPH/generalized polarization of Laurdan, calcein release from liposomes, fluorescence dequenching of octadecyl-rhodamine B and liposome-size, respectively. Both UA and OA showed a dose-dependent effect with membrane rigidification, increase of membrane permeabilization and fusion. Except for the effect on membrane fluidity, the effect of UA was consistently higher compared with that obtained with OA, suggesting the role of methyl group position. All together the data demonstrated the potential role of compounds acting on lipid membranes for enhancing the activity of other antibiotics, like ampicillin and inducing synergy. Such combinations offer an opportunity to explore a larger antibiotic chemical spac

Reply to the letter to the editor “Low‐dose hydroxychloroquine therapy and lower mortality in hospitalized patients with COVID‐19: association does not mean causality.”

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Reply to The perceived efficacy of hydroxychloroquine in observational studies: the results of the confounding effects of goals of care

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