Potentiel antibactérien de quelques plantes médicinales Camerounaises : Etude bio-guidée de Albizia adianthifolia (Fabaceae) vis-à-vis des bactéries multirésistantes Gram négatif

Abstract

The treatment of bacterial infections is currently a serious public health problem, mainly because of the emergence of multidrug-resistant pathogenic microorganisms. Among the solutions considered, natural substances from medicinal plants could therefore constitute a source of potential response to the fight against multidrug resistance.The present study was carried out in order to identify from the methanol extracts of six Cameroonian medicinal plants, some constituents that could justify their antibacterial activity for possible exploitation in a phytomedicine. These plants were: Albizia adianthifolia (Schum.)(Fabaceae), Alchornea laxiflora (Benth.) Pax & K Hoffm (Euphorbiaceae), Boerhavia diffusa Lin- (Nyctaginaceae), Combretum hispidum Laws (Combretaceae), Laportea ovalifolia (Schum.) Chew (Urticaceae) and Scoparia dulcis Linn. (Scrophulariaceae). The rapid colorimetric method with p-iodonitrotetrazolium chloride (INT) was used to determine the antibacterial activity of crude extracts, fractions, sub-fractions and isolated compounds. The bioguided purification of the most active plant (based on antibacterial screening) was carried out by a succession of column chromatography. The elucidation of the chemical structures of isolated compounds was done by spectroscopic methods (NMR, Infrared and HRESI-MS). The activity of the crude extract of theroots of Albizia adianthifolia and that of its most promising isolated compound were evaluated on the bacterial membrane, bacterial kinetics, biofilms and sheep erythrocytes. The effect of efflux pumps on the activity of the latter was performed by evaluation on 10 strains of Gram-negative multiresistant bacteria in vitro in the absence and in the presence of an efflux pumps inhibitor; Phenylalanine Arginine-β- Naphthylamide (PAβN). Based on the results obtained, Albizia adianthifolia (bark and root) extracts and the extract of Laportea ovalifolia were shown to be the most active on 93.33% (14/15) of the bacterial strains tested and the extract of Combretum hispidum was the least active with an activity percentage of 20% (3/15). Having presented the best antibacterial activity, the extract of the roots of Albizia adianthifolia was selected for the continuation of the work and was use to isolate the following compounds : stearic acid (1), stigmasterol and β-sitosterol (2+ 3) in a 1: 1 ratio,β-sitosterol 3-O-βD-glucopyranoside (4), palmatine (5), homomangiferin (6) , mangiferin (7), 3-O-{α-L-Oarabipyranosyl-(1→6)-β-D-glucopyranosyl) oleanolic acid or adianthifolioside GS1 (8) and 3-O- {β-D-glucopyranosyl- (1 → 2) -α-L-arabipyranosyl- (1 → 6) -β-D- glucopyranosyl} oleanolic (9). Compound 8 had been shown to be novel compound in the literature review till date. All compounds (except compound 1) showed antibacterial activity against multidrug-resistant strains used with Minimal Inhibitory Concentration (MIC) between 2 μg/mL and 128 μg/mL, Thèse de Doctorat/Ph.D en Biochimie, rédigée par TCHINDA FOSSI Cédric compound 8 being the most active because, it exhibited complete inhibition on the bacteria tested. The combination of all compounds (except compound 1) and the crude extract of A. adianthifolia roots at PAβN significantly improved the activity of the crude extract and 2 + 3, 4 and 8. The combination of compounds 2 + 3, 4 and 8 with antibiotics showed synergistic effects on the seven antibiotics tested. Adianthifolioside GS1 (8) and the crude extract of the roots of Albizia adianthifolia showed a bacteriostatic effect vis-a-vis the strains tested, and caused membrane lysis in K. pneumoniae strains KP55 and E. coli AG102 at concentrations tested (0.5 x MIC, MIC and 2 x MIC). The extract of the roots of Albizia adianthifolia showed a significant inhibition of the biofilms of the strains tested, contrary to the compound 8. The crude extract and more particularly its most active isolated compound (compound 8) showed a low toxicity towards erythrocyte cells. This study presents results that would augur the use of the substances derived from the roots of Albizia adianthifolia in the fight against infections caused by multidrug resistant Gram negative bacteria. Thus, the crude extract of the roots of this plant and the isolated compound 8 appear as good candidates for the preparation of phytomedicines and drug respectively, the combination of the compounds 2 + 3, 4 and 8 with the usual antibiotics can be used to potentiate the action of the latter.