Chemical composition, direct and indirect antibacterial activity against multidrug resistant bacteria and toxicity assessment of essential oil of Cymbopogon giganteus leaves

Because of antibioresistance rising, the search of new alternative strategies by combining classic antibiotics and essentials oils to restore antibiotics efficacy may be a promising approach. The aim of this work was to study the direct and indirect antimicrobial activity of Cymbopogon giganteus essential oil from Benin (EOCG) on multidrug resistant bacteria, its chemical composition and its oral acute toxicity. Direct antimicrobial activity was tested by determination of Minimal Inhibitory Concentration (MIC), and indirect activity, by calculating the Fractional Inhibitory Concentration Index using checkerboard (FICI; synergy: FICI ≤ 0.5; additivity: 0.5<FICI≤1) on reference but also on multidrug resistant clinical isolates. Composition was determined by GC-MS and GC-FID. Cytotoxicity was evaluated in vitro against human non-cancer fibroblast cell line (WI38) by MTT assay and oral acute toxicity by determination of the “limit dose test” at 2000mg/kg. Limonene (12.07%) and p-menthane derivatives (54.87%) were the major components. Our results confirmed the direct antimicrobial activity of EOCG, but here on clinical resistant strains (MIC from 0.125%v/v to 0.5%v/v). We also observed, for the first time, the synergistic effects between EOCG and amoxicillin with FICI between 0.12-0.5 against two Escherichia coli amoxicillin-resistant clinical strains, synergistic to additive effects between EOCG and colistin or oxacillin/ampicillin respectively against Pseudomonas aeruginosa PA544 and Staphylococcus epidermidis SE361 (two multiresistant clinical isolates). EOCG had a low cytotoxicity (IC50: 67.06±2.69 μg/ml) and no acute toxicity at the dose of 2000mg/kg per os. This is the first report of oral acute toxicity assessment of this essential oil.