Cymbopogon giganteus Chiov. essential oil: Direct effects or activity in combination with antibiotics against multi-drug resistant bacteria

The discovery of new antimicrobial agents is necessary due to the emergence of multi-drug bacterialresistance. The aim of this work was to study the direct and indirect antimicrobial activity of a Beninesesample of Cymbopogon giganteus essential oil (EOCG) on multi-drug resistant clinical bacteria, its chemicalcomposition, and its cytotoxicity. Direct antimicrobial activity was tested by determination of minimalinhibitory concentration (MIC), and indirect activity, by determining Fractional Inhibitory ConcentrationIndex using checkerboard [fractional inhibitory concentration indices (FICI); synergy: FICI ≤ 0.5; additivity:0.5 < FICI ≤ 1]. EOCG composition was determined by GC-MS and GC-FID and cytotoxicity was determinedby 3-(4,5-dimethylthiazol-2-yl)-2,5-diphényltetrazolium bromide (MTT) assay. p-Menthane derivatives(54.87%) and limonene (12.07%) were detected as major compounds by GC analysis. Our results confirmedthe direct antimicrobial activity of EOCG, but here on clinical resistant strains (MIC from 0.125% v/v to 0.5%v/v). We also show synergistic effects between EOCG and amoxicillin with FICI ranges of 0.12–0.5 againsttwo Escherichia coli resistant clinical strains, synergistic to additive effects between EOCG and colistin oroxacillin/ampicillin, respectively, against Pseudomonas aeruginosa PA544 and Staphylococcus epidermidisSECN361 (two resistant clinical isolates). Our results also indicate that EOCG had low cytotoxicity(IC50: 67.06 ± 2.694 μg/ml).