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 bacterial resistance. The aim of this work was to study the direct and indirect antimicrobial activity of a Beninese sample of Cymbopogon giganteus essential oil (EOCG) on multi-drug resistant clinical bacteria, its chemical composition, and its cytotoxicity. Direct antimicrobial activity was tested by determination of minimal inhibitory concentration (MIC), and indirect activity, by determining Fractional Inhibitory Concentration Index 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 determined by 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 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 show synergistic effects between EOCG and amoxicillin with FICI ranges of 0.12–0.5 against two Escherichia coli resistant clinical strains, synergistic to additive effects between EOCG and colistin or oxacillin/ampicillin, respectively, against Pseudomonas aeruginosa PA544 and Staphylococcus epidermidis SECN361 (two resistant clinical isolates). Our results also indicate that EOCG had low cytotoxicity (IC50: 67.06 ± 2.694 μg/ml).