Chemical composition and modulation of bacterial drug resistance of the essential oil from leaves of Croton grewioides

The essential oil from leaves of Croton grewioides Baill was obtained by hydrodistillation using Clevenger apparatus, and its chemical composition was analyzed by GC-MS, where 18 compounds were identified, mostly as monoterpenes (55.56%) and sesquiterpenes (44.44%), in which the major constituent was the alpha-pinene (47.43%). The essential oil of Croton grewioides (EOCg) and its major compound (alpha-pinene) were evaluated as modulators of antibiotic resistance in strain SA-1199B and IS-58 of Staphylococcus aureus that overexpresses efflux protein. The minimum inhibitory concentrations (MICs) of the antibiotics were determined by the microdilution assay in the absence and in the presence of sub-inhibitory concentration of EOCg and alpha-pinene. Although the EOCg and alpha-pinene did not indicate relevant antibacterial activity in vitro, they acted as antibiotic resistance modulators, i.e., EOCg in combination with norfloxacin, reducted its MIC, by 64x whereas in combination with tetracycline it was observed a reduction of 4x. Additionally, it was observed a MIC reduction of tetracycline by 32x, when combined with alpha-pinene. The results suggest that EOCg and alpha-pinene modulate or even reverse bacterial resistance as a putative efflux pump inhibitor. (C) 2017 Elsevier Ltd. All rights reserved.