Combination of Chemically-Characterized Essential Oils from Eucalyptus polybractea, Ormenis mixta, and Lavandula burnatii: Optimization of a New Complete Antibacterial Formulation Using Simplex-Centroid Mixture Design

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Abstract
This study aims to identify the volatile profile of three essential oils obtained from Eucalyptus polybractea cryptonifera (EPEO), Ormenis mixta (OMEO), and Lavandula burnatii briquet (LBEO) and to examine their combined antibacterial activity that affords the optimal inhibitory ability against S. aureus and E. coli using simplex-centroid mixture design and checkerboard assay. Essential oils (EOs) were isolated by hydrodistillation and characterized using gas chromatography-mass spectrometry (GC-MS) and gas chromatography coupled with flame-ionization detector (GC-FID). The antibacterial activity was performed using disc diffusion and microdilution assays. The chemical analysis revealed that 1,8-cineole (23.75), p-cymene (22.47), and α-pinene (11.20) and p-menthane-1,8-diol (18.19), α-pinene (10.81), and D-germacrene (9.17) were the main components detected in E. polybractea and O. mixta EOs, respectively. However, L. burnatii EO was mainly represented by linalool (24.40) and linalyl acetate (18.68). The EPEO, LBEO, and OMEO had a strong antibacterial effect on S. aureus with minimal inhibitory concentrations (MICs) values ranging from 0.25 to 0.5 (v/v). Furthermore, the combination of 1/2048 MICEPEO+1/4 MICLBEO showed a synergistic antibacterial effect on S. aureus with a FIC index of 0.25, while the formulation of 1/4 MICEPEO+1/4 MICOMEO demonstrated an antibacterial synergistic activity on E. coli with a FIC index of 0.5. Moreover, the simplex-centroid mixture design reported that the most effective combinations on E. coli and S. aureus correspond to 32/28/40 and 35/30/35 of E. polybractea, O. mixta, and L. burnatii, respectively. Presented information highlights the action of antibacterial formulations of these EOs and suggests their potential applications as alternatives to commercialized drugs to contract the development of bacteria causing serious infections and food deterioration.

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