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Abstract
This study aimed to determine the volatile oil composition of the fresh leaves of dill, celery, coriander, and fennel, grown in Jordan, and to evaluate the antimicrobial activity of the essential oils individually and in binary (50:50) and quaternary (25:25:25:25) mixtures. The volatile oils, obtained by hydrodistillation, were analysed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). In vitro antimicrobial activities of the essential oils were evaluated against Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, and Candida albicans. Using the microtiter plate dilution method minimal inhibitory concentrations (MICs) of the essential oils were determined and the checkerboard method was then used to investigate the antimicrobial efficacy of the essential oil combinations by means of the fractional inhibitory concentration index (FICI). Dill and celery samples were rich in monoterpene hydrocarbons, while the oils of fennel and coriander were dominated by non-terpenoid compounds. In antimicrobial experiments, the essential oil of dill yielded the best activity against all tested microorganisms. In binary combination of the essential oils, best results were detected in a combination of dill with coriander against B. subtilis and P. aeruginosa, and fennel with coriander against E. coli and P. aeruginosa. The present study is the first evaluation of the volatile oil composition and determination of the antimicrobial activities of the fresh leaves of dill, celery, coriander, and fennel in binary and quaternary combinations. Additive or synergistic effects were detected in certain binary oil combinations while indifferent and antagonist effects were noticed in other mixtures.