Chemical composition and antibacterial activity of essential oils and major fractions of four Achillea species and their nanoemulsions against foodborne bacteria

•Essential oils of four Achillea sp. were isolated and analyzed by GC and GC/MS.•The oils showed antimicrobial activity against five species of foodborne bacteria.•The oils of Achillea biebersteinii and Achillea fragrantissima were the most effective.•Escherichia coli and Staphylococcus aureus were the most susceptible to the oils and their fractions.•Nanoemulsions of the oils showed increased antibacterial activity with lower MICs.

Essential oils (EOs) of Achillea biebersteinii, Achillea fragrantissima, Achillea santolina and Achillea millefolium were obtained by hydrodistillation and analyzed using Gas Chromatography (GC) and GC/Mass Spectrometry (MS). Nanoemulsions from EOs and major fractions were prepared using the High Pressure Homogenization technique (HPH). EOs and their nanoemulsions were tested for their antibacterial activity against two Gram-positive foodborne bacteria (Staphylococcus aureus and Listeria monocytogenes) and three Gram-negative species (Escherichia coli, Pseudomonas aeruginosa and Salmonella enteritidis). The Gram-positive bacteria were more susceptible than the Gram-negative ones, where P. aeruginosa was the most resistant. A plant oil was also more active than its major components and the oils of A. biebersteinii and A. fragrantissima were the most active regardless of the microorganism tested (diameter of inhibition zones ranged between 6.0 and 21.5 mm). The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) ranged between 60.0 and 480.0 μg/ml. When tested as nanoemulsions, activity of EOs and fractions was increased dramatically (diameter of inhibition zones reached 34.5 mm and (MIC) and (MBC) reached 15.0 μg/ml with A. biebersteinii nanoemulsions against S. aureus). The study recommends the use of the test plant oils as antimicrobial biorationals, especially at their nanoscale after the required toxicological assessments.

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