Superior antibacterial activity of nanoemulsion of Thymus daenensis essential oil against E. coli

Natural preservatives are being extensively investigated for their potential industrial applications in foods and other products. In this work, an essential oil (Thymus daenensis) was formulated as a water-dispersible nanoemulsion (diameter=143nm) using high-intensity ultrasound. The antibacterial activity of the essential oil in both pure and nanoemulsion forms was measured against an important food-borne pathogen bacterium, Escherichia coli. Antibacterial activity was determined by measuring the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The antibacterial activity of the essential oil against E. coli was enhanced considerably when it was converted into a nanoemulsion, which was attributed to easier access of the essential oils to the bacterial cells. The mechanism of antibacterial activity was investigated by measuring potassium, protein, and nucleic acid leakage from the cells, and electron microscopy. Evaluation of the kinetics of microbial deactivation showed that the nanoemulsion killed all the bacteria in about 5min, whereas only a 1-log reduction was observed for pure essential oil. The nanoemulsion appeared to amplify the antibacterial activity of essential oils against E. coli by increasing their ability to disrupt cell membrane integrity.