Combination effects of ultrasound and citral nanoemulsion against Shigella flexneri and the preservation effect on fresh-cut carrots

Recent studies indicated the role of ROS toward antibacterial activity. In our study we report ROS mediated membrane lipid oxidation of Escherichia coli treated with ZnO nanoparticles (NPs) as supported by detection and spectrophotometric measurement of malondialdehyde (MDA) by TBARS (thiobarbituric acid-reactive species) assay. The antibacterial effects of ZnO NPs were studied by measuring the growth curve of E. coli, which showed concentration dependent bacteriostatic and bacteriocidal effects of ZnO NPs. The antibacterial effects were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Further, antibacterial effect of ZnO NPs was found to decrease by introducing histidine to the culture medium treated with ZnO NPs. The ROS scavenging action of histidine was confirmed by treating histidine to the batch of Escherichia coli+ZnO NPs at the end of the lag phase of the growth curve (Set-I) and during inoculation (Set-II). A moderate bacteriostatic effect (lag in the E. coli growth) was observed in Set-II batch while Set-I showed no bacteriostatic effect. From these evidences we confirmed that the antibacterial effect of bare as well as TG capped ZnO NPs were due to membrane lipid peroxidation caused by the ROS generated during ZnO NPs interaction in culture medium. Copyright © 2012 Elsevier B.V. All rights reserved.

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.