Abstract
Essential oils (EOs) or their components represent one of the most promising natural, safe, and feasible alternatives to prevent the growth of food-borne pathogens like Listeria monocytogenes and Escherichia coli in food matrices. Although antimicrobial properties of EOs and their components are well-documented, limited and fragmented information is available on the changes induced by these compounds, even at sub-lethal concentrations, in the physiological properties of microbial cells. The aim of this study was to explore the morpho-physiological changes of L. monocytogenes Scott A and E. coli MG 1655 induced after 1 h exposure to different sub-lethal and lethal concentrations of citral, carvacrol, (E)-2-hexenal, and thyme EO. For this purpose, different cell viability parameters such as membrane integrity, esterase activity, and cytoplasmic cell membrane potential were measured by flow cytometry. Flow cytometric data revealed specific response patterns in relation to the strain, the natural antimicrobial and its concentrations. Both the target microbial strains showed an increased cell membrane permeabilization without a loss of esterase activity and cell membrane potential with increasing citral, carvacrol and thyme EO concentrations. By contrast, (E)-2-hexenal did not significantly affect the measured physiological properties of L. monocytogenes Scott A and E. coli MG 1655. The used approach allowed identifying the most effective natural antimicrobials in relation to the microbial target.
Highlights
Consumer’s demand for minimally processed and ready-to-eat foods with a reduced content of synthetic preservatives has stimulated the research of alternative preservation strategies
The exposure of L. monocytogenes cells to the natural antimicrobials at different concentrations was performed in the middle of the exponential growth phase (OD = 0.4; λ = 600 nm)
The flow cytometry approach allowed to understand the targets of sub-lethal concentrations of citral, carvacrol, (E)-2hexenal and thyme Essential oils (EOs) to L. monocytogenes and E. coli cells
Summary
Consumer’s demand for minimally processed and ready-to-eat foods with a reduced content of synthetic preservatives has stimulated the research of alternative preservation strategies. Essential oils (EOs) or their components represent one of the most promising natural feasible alternatives to improve food safety, shelf-life and quality. Thyme EO, and some components of citrus and officinal EOs, such as citral, carvacrol, and (E)-2-hexenal, are very promising alternatives to traditional preservatives (Ivanovic et al, 2012). They are widely reported to be able to improve safety and shelf-life of several foods even when used at concentrations lower than their minimal bactericidal concentrations. As described by Burt (2004) and Hyldgaard et al (2012), the primary site of toxic action of terpenes like citral and carvacrol is represented by the cell membrane due to their hydrophobic properties
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