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- ItemFlow Cytometric Assessment of the Morphological and Physiological Changes of Listeria monocytogenes and Escherichia coli in Response to Natural Antimicrobial Exposure(Lausanne : Frontiers Media, 2018-11-14) Braschi, Giacomo; Patrignani, Francesca; Siroli, Lorenzo; Lanciotti, Rosalba; Schlueter, Oliver; Froehling, AntjeEssential 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. Copyright © 2018 Braschi, Patrignani, Siroli, Lanciotti, Schlueter and Froehling.
- ItemOptimizing the application of plasma functionalised water (PFW) for microbial safety in fresh-cut endive processing(New York, NY [u.a.] : Elsevier Science, 2021) Schnabel, Uta; Balazinski, Martina; Wagner, Robert; Stachowiak, Jörg; Boehm, Daniela; Andrasch, Mathias; Bourke, Paula; Ehlbeck, JörgThe microbiological profiles and responses of native microflora of endive were investigated using a model process line, to establish where a defined PFW should be optimally applied to retain or improve produce microbiological quality. The PFW processes were compared with tap water and ClO2. The antimicrobial efficacy of PFW was quantified by determining the reduction in microbial load, the microbial viability and vitality. Depending on the stage of application of PFW, up to 5 log10-cycles reduction was achieved, accompanied by a reduction of metabolic activity, but not necessarily with a decrease in metabolic vitality. Multiple application (3-step-PFW-application) was more effective than single application (1-step-PFW-application) and PFW showed stronger antimicrobial effect in pre-cleaned endive. High concentrations of nitrite (315 mg l−1) and nitrate (472 mg l−1) in PFW were the main factors for the antimicrobial efficacy of PFW against bacteria. Furthermore, H2O2 and an acidic pH supported the mechanism of action against the endive microflora. These results identify the pathway to scale up successful industrial application of PFW targeting microbiological quality and safety of fresh leafy products.Industrial relevance The safety, quality and shelf life of freshly cut vegetables, e.g. lettuce, are strongly influenced by the microbial load. In addition, the hygienic design of production line, and a good handling/ production practice are indispensable. This study shows that the application of PFW, as a promising non-thermal sanitation technology, enables the inactivation of native microbial contamination on fresh-cut endive depending on the process stage of application. It further describes the impact of PFW on the metabolic activity and metabolic vitality of the lettuce-associated microflora. For higher acceptance, the mechanism of action of PFW was assumed based on previous chemical analyses and compared to the industrial standard of ClO2. The results contribute to the understanding and product-specificity of PFW-induced effects on safety, quality and shelf life of fresh cut lettuce and could be a basis for a possible industrial implementation and complement of common technologies.
- ItemScreening of microbial communities associated with endive lettuce during postharvest processing on industrial scale(Amsterdam : Elsevier B.V., 2018) Fröhling, A.; Rademacher, A.; Rumpold, B.; Klocke, M.; Schlüter, O.In this study, the composition of the microbial community on endive lettuce (Cichorium endivia) was evaluated during different postharvest processing steps. Microbial community structure was characterized by culture-dependent and culture-independent methods. Endive lettuce was sampled exemplarily at four different stages of processing (raw material, cut endive lettuce, washed endive lettuce, and spin-dried (ready to pack) endive lettuce) and analysed by plate count analysis using non-selective and selective agar plates with subsequent identification of bacteria colonies by matrix-assisted laser desorption/ionization time-of light mass spectrometry (MALDI-TOF MS). Additionally, terminal-restriction fragment length polymorphism (TRFLP) analysis and 16S rRNA gene nucleotide sequence analysis were conducted. The results revealed structural differences in the lettuce microbiomes during the different processing steps. The most predominant bacteria on endive lettuce were detected by almost all methods. Bacterial species belonging to the families Pseudomonadaceae, Enterobacteriaceae, Xanthomonadaceae, and Moraxellaceae were detected in most of the examined samples including some unexpected potentially human pathogenic bacteria, especially those with the potential to build resistance to antibiotics (e.g., Stenotrophomonas maltophilia (0.9 % in cut sample, 0.4 % in spin-dried sample), Acinetobacter sp. (0.6 % in raw material, 0.9 % in cut sample, 0.9 % in washed sample, 0.4 % in spin-dried sample), Morganella morganii (0.2 % in cut sample, 3 % in washed sample)) revealing the potential health risk for consumers. However, more seldom occurring bacterial species were detected in varying range by the different methods. In conclusion, the applied methods allow the determination of the microbiome's structure and its dynamic changes during postharvest processing in detail. Such a combined approach enables the implementation of tailored control strategies including hygienic design, innovative decontamination techniques, and appropriate storage conditions for improved product safety.