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RESEARCH PRODUCT

Impact of relative air humidity fluctuations on the survival of Listeria monocytogenes : application to improve hygiene in food processing environment

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Environmental hydration fluctuations influence microorganism viability and activity. The air relative humidity (RH) is a potentially effective parameter to control the development and the persistence of pathogenic microorganisms. However, efficiency of this parameter remains not well characterized compared to others environmental factors such as pH or temperature. Drying is the final step of cleaning and disinfection processes used in the food industry. However, if this process is empirically used, it could also be used to improve the decontamination of surfaces in premises and reduce the persistence of microorganisms. Despite cleaning and disinfection, Listeria monocytogenes is commonly found in the food industry and persists during long periods. Thus, the main objective of this thesis is to evaluate and understand the impact of hydric fluctuations on L. monocytogenes survival to optimize its destruction on a surface.First, thirty L. monocytogenes strains, isolated from different food processing environment and belonging to different serotypes and levels of virulence, were exposed to hyperosmotic stress and drying process. Differences in resistances among the strains were analyzed and four strains with different resistance to hydric stress were selected to continue this study.To define the most lethal hydric fluctuation conditions for L. monocytogenes strains, various parameters, such as the dehydration kinetic, relative humidity level, the rehydration kinetic, the application of successive dehydration and rehydration cycles and drying medium, have been studied.Finally, cellular mechanisms inducing the cell death during hydric fluctuations were investigated by Fourier transform infrared spectroscopy, atomic force microscopy, and flow cytometry and anaerobic drying. These different methods revealed that mechanical and structural stresses are primarily responsible for the death of L. monocytogenes.This work demonstrates that the control of RH level and variation is an effective means for the destruction of L. monocytogenes and offers real perspectives to improve hygiene in food production premises.