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Sujet de la thèse :

Biofilm, virulence and control strategies of Listeria monocytogenes
Growth of the foodborne pathogen, Listeria monocytogenes, to high numbers in ready-to-eat food products importantly increases the infection risk of immunocompromised consumers. As it is very difficult to avoid cross-contamintation during processing and formation of bacterial biofilms, this study focused on the initial steps of biofilm formation and biofilm stucture, the relationship between virulence genes involved in the adhesion to inert surfaces and virulence and impact of Carnobacterium strains on Listeria monocytogenes infection using in vitro cell culture method.
In the first part we observed an initial 2-D biofilm structure including cell cluster formation with attached cells side-by-side, branch elongation by cell division, open geometric forms connected through branches and honeycomb-like structure which was reached with three out of four tested strains. Only EGD-e was not able to enter to the second step impairing the formation of an honeycomb-like structure and mature biofilm. A temperature of 20°C over the whole study was used to mimic the environment in food processing plants. Under pH 5, the impairment of the initial biofilm structure for all strains was observed which could constitute a way of biofilm development prevention.
In the second part, we screened a collection of Listeria monocytogenes seafood isolates for their adhesion abilities and in vivo and in vitro virulence level. Focusing on two low-virulent strains (EU 2153 and EU 2154) we showed that they have the highest adhesion rate among strains. We succesfully amplified and sequenced prfA and inlA for these two strains and found one nonsense mutation of inlA in EU 2153 and one point mutation in EU 2154 explaining their low virulence and invasion rate.
The third part investigated an effect of bacteriocin-producing and nonbacteriocinogenic Carnobacterium strains against Listeria monocytogenes infection using in vitro HT29 and HT29-MTX models. All strains tested were able to reduce the epithelial cells infection caused by L. monocytogenes Scott A. Depth analysis with various pretreatments of Carnobacteria strains indicated the influence of bacteriocins as well as the nonactive mechanism of cell monolayers protection suggesting the possible use of protective bacteria to reduce L. monocytogenes infection.