The genus Legionella includes bacteria that can cause a severe human pneumonia, called legionellosis, that is lethal in 10% of the cases. They are able to colonize all water systems. Their genomes reveal a high plasticity, at both functional and structural levels. The first part of this thesis analyses the impact of histone-like proteins on the regulation of virulence in L. pneumophila. These proteins are involved both in the structuration of the bacterial chromosome and in global regulation of gene expression. Mutants for the genes encoding the Dps and IHF proteins were constructed in L. pneumophila and analyzed for both their sensitivity to different stress conditions and virulence properties. Both IHF and Dps are involved in the regulation of virulence in Legionella. Moreover, Dps reduces sensitivity to oxidative stress and IHF may regulate the entry into the VBNC state (viable non-culturable), a particular physiological state in which bacterial cells are viable but can not grow anymore. The second part aims at using the genomic structural plasticity, especially the one induced by IS mobile genetic elements, as epidemiological tool. Currently, identification methods are unable to discriminate isolates from the same species. To prevent new infections, it is however crucial to quickly and accurately identify the contaminated settings, using clinical samples from patients as comparative elements. The development of an RFLP-IS method revealed an IS element, ISLpn11, as a highly resolutive tool with a discrimination rate of 80% in the strain L. pneumophila Paris, which is responsible for >10 % of the European outbreaks.
