Phd in CELL AND DEVELOPMENTAL BIOLOGY

Thesis title: Widening the landscape of Shigella flexneri virulence factors: the key roles of YbjX, VirK and the AcrAB efflux pump

The leading cause of bacillary dysentery in developing countries is Shigella flexneri, an enteropathogen highly adapted to the human host. The infectious strategy is complex and relies on the regulated expression of virulence factors encoded by chromosomal genes and genes harboured by a conserved and large virulence plasmid (pINV). It involves intracellular life stages within macrophages and colonic epithelial cells; the result is the induction of a strong inflammatory response, the destruction of the epithelium and, hence, a diarrhoeal syndrome. Due to the serotype diversity, a vaccine is unavailable yet, and antibiotic-resistant strains are frequently emerging worldwide. Identifying factors that play a significant role in pathogenicity could provide novel targets for virulence-directed therapeutic alternatives to antibiotics. The cell envelope is a promising source of those targets, as it is a bacterial structure involved in interaction with the extracellular environment, resistance to antimicrobial compounds, evasion of the host immune system and many other biological processes. In the first chapter of this thesis, I report my work on the characterisation of the cytoplasmic proteins YbjX and VirK and their genetic loci. They have similar structures and were revealed to be relevant in maintaining the integrity of the bacterial envelope and modulating the host immune response. The second chapter focuses on the study of the role played by the multidrug-resistance efflux pump (MDR EPs) AcrAB-TolC in the invasion of epithelial cells. My work contributes to the increasing evidence that MDR EPs have pivotal roles in bacterial physiology and virulence, in addition to the already established involvement in the extrusion of antibiotics. Finally, two publications are presented regarding the widespread two-component signal transduction systems. More specifically, on the role TCSs have in the regulation of virulence in S. flexneri and the development of inhibitors which may prove to be valuable resources in the era of antibiotic resistance.