The Role of Flagella and Bacterial Motility in Virulence of Salmonella

Flagella are sophisticated nanomachines and important virulence factors of many pathogenic bacteria. Besides the ability to swim through liquid environments, the flagellum also contributes to successful infection of the host cell by enhancing cell adhesion and invasion. The flagellum consists out of three main parts: (i) the basal body, (ii) a flexible hook, and (iii) a long helical filament. Due to its purposes in distinct infection phases, flagellar gene expression, assembly, and functionality has to be regulated tightly in response to environmental cues. In this thesis, new motility regulators have been identified and mechanisms of flagella modification have been characterised according to their contribution to motility and pathogenesis of Salmonella. In the first chapter, five genes (rflP, yjcC, STM1267, STM3363, and rfaG) have been shown to influence swimming and/or swarming motility via alterations of flagellar gene expression or assembly. In the second chapter, a recently described transcriptional activator of flagellar gene expression, HilD, was characterised regarding its contribution to bacterial motility. HilD overexpression resulted in non-motile bacteria independently of flagellar assembly. It is likely that metabolic changes through collapse of the proton-motive force led to the described motility defect. Further, HilD was shown to activate expression of numerous fimbrial structures, such as Pef, Saf, or curli fimbriae. In the third chapter, alternate expression of the two antigenically distinct flagellins, FliC and FljB, was investigated regarding its importance for Salmonella physiology. FliC flagellin variants facilitated target-site selection and Spi-1 injectisome-dependent invasion during swimming on epithelial host cell surfaces. FljB-expressing bacteria were outcompeted by FliC-expressing bacteria during eukaryotic cell invasion and colonisation in the gastroenteritis mouse model. The fourth chapter focused on the posttranslational modification of the filament by the methylase FliB. FliB methylated lysine residues of the surface-exposed D2 and D3 domains of both flagellins, FliC and FljB. Moreover, flagellin methylation affected epithelial cell adhesion and invasion in a mannose-dependent manner. Consequently, strains deficient in flagellin methylation were outcompeted during dissemination in the gastroenteritis mouse model.