Le flagelle de Brucella melitensis : caractérisation de la structure et de la régulation flagellaire

The bacterial flagellum is a rotative filamentous appendage which is anchored in the bacterial membranes and allows the bacterium to move. Besides its main locomotive function, the bacterial flagellum fulfills other functions such as adhesion and secretion. The filament, which comprises most of the flagellum structure, is formed by an assembly of proteins called flagellins. It is estimated that nearly 80% of the bacteria produce a flagellum. Brucella is a mammal pathogens bacterium that had long been designated as non-flagellated. However, recent discoveries have shown that all flagellar genes were present in its genome and were necessary to Brucella’s pathogenic ability. Several regulators that control the production of flagellar proteins have also been uncovered. However, no flagellum was viewed on Brucella until today. In this work, we showed for the first time through the optical and electron microscopy that B. melitensis produces transitionally a polar sheathed flagellum. On the other hand, we also investigated the regulation of flagellar system. Firstly, we studied the role of sigma factor RpoE1 on flagellar genes. The rpoE1 mutant overexpresses flagellar genes and produces a longer flagellum. Secondly, we showed that the production of flagellin was controlled by specific regulators named FlbT and FlaF. FlbT is a flagellin activator while FlaF is a flagellin inhibitor. In addition, we showed that the function of FlbT appears to be conserved among the Rizhobiales. We also showed that B. melitensis is more virulent in the absence of flagellin, which indicates a specific role of flagellin. The function of the flagellin of Brucella remains to be investigated. Finally, a preliminary in silico analysis of the genome of Brucella has enabled us to target other potential regulators of the flagellar system. The data gathered during this work have allowed us to draw a hierarchical regulatory cascade of flagellar genes of Brucella in 3 classes.