Assembly and dynamic regulation of the tip filament of the <i>Bordetella</i> type III secretion system injectisome

<i>Bordetella bronchiseptica</i> and <i>Bordetella pertussis</i> are two closely related respiratory pathogens that employ their T3SS injectisome to deliver the BteA effector into host cells. In this study, we visualized the needle tip filament of their T3SS injectisome, a structure formed by the Bsp22 protein. We demonstrate that during <i>Bordetella</i> cultivation in Stainer-Scholte medium, Bsp22 filaments are abundant and can dynamically extend up to several micrometers in length through the incorporation of new subunits at their distal ends. In contrast, these filaments become shorter and/or less abundant during infection of host cells. This reduction correlates with decreased <i>bsp22</i> mRNA expression and lower Bsp22 protein levels, while the levels of <i>bscD</i> mRNA, which encodes the inner membrane ring protein of the injectisome, remain stable. These results highlight the adaptability of the <i>Bordetella</i> T3SS injectisome and show how its tip filament structure changes in response to different environments.