Type IV Pili in Francisella tularensis: Roles of pilFand pilTin Fiber Assembly, Host Cell Adherence, and Virulence

ABSTRACTFrancisella tularensis, a highly virulent facultative intracellular bacterium, is the causative agent of tularemia. Genome sequencing of all F. tularensissubspecies revealed the presence of genes that could encode type IV pili (Tfp). The live vaccine strain (LVS) expresses surface fibers resembling Tfp, but it was not established whether these fibers were indeed Tfp encoded by the pilgenes. We show here that deletion of the pilFputative Tfp assembly ATPase in the LVS resulted in a complete loss of surface fibers. Disruption of the pilTputative disassembly ATPase also caused a complete loss of pili, indicating that pilTfunctions differently in F. tularensisthan in model Tfp systems such as those found in Pseudomonas aeruginosaand Neisseriaspp. The LVS pilFand pilTmutants were attenuated for virulence in a mouse model of tularemia by the intradermal route. Furthermore, although absence of pili had no effect on the ability of the LVS to replicate intracellularly, the pilFand pilTmutants were defective for adherence to macrophages, pneumocytes, and hepatocytes. This work confirms that the surface fibers expressed by the LVS are encoded by the pilgenes and provides evidence that the Francisellapili contribute to host cell adhesion and virulence.