Type IV pili in Francisella tularensis: roles of pilF and pilT in fiber assembly, host cell adherence and virulence

Center for Infectious Diseases, Department of Molecular Genetics and Microbiology, Stony Brook University, Stony Brook, NY 11794-5120; Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, Milwaukee, WI 53226

^* To whom correspondence should be addressed. Email: David.Thanassi{at}stonybrook.edu.

	Abstract

Francisella tularensis, a highly virulent facultative intracellular bacterium, is the causative agent of tularemia. Genome sequencing of all F. tularensis subspecies revealed the presence of genes that could encode for 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 the pil genes. Here, we show that deletion of the pilF putative Tfp assembly ATPase in the LVS resulted in a complete loss of surface fibers. Disruption of the pilT putative disassembly ATPase also caused a complete loss of pili, indicating that pilT functions differently in F. tularensis compared to model Tfp systems such as found in Pseudomonas aeruginosa and Neisseria spp. The LVS pilF and pilT mutants 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 pilF and pilT mutants were defective for adherence to macrophages, pneumocytes and hepatocytes. This work confirms that the surface fibers expressed by the LVS are encoded by the pil genes and provides evidence that the Francisella pili contribute to host cell adhesion and virulence.