Francisella tularensis is a highly virulent intracellular human pathogen that is capable of rapid proliferation in the infected host. Mutants affected in intracellular survival and growth are highly attenuated which highlights the importance of the intracellular phase of the infection. Genomic analysis has revealed that Francisella encodes all genes required for expression of functional type IV pili (Tfp), and in this focused review we summarize recent findings regarding this system in the pathogenesis of tularemia. Tfp are dynamic adhesive structures that have been identified as major virulence determinants in several human pathogens, but it is not obvious what role these structures could have in an intracellular pathogen like Francisella. In the human pathogenic strains, genes required for secretion and assembly of Tfp and one pilin, PilA, have shown to be required for full virulence. Importantly, specific genetic differences have been identified between the different Francisella subspecies where in the most pathogenic type A variants all genes are intact while several Tfp genes are pseudogenes in the less pathogenic type B strains. This suggests that there has been a selection for expression of Tfp with different properties in the different subspecies. There is also a possibility that the genetic differences reflect adaptation to different environmental niches of the subspecies and plays a role in transmission of tularemia. This is also in line with recent findings where Tfp pilins are found to be glycosylated which could reflect a role for Tfp in the environment to promote survival and transmission. We are still far from understanding the role of Tfp in virulence and transmission of tularemia, but with the genomic information and genetic tools available we are in a good position to address these issues in the future.
Keywords: Francisella tularensis, type IV pili, virulence, type II secretion
Citation: Näslund Salomonsson E, Forslund A-L and Forsberg Å (2011) Type IV pili in Francisella – a virulence trait in an intracellular pathogen. Front. Microbio. 2:29. doi: 10.3389/fmicb.2011.00029
Received: 02 December 2010;
Accepted: 04 February 2011;
Published online: 15 February 2011.
Edited by:Anders Sjostedt, Umeå University, Sweden
Reviewed by:David Thanassi, Stony Brook University, USA
Copyright: © 2011 Näslund Salomonsson, Forslund and Forsberg. This is an open-access article subject to an exclusive license agreement between the authors and Frontiers Media SA, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.
*Correspondence: Åke Forsberg, Department of Molecular Biology, UmeÅ University, 901 87 UmeÅ, Sweden. e-mail: firstname.lastname@example.org