Francisella tularensis group Database

Francisella tularensis subsp. holarctica 257


 

Francisella tularensis is a small Gram-negative intracellular bacterium. It is the causative agent of tularemia, and is considered a potential bioterrorism agent. Tularemia has been known to occur almost exclusively in the Northern Hemisphere, and the severity of the disease depends both on the route of infection and the virulence of the organism causing the infection. In spite of some recent progresses, many aspects of the Francisella biology, evolution and virulence mechanisms remain poorly understood.

In this project, we are sequencing multiple Francisella strains and performing comparative genomics analysis to understand the evolution, global distribution and virulence mechanisms of Francisella species.

Project Information

The Francisella tularensis sequencing project is part of the NIAID funded Microbial Sequencing Center at the Broad Institute. Its goal is to release annotated genome assemblies from several Francisella tularensis isolates.

What is Francisella tularensis?

Francisella tularensis is the causative agent of tularemia, a zoonotic disease that has gained special interest of late due in part to its very high infectivity that enables airborne transmission and a very low infection dose. As few as ten bacteria by the respiratory route can cause disease in humans (1). For this reason, F. tularensis is considered a potential bioterrorism agent and it is one of only three bacterial pathogens included in the NIAID category A list of select agents. Although the more common form of the disease (ulceroglandular tularemia) is contracted from the bite of ticks or other arthopods that have fed on infected animals (rabbits, ground squirrels, and other rodents), the disease can also be transmitted by airborne route. Pneumonic tularemia is contracted through aerosol generated by farming activities in areas populated by infected animals. The severity of the disease is variable and depends both on the route of infection and the virulence of the organism. Interestingly, tularemia has been known to occur only in the Northern Hemisphere, although recently, a case has been reported in Australia and a Francisella novicida-like strain has been implicated (2).

F. tularensis is a very small Gram-negative intracellular gamma-proteobacterium that is widely dispersed. A substantial effort of the scientific research studies on Francisella has focused on classification and typing. The genus Francisella has only been commonly accepted since the mid-to-late 1960s. Although the classification of this group of organisms is still in flux, it is generally accepted that the genus Francisella encompasses two species, F. tularensis and F. philomiragia. While the F. tularensis group has been generally divided into two main biotypes (type A and B), four subspecies (tularensis - type A, holarctica - type B, mediaasiatica, and novocida) have recently been recognized. Recent microarray work performed by Broekhuijsen et al (2003) have extended our knowledge of the genetic conservation existent among the F. tularensis strains while uncovering regions unique to the highly virulent strain. In spite of this work there remains a general lack of full understanding regarding the exact mechanism(s) of virulence employed by F. tularensis during the infection of its mammalian hosts (3).

In the this sequencing project, we targeted strains that will reveal F. tularensis variation and diversity (useful for genotyping, phylogenetics and bioforensics) as well as those that will shed light on the issues of virulence and the pathogenic processes of this important group of pathogens.

Project Collaborators

References

  1. McCrumb, F.R. (1961) Aerosol infection of man with Pasteurella tularensis. Bacteriol. Rev. 25, 262-267.
  2. M. J. Whipp, Jennifer M. Davis, Gary Lum, Jim de Boer, Yan Zhou, Scott W. Bearden, Jeannine M. Petersen, May C. Chu, and Geoff Hogg (2003). Characterization of a novicida-like subspecies of Francisella tularensis isolated in Australia. J. Med. Microbiol., 52:839-842.
  3. Broekhuijsen, et al. 2003. Genome-Wide DNA Microarray Analysis of Francisella tularensis Strains Demonstrates Extensive Genetic Conservation within the Species but Identifies Regions That are Unique to the Highly Virulent F. tularensis subsp. tularensis. J. Clinical Microbiol. 41:2924-2931