Francisella tularensis

Francisella tularensis is a pathogenic species of gram-negative bacteria and the causative agent of tularemia or rabbit fever.^[1] Due to its ease of spread by aerosol and its high virulence, F. tularensis is classified as a Class A agent by the U.S. government.^[2]

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Subspecies

Four subspecies (biovars) of F. tularensis have been classified. The biovar tularensis (or type A) is found predominantly in North America and is the most virulent of the four known subspecies and is associated with lethal pulmomary infections. Biovar palearctica (also known as biovar holarctica or type B) is found predominantly in Europe and Asia but rarely leads to fatal disease. An attenuated live vaccine strain of subspecies palearctica has been described, though it is not yet fully licensed by the FDA as a vaccine. Subspecies novicida was characterized as a relatively nonvirulent strain; only two tularemia cases in North America have been attributed to novicida and these were only in severely immunocompromised individuals. The fourth, biovar mediasiatica, is found primarily in central Asia; little is currently known about this subspecies or its ability to infect humans.

Pathogenesis

F. tularensis is capable of infecting a number of small mammals such as voles, rabbits, and muskrats, as well as humans. Despite this, no case of tularemia has been shown to be initiated by human-to-human transmission. Rather, tularemia is caused by contact with infected animals or vectors such as ticks, mosquitos, and deer flies.

Infection with F. tularensis can occur via several routes. The most common occurs via skin contact, yielding an ulceroglandular form of the disease. Inhalation of bacteria - particularly biovar tularensis, leads to the potentially lethal pneumonic tularemia. While the pulmonary and ulceroglandular forms of tularemia are more common, other routes of inoculation have been described and include oropharyngeal infection due to consumption of contaminated food and conjunctival infection due to inoculation at the eye.

F. tularensis is capable of surviving outside of a mammalian host for weeks at a time and has been found in water, grassland, and haystacks. Aerosols containing the bacteria may be generated by disturbing carcasses due to brushcutting or lawn mowing; as a result, tularemia has been referred to as lawnmower disease. Recent epidemiological studies have shown a positive correlation between occupations involving the above activities and infection with F. tularensis.

Life Cycle

F. tularensis is a facultative intracellular bacterium that is capable of infecting most cell types but primarily infects macrophages in the host organism. F. tularensis entry into the macrophage occurs via phagocytosis and the bacterium is sequestered from the interior of the infected cell by a phagosome. F. tularensis then breaks out of this phagosome into the cytosol and rapidly proliferates. Eventually the infected cell undergoes apoptosis, and the progeny bacteria are released to initiate new rounds of infection.

Virulence Factors

  The virulence mechanisms for F. tularensis have not been well characterized. Like other intracellular bacteria that break out of phagosomal compartments to replicate in the cytosol, F. tularensis strains produce different hemolytic agents, which facilitate degradation of the phagosome. Specifically, a hemolysin protein NlyA with similarity to Pseudomonas aeruginosa HlyA was characterized in biovar novicida whereas an acid phospholipase C AcpA has been found in other strains to act as a hemolysin.

While F. tularensis does not contain virulence secretion systems typical of some better-characterized pathogenic bacteria, it does contain a number of ATP binding cassette (ABC) proteins that may be linked to the secretion of virulence factors.^[3] In addition, F. tularensis uses type IV pili to bind to the exterior of a host cell and thus become phagocytosed. Mutant strains lacking pili show severely attenuated pathogenicity.

The expression of a 23-kD protein known as IglC is required for F. tularensis phagosomal breakout and intracellular replication; in its absence mutant F. tularensis die and are degraded by the macrophage. This protein is located in a putative pathogenicity island regulated by the transcription factor MglA.

F. tularensis, in vitro, downregulates the immune response of infected cells, a tactic used by a significant number of pathogenic organisms to ensure that replication is (albeit briefly) unhindered by the host immune system by blocking the warning signals from the infected cells. This downmodulation of the immune response requires the IglC protein, though again it is not clear what the contributions of IglC and other genes are.

Several other putative virulence genes exist but have yet to be characterized for function in F. tularensis pathogenicity.

Genetics

Like many other bacteria, F. tularensis undergoes asexual replication. Bacteria will divide into two daughter cells, each of which contains identical genetic information. Genetic variation may be introduced via mutation or horizontal gene transfer.

The genome of F. tularensis biovar tularensis strain SCHU4 has been sequenced.^[4] The studies resulting from the sequencing suggest that a number of gene coding regions in the F. tularensis genome are disrupted by mutations and thus create blocks in a number of metabolic and synthetic pathways that are required for survival. This indicates that F. tularensis has evolved to depend on the host organism for certain nutrients and other processes ordinarily taken care of by these disrupted genes.

The F. tularensis genome contains unusual transposon-like elements resembling counterparts that normally are found in eukaryotic organisms.

Genomics

• Francisella Genome Projects (from Genomes OnLine Database)
• Comparative Analysis of Francisella Genomes (at DOE's IMG system)

References