Francisella tularensis

Francisella tularensis is a pathogenic species of gram-negative bacteria and the causative agent of tularemia or rabbit fever. It is a facultative intracellular bacterium. It is classified as a Class A agent by the U.S. government due to its ease of spread by aerosol and its high virulence.

In 1911 the species was found in ground squirrels in California. There are four subspecies that have been classified. Biovar tularensis is found mostly in North America. Biovar palearctica is found predominately in Europe and Asia but rarely leads to fatal disease. Novicida, a nonvirulent strain, has only been diagnosed twice in North America. Biovar mediasiatica is found in central Asia and little is known about it.

It is capable of infecting voles, rabbits, muskrats, and humans. No case has been shown to transfer from human-to-human transmission but is more commonly spread by ticks, mosquitoes, and deer flies.
Infection can occur via several routes, most commonly through skin contact leading to ulceroglandular form of the disease. Inhalation leads to potentially lethal pneumonic tularemia. While these forms of tularemia are common other routes include oropharyngeal infection due to consumption of contaminated food and conjunctival infection due to inoculation at the eye.

It is capable of surviving outside of a mammalian host for weeks at a time and is found in water, grass, and haystacks. Aerosols of the bacteria may be spread when carcasses are disturbed due to mowing or brush-cutting.

It can infect most cell types but infects primarily macrophages in the host organism. The infected cell undergoes apoptosis which leads to the progeny bacteria being released to initiate new rounds of infection.

F. tularensis doesn’t contain any virulence secretion systems but it does contain a number of ATP binding cassette proteins that may be linked to the secretion of virulence factors. Several putative virulence genes exist but have yet to be characterized for function in F. tularensis pathogenicity.

It undergoes asexual replication where it will divide into two daughter cells, which both contain identical genetic information. Mutation or horizontal gene transfer may cause genetic variation.