Francisella

Francisella is a genus of Gram-negative bacteria. The most well-known species, Francisella tularensis, is the causative agent of tularemia, also known as rabbit fever or deer fly fever. This bacterium is a facultative intracellular pathogen, meaning it can survive and replicate both inside and outside of host cells. Francisella tularensis is highly infectious and can infect a wide range of mammals, particularly rodents and lagomorphs, but it can also infect birds, fish, and even invertebrates. Humans can contract tularemia through various routes, including tick and deer fly bites, handling infected animals, inhaling contaminated dust or aerosols, or ingesting contaminated food or water.

The genus Francisella was first described in 1941. Other species within the Francisella genus, such as Francisella novicida and Francisella philomiragia, are also pathogenic but are generally considered to be less virulent or infect a more restricted range of hosts than F. tularensis. Francisella bacteria are small, pleomorphic coccobacilli, typically measuring 0.2 to 0.7 micrometers in width and 0.8 to 2.0 micrometers in length. They are non-motile and do not form spores. Their growth is slow, requiring enriched media and incubation at 35-37 degrees Celsius. The characteristic symptoms of tularemia in humans can include fever, chills, headache, muscle pain, and a skin ulcer at the site of infection, along with swollen lymph nodes. Different subspecies and strains of F. tularensis exhibit varying degrees of virulence and geographical distribution.