Francisella

Francisella is a genus of pathogenic, Gram-negative bacteria. They are small coccobacillary or rod-shaped, non-motile organisms, which are also facultative intracellular parasites of macrophages.^[1] Strict aerobes, Francisella colonies bear a morphological resemblance to those of the genus Brucella.^[2]

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The genus was named in honor of American bacteriologist Edward Francis, who, in 1922, first recognized F. tularensis (then named Bacterium tularensis) as the causative agent of tularemia.^[3]

Pathogenesis

The type species, F. tularensis, causes the disease tularemia or rabbit fever.^[4] F. novicida and F. philomiragia (previously Yersinia philomiragia) are associated with septicemia and invasive systemic infections. It should be noted that the taxonomy of the genus is somewhat uncertain, especially in the case of F. novicida (may be a subspecies of F. tularensis). In general, identification of species is accomplished by biochemical profiling or 16S rRNA sequencing.

Laboratory characteristics

Francisella can survive for several weeks in the environment; paradoxically, they can be difficult to culture and maintain in the lab.^[5] Growth is slow (though increased by CO₂ supplementation) and the organisms are fastidious, with most Francisella strains requiring cystine and cysteine media supplementation for growth. Growth has been successful on several media types, including chocolate agar and Thayer-Martin medias with appropriate additives as noted above. Attempted isolation on MacConkey agar is not reliable or generally successful.^[4]

After 24 hours of incubation on appropriate solid media, Francisella colonies are generally small (1 to 2 mm), opaque, and white-gray to bluish-gray in color. Colonies are smooth, with clean edges and, after a 48 hours of growth, tend to have a shiny surface.

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