Francisella Tularensis Spp.

''Francisella'' is a genus of Gram-negative bacteria. They are small coccobacillary or rod-shaped, nonmotile organisms, which are also facultative intracellular parasites of macrophages. Strict aerobes, ''Francisella'' colonies bear a morphological resemblance to those of the genus ''Brucella''. Some ''Francisella'' species are pathogenic bacteria but some others are endosymbionts of ticks. Ticks do not use any other food source than vertebrate blood and therefore ingest high levels of protein, iron and salt, but few vitamins. To overcome these nutritional deficiencies, ticks have evolved obligate interactions with nutritional endosymbionts, including ''Francisella'' endosymbionts. Their experimental elimination typically results in decreased tick survival, molting, fecundity and egg viability, as well as in physical abnormalities, which all are fully restored with an oral supplement of B vitamins. The genome sequencing of ''Francisella'' endosymbionts confirmed that they consistently produce three B vitamin types, biotin (vitamin B₇), riboflavin (B₂) and folate (B₉). ''Francisella'' endosymbionts are often misidentified as ''Francisella tularensis''; however, ''Francisella'' endosymbionts lack virulence genes and cannot infect humans.

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.

Pathogenesis

The type species, '' F. tularensis'', causes the disease tularemia or rabbit fever. '' F. novicida'' and '' F. philomiragia'' (previously ''Yersinia philomiragia'') are associated with sepsis and invasive systemic infections.

Francisella has been detected in ticks Vilcins I, Fournier P, Old JM, Deane EM (2009). Evidence for the presence of Francisella and spotted fever group Rickettsia DNA in the Tick ''Amblyomma fimbriatum'' (Acari: Ixodidae), Northern Territory, Australia. Journal of Medical Entomology. 46(4), 926-933. doi: 10.1186/s13071-015-0719-3

Taxonomy

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. An updated phylogeny based on whole genome sequencing has recently been published showing the genus ''Francisella'' could be divided into two main genetic clades: one including ''F. tularensis'', ''F. novicida'' and '' F. hispaniensis'', and another including '' F. philomiragia'' and '' F. noatunensis''.

Laboratory characteristics

''Francisella'' species can survive for several weeks in the environment; paradoxically, they can be difficult to culture and maintain in the laboratory. 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 medium with appropriate additives as noted above. Attempted isolation on MacConkey agar is not reliable or generally successful.

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.

References

External links

Francisella
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Thiotrichales
Bacteria genera