''Francisella'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. They are small coccobacillary or

rod-shaped Bacterial cellular morphologies are the shapes that are characteristic of various types of bacteria and often key to their identification. Their direct examination under a light microscope enables the classification of these bacteria (and archae ...

, non

motile Motility is the ability of an organism to move independently using metabolic energy. This biological concept encompasses movement at various levels, from whole organisms to cells and subcellular components. Motility is observed in animals, mi ...

organisms, which are also facultative

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

parasites of

macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

s. Strict

aerobes An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. The ability to exhibit aerobic respiration may yield benefits to the aerobic organism, as aerobic respiration yields more energy than anaerobic ...

, ''Francisella'' colonies bear a morphological resemblance to those of the genus ''

Brucella ''Brucella'' is a genus of Gram-negative bacterium, bacteria, named after David Bruce (microbiologist), David Bruce (1855–1931). They are small (0.5 to 0.7 by 0.6 to 1.5 μm), non-Bacterial capsule, encapsulated, non-motile, facultatively ...

''. Some ''Francisella'' species are pathogenic bacteria but some others are

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s 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 vitamin B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells. They are a chemically diverse class of compounds. Dietary supplements containing all eight are referred to as a vit ...

s. The genome sequencing of ''Francisella'' endosymbionts confirmed that they consistently produce three B vitamin types,

biotin Biotin (also known as vitamin B7 or vitamin H) is one of the B vitamins. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. ...

(vitamin B₇),

riboflavin Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...

(B₂) and

folate Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...

(B₉). ''Francisella'' endosymbionts are often misidentified as ''

Francisella tularensis ''Francisella tularensis'' is a pathogenic species of Gram-negative coccobacillus, an aerobic bacterium. It is nonspore-forming, nonmotile, and the causative agent of tularemia, the pneumonic form of which is often lethal without treatment. It i ...

''; however, ''Francisella'' endosymbionts lack virulence genes and cannot infect humans. The genus was named in honor of American

bacteriologist A bacteriologist is a microbiologist, or similarly trained professional, in bacteriology— a subdivision of microbiology that studies bacteria, typically Pathogenic bacteria, pathogenic ones. Bacteriologists are interested in studying and learnin ...

Edward Francis, who, in 1922, first recognized ''F. tularensis'' (then named ''Bacterium tularensis'') as the causative agent of

tularemia Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium '' Francisella tularensis''. Symptoms may include fever, skin ulcers, and enlarged lymph nodes. Occasionally, a form that results in pneumonia or a throat ...

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 Sepsis is a potentially life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage of sepsis is followed by suppression of the immune system. Common signs and s ...

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 Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

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 Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mec ...

and

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

media supplementation for growth. Growth has been successful on several media types, including

chocolate agar Chocolate agar (CHOC) or chocolate blood agar (CBA) is a nonselective, Growth medium#Enriched media, enriched growth medium used for isolation of pathogenic bacteria. It is a variant of the blood agar plate, containing red blood cells that have ...

and Thayer–Martin medium with appropriate additives as noted above. Attempted isolation on

MacConkey agar MacConkey agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentatio ...

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 Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...

, 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
genomes and related information a
PATRIC
a Bioinformatics Resource Center funded b
NIAID

{{Authority control Thiotrichales Bacteria genera