Coxiella burnetii

Coxiella burnetii
A dry fracture of a Vero cell exposing the contents of a vacuole where Coxiella burnetii is growing
Scientific classification
Domain:	Bacteria
Phylum:	Pseudomonadota
Class:	Gammaproteobacteria
Order:	Legionellales
Family:	Coxiellaceae
Genus:	Coxiella
Species:	C. burnetii
Binomial name
Coxiella burnetii (Derrick 1939) Philip 1948

Coxiella burnetii is an

The RML team proposed the name Rickettsia diaporica, derived from the Greek word for having the ability to pass through filter pores, to avoid naming it after either Cox or Davis if indeed Noguchi's description had priority. Around the same time, Derrick proposed the name Rickettsia burnetii, in recognition of Burnet's contribution in identifying the organism as a Rickettsia. As it became clear that the species differed significantly from other Rickettsia, it was first elevated to a subgenus named after Cox, Coxiella, and then in 1948 to its own genus of that name, proposed by Cornelius B. Philip, another RML researcher.^[4] Research in the 1960s–1970s by French Canadian-American microbiologist and virologist Paul Fiset was instrumental in the development of the first successful Q fever vaccine.^[5]

Coxiella was difficult to study because it could not be reproduced outside a host. However, in 2009, scientists reported a technique allowing the bacteria to grow in an axenic culture and suggested the technique may be useful for study of other pathogens.^[6]

Pathogenesis

Of the many C. burnetii strains, two of the most studied are the Nine Mile phase I and Priscilla phase I strain. In recent years, more strains have been studied. Nonetheless, it has been demonstrated that the Nine Mile strain is one of the most virulent strains of C. burnetti with as few as four organisms needed to cause infection. This is particularly relevant as murine rodents are poorly susceptible to C. burnetii, necessitating a higher dose and a more virulent dose to inoculate murine rodents for disease study. ^[7]

The ID₅₀ (the dose needed to infect 50% of experimental subjects) is one via inhalation; i.e., inhalation of one organism will yield disease in 50% of the population. This is an extremely low infectious dose (only 1-10 organisms required), making C. burnetii one of the most infectious known organisms.^[8][9] Disease occurs in two stages: an acute stage that presents with headaches, chills, and respiratory symptoms, and an insidious chronic stage.

C. burnettii infections begins within the

Following infection, C. burnetii has a biphasic developmental cycle, which consists of small cell variant (SCV) and large cell variant (LCV) morphological forms, which are both infectious. As the SCV is metabolically repressed and resistant to many environmental stressors, it is likely the form that initiates natural infections. Having entered a host cell, C. burnetii SCVs transit through the phagolysosomal maturation pathway. In the first six hours post-infection, endosomes, autophagosomes, and lysosomes containing acid phosphatase fuse with the nascent phagosome to form early PV, which fosters the transition from SCV to LCV. Resultantly, C. burnetii is metabolically activated and produces the T4SS to translocate effector proteins into the host cytoplasm. After 6 days, C. burnetii transitions back to SCV.^[7][11]

While most infections clear up spontaneously, treatment with tetracycline or doxycycline appears to reduce the symptomatic duration and reduce the likelihood of chronic infection. A combination of erythromycin and rifampin is highly effective in curing the disease, and vaccination with Q-VAX vaccine (CSL) is effective for prevention of it.^{[citation needed]}

The bacteria use a type IVB secretion system known as Icm/Dot (intracellular multiplication / defect in organelle trafficking genes) to inject over 100 effector proteins into the host. These effectors increase the bacteria's ability to survive and grow inside the host cell by modulating many host cell pathways, including blocking cell death, inhibiting immune reactions, and altering vesicle trafficking.^[12][13][14] In Legionella pneumophila, which uses the same secretion system and also injects effectors, survival is enhanced because these proteins interfere with fusion of the bacteria-containing vacuole with the host's degradation endosomes.^[15]

Use as a biological weapon

The United States ended its biological warfare program in 1969. When it did, C. burnetii was one of seven agents it had standardized as biological weapons.^[16]

Genomics

At least 75^[17] completely sequenced genomes of Coxiella burnetii strains exist,^[18] which contain about 2.1 Mbp of DNA each and encode around 2,100 open reading frames; 746 (or about 35%) of these genes have no known function.

In bacteria small regulatory RNAs are activated during stress and virulence conditions. Coxiella burnetii small RNAs (CbSRs 1, 11, 12, and 14) are encoded within intergenic region (IGR). CbSRs 2, 3, 4 and 9 are located antisense to identified ORFs. The CbSRs are up-regulated during intracellular growth in host cells.^[19]

All C. burnetii isolates either carry one of four conserved independently-replicating large plasmids (QpH1, QpDG, QpRS, or QpDV) or a chromosomal element derived from QpRS. QpH1 carries virluence factors important for the bacterium's survival inside mouse macrophages^[20] and Vero cells; growth on axenic media is unaffected. QpH1 also contains a toxin-antitoxin system.^[21] Among all plasmids, 8 conserved genes code for proteins that are inserted into the host cell via the secretion system.^[21]

Additional images

• 
  C. burnetii, the causative agent of Q fever

References

External links

Wikimedia Commons has media related to Coxiella burnetii.

• Coxiella burnetii genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID