Species of bacterium
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Coxiella burnetii is an obligate intracellular bacterial pathogen, and is the causative agent of Q fever. The genus "Coxiella" is morphologically similar to "Rickettsia", but with a variety of genetic and physiological differences. "C. burnetii" is a small Gram-negative, coccobacillary bacterium that is highly resistant to environmental stresses such as high temperature, osmotic pressure, and ultraviolet light. These characteristics are attributed to a small cell variant form of the organism that is part of a biphasic developmental cycle, including a more metabolically and replicatively active large cell variant form. It can survive standard disinfectants, and is resistant to many other environmental changes like those presented in the phagolysosome.
History and naming.
Research in the 1920s and 1930s identified what appeared to be a new type of "Rickettsia", isolated from ticks, that was able to pass through filters. The first description of what may have been "Coxiella burnetii" was published in 1930 by Hideyo Noguchi, but since his samples did not survive, it remains unclear as to whether it was the same organism. The definitive descriptions were published in the late 1930s as part of research into the cause of Q fever, by Edward Holbrook Derrick and Macfarlane Burnet in Australia, and Herald Rea Cox and Gordon Davis at the Rocky Mountain Laboratory (RML) in the United States.
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. 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.
"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.
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. 
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. 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 alveoli. Upon inhalation, it targets alveolar macrophages and passively enters them via actin-dependent phagocytosis. After initial binding, it is suggested that "C. burnetii" enters phagocytotic cells via passive actin-dependent phagocytosis and enters non-professional phagocytes via an active zipper mechanism. "C. burnetii" exploits the αVβ3 integrin to enter using RAC1-dependent phagocytosis, which is believed to have evolved as a mechanism to avoid the induction of an inflammatory response.
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.
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.
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. 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.
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.
Genomics.
At least 75 completely sequenced genomes of "Coxiella burnetii" strains exist, 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.
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 and Vero cells; growth on axenic media is unaffected. QpH1 also contains a toxin-antitoxin system. Among all plasmids, 8 conserved genes code for proteins that are inserted into the host cell via the secretion system.
References.
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