Coxiella burnetii
A Microbial Biorealm page on the genus Coxiella burnetii
Classification
Higher order taxa
Bacteria; Proteobacteria; Gammaproteobacteria; Legionellales; Coxiellaceae; Coxiella (1)Bacteria; Proteobacteria; Gammaproteobacteria; Legionellales; Coxiellaceae; Coxiella (1)
Species
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NCBI: Taxonomy |
Coxiella burnetii
Description and significance
Coxiella burnetii is an obligate intracellular Gram-negative coccobacillus bacterium that is known to be the main pathogen that causes Q fever in mammals and humans. (3) Harold Cox and MacFarlane Burnet initially identified Q fever as “query fever” in 1935 when a number of infections were found to be from an Australian slaughterhouse. Once this disease was elucidated to be a human pathogen, the name was properly changed to Q fever. (4) Its global pathogenic effect demonstrates the need for preventive measures to control the rate of infection worldwide and its potential use for bioterrorism. The significance of a completed sequenced genome is the benefit of being able to further understand the mechanisms of pathogenesis and use this knowledge to fight against this infectious disease. Sheep, cattle, and goats are major sources of Coxiella burnetii that can potentially help spread the disease to other organisms. The most common mode of transmission to humans is primarily through external waste excretions from infected animals. The aerosol route to infection is frequent by inhalation of contaminated air that contains many of these organisms or through an insect vector. (5) Coxiella burnetii live in a wildlife livestock environment and can withstand heat, dryness, and antibacterial compounds, allowing this bacterium to persist outside the host for an extensively long period of time. It is an acidophile, meaning it tends to surround itself in an environment with low pH. Uniquely enough, it can be endocytosed by a macrophage and complete replication inside the phagolysozyme during its life cycle. (3)
Genome structure
Random shotgun method was used to sequence the genome of Coxiella burnetii. (3) The genome of this organism contains a single 1,995,281 base-pair circular chromosome and a single 37,393 base-pair QpH1 circular plasmid. (1) In the chromosome, there are 1,022 protein-coding genes found for known proteins, 179 genes for proteins of unknown function, 3 stable rRNAs, and 42 stable tRNAs. Percent of G+C content is approximately 42.6% and percent coding is approximately 89.1% in the chromosome. As for QpH1, there are 11 genes found for known proteins, 5 for proteins of unknown function, and no stable RNAs. 39.3% represents the percent of G+C content and 78.8% is the percent coding in the plasmid. (6) Examining 20 highly conserved proteins in 16S rRNA gene sequence analysis proved the fundamental phylogenetic difference with the α-proteobacterial Rickettsia organisms and confirmed that Coxiella are truly γ-proteobacterial. In genomic comparison with other similar obligate parasites such as those from Rickettsia, the genome of Coxiella was found to contain mobile elements, metabolic and transport capabilities not typically found for the kind of bacteria they are. In addition, 29 insertion sequences were located in the genome; this is noteworthy information because other obligate parasites have very little or none of these elements. However, the presence of pathogenecity islands along with the unique transposons found in the genome does not suggest recent exchange of mobile genetic elements with other organisms. A notion that Coxiella burnetii are enduring a genome reduction, in which certain genes that encode important genetic information ultimately lose their functionality and degrade, has been proposed due to the fact that 83 pseudogenes have been identified. Proteins synthesized by this bacterium has a particularly high pI, which may be due to the acidic environments Coxiella burnetii reside in. (3)
Cell structure and metabolism
Being that Coxiella burnetii is a Gram-negative bacterium, this distinction marks important properties about the cell structure. Gram-negative bacteria have two membranes, an inner and outer membrane. The outer membrane lacks an energy source, but compensates by having porins fused into the membrane. The organism does not have a thick cell wall composed of peptidoglycan like Gram-positive bacteria. In between the two membranes lies the periplasmic space. Lipopolysaccharides are anchored to the membrane. During its life cycle, phagocytosis brings the bacteria into the host cell, where it remains in phagocytic vacuoles and replicates in the phagolysozyme.
Compared to free-living bacteria, Coxiella burnetii does not have a broader range of transport capacities. In spite of this limitation, this bacterium possesses more transport capacities than other obligate parasites like Rickettsia organisms. For active transport mechanisms by ion-coupled transport, Na+/H+ exchangers are thought to be significant in maintaining pH in the acidic environment Coxiella burnetii thrive in. Although group transfer mechanisms via the phosphotransferase system are not evident in Coxiella, enzymes involved in the system, such as HPr and enzyme 1, are present and play a regulatory role. (3)
Unlike the other obligate parasites, the greater metabolic capacities of Coxiella burnetii permit the bacteria to undergo glycolysis, gluconeogenesis, pentose phosphate pathway, and TCA cycle. They lack ATP/ADP exchangers in their transport system. The bacterium utilizes few sugars, including xylose and glucose, and their uptake is aided by a membrane gradient. Amino acids are taken into the cell by an outstanding number of 15 transporters and peptides have three. Due to the copious number of transporters required for the uptake of these organic nutrients, amino acids and peptides may be the major carbon sources for this bacterium. (3)
Ecology
Coxiella burnetii does not display a pathogenic effect for all organisms it encounters. For example, B. La Scola and D. Raoult discovered that Coxiella burnetii and free-living amoeba Acanthamoeba castellanii are able to co-exist in a benign relationship upon infection. The bacterium was able to produce spore-like structures in vacuoles within the amoeba, demonstrating its ability to differentiate without any interference from the other species. The bacterium does not cause any diseases in the amoeba. Likewise, the amoeba did not utilize the bacterium as food, so it contributes to the survival of Coxiella burnetii by supplying an intracellular niche for it to differentiate into a structure similar to a spore and resist harmful environmental conditions. (7)
Besides surviving in a host cell, Coxiella burnetii can adapt to a shared intracellular niche with another parasite without inhibition of development for either. According to research accomplished by Andreoli and colleagues, Vero cells that were previously inhabited by Coxiella burnetii were co-infected with another type of parasite such as Trypanosoma cruzi. Parasitophorous vacuoles (PV) of T. cruzi trypomastigotes and bacterial vacuoles fused together upon co-infection. T. cruzi was able to proceed with differentiation into amastigotes and division in Coxiella burnetii vacuoles. The co-existence of both parasites depends on the ability of Coxiella burnetii to provide sufficient amount of nutrients for T. cruzi. (8)
Pathology
Q fever is a global disease caused by the pathogen Coxiella burnetii. Without any symptoms and a low dosage that leads to infection, the disease can go unnoticed until serious health consequences begin to present themselves. Because of its natural high resistance to harsh environmental conditions, including dessication, heat, and antibacterial compounds, the transmission of Q fever to other organisms is very effective through contaminated air, the main mode of transmission. In reference to virulence factors, genes encoding adhesive structures in the genome such as pili are absent, but there are 13 ankyrin domains that may assist in the bacterium’s attachment to its host. (3) The method in which humans get infected is by infected animals such as sheep, cattle, goat, dogs, and cats. These infected animals can produce excretions through urine, feces, and milk that contain infectious dosages of this pathogenic bacterium, which can be dangerously mistakenly inhaled, consumed, or be in contact with. The bacterium can be isolated in the placentas of infected animals and can cause abortions due to inflammation. Not just livestock and domestic animals can get infected. Even fish and rodents can acquire Q fever as well. (9)
Humans who are exposed to or handle infected animals, such as farmers or veterinarians, have a higher risk of infection and then the disease develops. Typically, there are no obvious symptoms after infection. (9) Only about 50% of people who are infected show signs of the disease. Symptoms similar to the flu may appear, but it is not specific and not always diagnosed as Q fever. The disease takes the form of pneumonia or hepatitis commonly. As the infection becomes more serious, chronic Q fever develops. Endocarditis, inflammation of the aortic heart valves, has been associated with the chronic complications of the disease. The survival rate is higher for those who do not suffer from chronic
Q fever. (5)
Application to Biotechnology
Coxiella burnetii can have a variety of biological uses in other organisms by producing many multiple clones of its own genes and inserting a gene of interest into the DNA of other organisms that may not have been able to express a certain function before. For example, the genome of Coxiella burnetii contains a gene mucZ that encodes for the production of capsule in E. coli. The gene mucZ can be cloned into a plasmid vector in the host E. coli. The plasmid DNA can be extracted and transferred to E. coli to express mucZ and allow it to produce a capsule for different functions such as motility, attachment, virulence, protection etc. Through PCR, DNA replication can be amplified and useful for studying gene expression or regulation in other organisms with the assistance of plasmids. (10)
Current Research
In April 2007, scientific research was accomplished by Jensen T.K. and colleagues regarding the presence of Coxiella burnetii in 90 aborted placentas of mammals over a two year period. Fluorescent in situ hybridization (FISH) assay can be implemented as a diagnostic tool to study and differentiate various microbes. They used this method to target 16S ribosomal RNA to identify Coxiella burnetii in formalin-fixed, paraffin-embedded tissue samples. Positive controls were taken from human serum for 12 cases of placentitis and 7 samples for negative controls and compared to the tissue samples. Each sample was hybridized with a specific oligonucleotide probe for the bacterium. All cases yielded a positive result for the bacterium, in which an enlarged trophoblast cytoplasm can be seen due to the presence of reddish brown stained bacteria. It was concluded that FISH was an effective tool in the detection of Coxiella burnetii. (11)
Scientific research performed by Kelly Cairns et al. in November 2006 investigated the prevalence of Coxiella burnetii DNA in uterine and vaginal samples from 50 shelter and 47 client-owned healthy female cats of north Colorado with polymerase chain reaction (PCR) assay. The life span of most cats did not reach more than three years. Transmission of Q fever from domestic animals to humans has been reported in past cases, supporting the perception that cats harbor a good source of this bacterium. Antibodies against this bacterium have been found in cats from different countries, including North America, Japan, and Korea. The abundance of antibodies worldwide suggests a global threat to humans who are in contact with cats. Positive controls for the presence of Coxiella burnetii originated from immunofluorescence assay slides. The difficulty in studying this organism in culture points to the use of PCR to track infection in domestic animals and humans. The PCR assay utilized primers specific for Trans1 and Trans2 transposon-like region. The uterine samples that tested positive for Coxiella burnetii DNA came from cats that had a home and owners to take care of them. Thus, it has been shown that healthy cats can harbor this pathogenic bacterium even if they look “clinically normal.” It is stated that parturient cats are more likely to be associated with the transmission of Q fever through cat contact. (12)
Not only are methods of isolation and detection of bacteria popular in current microbiology research, the significance of regulation of food safety has not gone unstudied. Because Coxiella burnetii has been known to possibly be found in milk, milk pasteurization has never been so crucial in the promotion of sanitary food production. Milk pasteurization aims to kill all harmful bacteria through extensive heat treatment. Due to the fact that this bacterium is perceived to be one of the toughest heat resistant organisms existing, questions were raised from O. Cerf and R. Condron involving the effectiveness of upholding the requirement for milk to be pasteurized under rigorous temperature and time-sensitive conditions because of previous notions that Q fever could possibly be transmitted through food. The conclusion was that infection arising from the consumption of unpasteurized milk does not inevitably lead to the clinical disease of Q fever, brought on by either inhalation or arthropod bites. (13)
References
1. ExPASy. Swiss Institute of Bioinformatics. 02 May 2007 <http://expasy.org/sprot/hamap/COXBU.html>.
2. NIAID Biodefense Image Library. National Institute of Allergy and Infectious Diseases. 02 May 2007 <http://www3.niaid.nih.gov/biodefense/Public/Images.htm>.
3. Seshadri R, Paulsen IT, Eisen JA, et al. “Complete genome sequence of the Q-fever pathogen coxiellaburnetii”. PNAS. 2003. Volume 100. p. 5455-5460.
4. Q-fever (Coxiella burnetii) Zoonoses. Animal Disease Diagnostic Laboratory. 02 May 2007 < http://www.addl.purdue.edu/newsletters/2004/spring/qfever.htm>.
5. Q fever. Centers for Disease Control and Prevention. 02 May 2007 <http://0-www.cdc.gov.mill1.sjlibrary.org/ncidod/dvrd/qfever/>.
6. Genome Project. <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=Add%20to%20Clipboard&DB=genomeprj>.
7. B. La Scola, D. Raoult. “Survival of Coxiella burnetii within free-living amoeba Acanthamoeba castellanii”. Clinical
Microbiology and Infection. 2001. Volume 7 (2). p. 75–79.
8. Andreoli W.K., Taniwaki N.N., Mortara R.A. “Survival of Trypanosoma cruzi metacyclic trypomastigotes within Coxiella burnetii vacuoles: Differentiation and replication within an acidic milieu”. Microbes and Infection. 2006. Volume 8 (1). p. 172-182.
9. Marrie, J. Thomas. “Q fever - a review”. Canadian Veterinary Journal. 1990. Volume 31 (8). p. 555-563.
10. M Zuber, T A Hoover, and D L Court. “Analysis of a Coxiella burnetii gene product that activates capsule synthesis in
Escherichia coli: requirement for the heat shock chaperone DnaK and the two-component regulator RcsC”. Journal of Bacteriology. 1995. Volume 177(15). p. 4238–4244.
11. Jensen TK, Montgomery DL, Jaeger PT, Lindhardt T, Agerholm JS, Bille-Hansen V, Boye M. “Application of fluorescent in situ hybridisation and immunohistochemistry for demonstration of Coxiella burnetii in placentas from ruminant abortions”. APMIS. 2007. Volume 115. p. 347–53.
12. Kelly Cairns, Melissa Brewer and Michael R. Lappin. “Prevalence of Coxiella burnetii DNA in vaginal and uterine samples from healthy cats of north-central Colorado”. Journal of Feline Medicine & Surgery. 2007. Volume 9. p. 196-201.
13. O. Cerf and R. Condron. “Coxiella burnetii and milk pasteurization: an early application of the precautionary principle?” Epidemiology and Infection. 2006. p. 946-951.
Edited by Lisa Leung, student of Rachel Larsen and Kit Pogliano
