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Coxiella burnetii (C.b, commonly known as Q fever rickettsia) is a Gram-negative bacterium that strictly parasitizes living cells. This pathogen is extremely resistant to physical and chemical factors and can survive for a long time in the external environment, humans and animals are generally susceptible and can be widely transmitted through aerosols, so the US counter-terrorism organization lists it as one of the biological warfare agents. Some domestic animals, pets, etc. are its main reservoir hosts. Coxiella burnetii can cause acute and chronic Q fever in humans. The clinical manifestations are non-specific. There are no symptoms in 60% of the body. Misdiagnosis and missed diagnosis are quite serious. During World War II, there were many outbreaks of the disease among the military. At present, Q fever has become one of the most widely distributed zoonosis in the world.
Figure 1. Bartonella-host-cell interactions and vascular tumor formation. (Dehio C.2005)
Previous classification methods classified Coxiella burnetii into the order Rickettsia, family Rickettsiaceae, and genus Rickettsia. At present, it has been separated from the order Rickettsia according to the latest genetic classification. This bacterium is a gram-negative bacterium, pleomorphic, non-flagellated and non-capsulated, with a size of approximately 0.2~0.4μm×0.4~1.0μm.Proliferates in the phagolysosome of host cells, and the optimal pH value is 4.5~5.0.The pleomorphism may be related to the different development cycles of the pathogen in lysosomes. The small cell variation (SCV) form is mostly located outside the host cell.After SCV is activated, it turns to the large cell variation (LCV) form to reproduce. Its pathogenicity and immunogenicity are closely related to its lipopolysaccharide, plasmids, surface proteins, etc. This bacterium can reproduce in many human and animal cells, and chicken embryos are an excellent host for cultivating the bacterium.
Coxiella burnetii lipopolysaccharide (LPS) is very similar to other gram-negative bacteria in that it is water-soluble and its terminal oligosaccharides are the main antigenic determinants. This bacterium has antigenic phase variation. The main reason is that the lipopolysaccharide (LPS) structure changes and adapts to different host environments to show two-phase antigenicity. Phase I rickettsiae are highly virulent, contain complete antigen components, and have smooth lipopolysaccharide I; phase II contain rough lipopolysaccharide II and are attenuated strains. Phase I rickettsiae are highly virulent and can induce phase II antibodies in the early stage of infection and produce phase I antibodies in the late stage. Therefore, phase II antigen can react with early and convalescent serum, while phase I antigen only reacts positively with late stage serum. The difference between the two-phase rickettsial LPS is mainly in the carbohydrate composition. Studying the relationship between the structure and function of LPS of Coxiella burnetii is of great significance for analyzing issues related to its pathogenicity and immunogenicity.
Plasmid
Coxiella burnetii is the only pathogenic bacterium in the Rickettsia family known to contain plasmids, and 4 plasmid types (QpHI, QpRS, QpDV and QpDG) and plasmidless types have been discovered. QpHI is 36kb, QpRS is 39kb, QpDV is 33.5kb, and QpDG is 51kb.Each plasmid has its own specific sequence, and most of its DNA is conserved. In the past, some scholars believed that acute and chronic isolates could be distinguished based on the plasmid. In recent years, some people have found that the type of plasmid has nothing to do with the clinical type of Q fever.
The main surface protein genes of Coxiella burnetii include heat shock protein gene, coml, p1, 17kDa protein gene, and omp34. Other related genes include gltA, sod, Cbmip, 23SrRNA, 16SrDNA and other genes.
The heat shock proteins of Coxiella burnetii mainly include Hsp60 and Hsp70, which belong to the Hsp60 and Hsp70 family members respectively. Hsp is a highly conserved molecule and an important protective antigen that can induce humoral immunity and cellular immunity. Hsp60 includes htpA and htpB. The two genes are closely connected with a spacing of 27 bp. These gene transcription in Escherichia coli is regulated by the promoter. Two proteins can be produced at 37°C. HtpA has no immunogenicity, the ORF is 291bp long, and the polypeptide molecular weight is 10.5kD; htpB ORF is 1656bp long, and the polypeptide molecular weight is 58.5kD. Through expression, it was found that HspB is located on the surface of Coxiella burnetii and Escherichia coli. HspB does not have an obvious transmembrane region, and it only reacts with the serum of acute convalescent and chronic Q fever patients. The Hsp70 gene is 1968bp long and the polypeptide molecular weight is 71kD. It is mainly located on the surface of the bacteria and has good antigenicity. Another heat shock protein, Gp96, is a 96kD glycoprotein in the endoplasmic reticulum. It is an important target molecule protein for intracellular parasitic bacteria to invade host cells and plays an important role in immunity.
The full length of the coml gene is 756bp, and the polypeptide molecular weight is 27kD.This gene is quite conservative, and the protein it encodes is located on the surface of the bacterial cell. Com1 is a membrane protein loosely connected to its cell membrane. It has three strongly hydrophilic regions and an isoelectric point of 8.8.The function of Com1 can change the redox potential within phagolysosomes, which may be related to pathogenicity.
The full length of the p1 gene is 759bp, encoding 252aa, and the molecular weight of the polypeptide is 29.5kD, which is highly conserved. P1 protein is a surface protein of Coxiella burnetii that is present in LCV of Coxiella burnetii but not in SCV. Phase I and phase II of Coxiella burnetii contain P1 outer membrane protein, which has good immunogenicity and immunoreactivity.
The protein encoded by the 17kDa outer membrane protein gene was discovered through the construction of a gene library of EP21/MP29/EPi. It can react with the multivalent serum of EP21/MP29/EPi strains and the monoclonal antibody against the 17kDa outer membrane protein, confirming that the protein is EP21/ The outer membrane protein of MP29/EPi is immunoreactive.
The full length of this gene is 903bp, encoding 300aa, and the molecular weight of the polypeptide is 34kD. The biological characteristics of this protein are not yet clear.
The citrate synthase (gltA) gene of Coxiella burnetii has a full length of 1290bp and encodes a 46kDa polypeptide. gltA plays an important role in the biosynthesis of the tricarboxylic acid cycle.
The ORF of the superoxide dismutase gene (sod) is 579bp long, encoding 193aa, and the polypeptide molecular weight is about 23kDa. SOD can remove the toxic effect of superoxide ions produced by phagocytes on bacteria by converting superoxide ions into hydrogen peroxide, thereby promoting the infection of the host by pathogens.
The Coxiella burnetii phagocyte infection factor (Cbmip) gene encodes 230aa, and the polypeptide molecular weight is 25.5kDa. The protein it encodes can react with specific antibodies against Legionella pneumoniae Mip, so the gene is called Cbmip and plays a certain role in the pathogenesis of Coxiella pneumoniae.
The 23SrRNA gene of C.b. contains a 444bp long insertion sequence (IVS), forming an open reading frame. The 16S-23SrRNA intergenic region (ISR), which has large variations in extracellular bacteria, tends to be conserved among different isolates. There is a 497-501 bp long intervening region (ITS) between the 16SrDNA and 23SrDNA of C.b. which is highly conserved. Therefore, the IVS and ITS can be used as genetic markers of Coxiella burnetii and can be used in PCR or DNA probes to identify the pathogen.
References
| Target | Cat. No. | Product Name | Host | Isotype | Application | |
| C. burnetii | DPAB-CS23067 | Goat Anti-C. burnetti Polyclonal antibody | Goat | IgG | ELISA | Inquiry |
| DPAB-CS23068 | Rabbit Anti-C. burnetti Polyclonal antibody | Rabbit | IgG | ELISA | Inquiry |
| Target | Cat. No. | Product Name | Expression System | Tag/Conjugate | Application | |
| C. burnetii | DAG-WT639 | Recombinant C. burnetii Coxiella Outer Membrane 1 (Com1) | E. coli | TBD | ELISA, CLIA | Inquiry |
| Target | Cat. No. | Product Name | Size | Species Reactivity | Application | Detection Sample | |
| C. burnetii | DEIA2186 | Coxiella burnettii ( Q-Fever ) Ph. 2 IgM ELISA Kit | 96T | Human | Qualitative | serum and plasma | Inquiry |
| DEIA2187 | Coxiella burnetii ( Q-Fever ) Phase 1 IgG ELISA Kit | 96T | Human | Qualitative | serum and plasma | Inquiry | |
| DEIA2188 | Coxiella burnettii ( Q-Fever ) Ph. 2 IgG ELISA Kit | 96T | Human | Qualitative | serum | Inquiry | |
| DEIA6296 | Coxiella burnetii (Q-Fever) Phase 1 IgG ELISA Kit | 96T | Human | Qualitative | serum | Inquiry |
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