Bartonella henselae: Difference between revisions
| Line 65: | Line 65: | ||
==Application to Biotechnology== | ==Application to Biotechnology== | ||
There is still much to discover about the pathogenicity of this organism. Because it is mostly found in human and cats, there have not been much discovery of biotechnology advantage with this organism. | |||
==Current Research== | ==Current Research== | ||
Revision as of 22:50, 29 August 2007
A Microbial Biorealm page on the genus Bartonella henselae
Classification
Bartonella henselae was formerly known as Rochalimaea henselae. It was re-classified in 1993 (11).
Higher order taxa:
Domain: Bacteria, Phylum: Proteobacteria, Class: Alphaproteobacteria, Order: Rhizobiales, Family: Bartonellaceae (1)
Species:
Bartonella henselae
There are two serotypes of Bartonella henselae. Seroptype I is Houston-1 and Serotype 2 is Marseille. The classification is based on the differences in the 16S ribosomal DNA sequences (8).
Description and significance
Bartonella henselae is an aerobic, oxidase-negative, and slow growing Gram negative rod, slightly curved. It does not have flagella to facilitate its movement; however, there have been evidence of twitching motility. It requires very exact and fastidious conditions to grow in vitro. The temperature for optimal growth is 37 degrees Celsius and is highly dependent on the form and quantity of heme available. It is also very sensitive to changes in pH and has an explicit pH range of 6.8 to 7.2 (3). Under the microscope, the colonies are cauliflower-like (9).
Bartonella henselae was first discovered in a patient suffering from Cat Scratch Disease, though not identified, in the 1950s by Debre et al. The bacteria is found and can be isolated from erythrocytes of cats as well as lymph nodes of humans (6). It is important to use blood agar or chocolate agar plates and provide carbon dioxide. Colonies usually takes two to six weeks to form. The slow growth contributes to frequent misdiagnonsis. That is perhaps the reason why the bacterium was not identified until the 1990s with extensive work by Hensel, even though the first case of Cat Scratch Disease was described four decades prior (3). It is later discovered that this bacterium is associated with many other symptoms found in HIV-positive individuals.
Genome structure
Bartonella henselae has a circular genome. It uses mainly chromosomal genes for its virulence, according to research up to date (4). However, a potential plasmid has also been discovered, although, further research is needed to determine the full functionality. The genome was completely sequenced in 2004. It has a genome size of approximately 1.9 Mbp with an estimated coding fraction of 72.3%, slightly larger than that of Bartonella quintana with a genome size of 1.5 Mbp. The origin of replication is characterized by excess guanine and thymine nucleotides on the leading strand. There are 301 genes unique to Bartonella henselae. Approximately sixty-two percent of the genes on this bacterium are located on four sectors. They include a prophage region of 55 kb and three genomic islands of 72, 34, and 9 kb. On one side of the genomic islands are tRNAs and the other side, integrase genes. The 34 and 70 kb genomic islands have many copies of fhaC/hecB and fhaB. The fhaC/hecB gene makes a molecule that controls the transport of filamentous hemagglutinin, which is encoded by fhaB (18). The Bartonella henselae genome also has an unusually high number of repeated genes. Genomic islands are not present in Bartonella quintana, therefore, it does not make filamentous hemagglutinin. The two species are 98.7% identical in the 16S rRNA gene sequence. These two species derived some of their genes from Brucella melitensis (16).
The housekeeping genes of Bartonella henselae are 16S rDNA, eno, ftsZ, gltA, groEL, ribC, and rpoB. These genes function in the growth and metabolism of the bacterium. The ftsZ is homologous to that of Bartonella bacilliformis. It is located at the end of the operon consisting of genes ddlB, ftsQ, and ftsA, 5’-ddlB-ftsQ-ftsA-ftsZ-3’. ddlB facilitates cell wall biosynthesis by coding for homologues of D-alanine D-alanine ligase. FtsQ and FtsA are critical as well because they are involved in cell division. Promoters are also located in the ddlB, ftsQ, and ftsA open reading frames. These are essential to the bacterium because promoters help maintain high levels of FtsZ activity, very much like E. Coli to enhance the transcription of the ftsZ mRNA (16).
The plasmid consists of genes ribD, ribC, and ribE that encode for riboflavin deaminase (RibD) and subunits of riboflavin synthetase, RibC and RibE. Riboflavin is the precursor to important cofactors such as flavin mononucleotide and flavin adenine dinucleotide. These two cofactors are essential in electron transport and contribute to the basic energy metabolism of the cell (17).
Both serotypes of Bartonella henselae have virB4 genes that produce other virulent factors. It has 331 bp and further research of the virB operon indicates that the one in Bartonella henselae is homologous to the one in Agrobacterium tumefaciens, a bacterium known for its pathogenesis with the Type IV pili, however, their roles in virulence are still unknown (19). As mentioned earlier, there are different genotypes of Bartonella henselae. The lack of congruence between 16S rDNA shows that horizontal gene transfers occur between different B. henselae strains (8).
Cell structure and metabolism
The average size of the bacterium is 2 micrometers in length by 0.5 to 0.6 micrometers in width. There is no evidence of flagella and they are composed of octadecenoic and hexadecanoic fatty acids, similar to Bartonella quintana (9). The lipopolysaccharide from Bartonella henselae does not participate directly with pathogenicity, as most Gram negative bacteria secrete endotoxins through this structure (21).
Bartonella henselae has Type IV secretion systems (T4SSs) which are transporter complexes on membranes that help transport substrate molecules to target cells (14). The Type IV pili participate in the attachment to target cells. These pili undergo phase variation and may be the pathogenic determinant for Bartonella species. Different strains undergo different phase variations. Some are more pathogenic than others, such as Bartonella henselae strain 87-66 compared to ATCC 49793. Apparently, there is a positive correlation between the expression of pili and pathogenicity. Three important proteins released are BepD, BepE, BepF. BepD is phosphorylated after transmission into host cells. The effect of BepD is not clear as yet (14).
An unusual feature of Bartonella henselae is its inability to use glucose to derive energy, since glucose is abundant in mammalian hosts. This is due to the fact that it has an incomplete glycolysis pathway. Another closely related species Bartonella quintana uses a similar mechanism for metabolism as well. Both bacteria use amino acid catabolism to generate energy.This was confirmed with oxygen consumption and the production of carbon dioxide (3). It has been determined that Bartonella quintana metabolizes mainly succinate and glutamate. A few years later, it was determined that Bartonella henselae uses succinate and glutamate as well, along with histidine, asparigines, glycine, and serine from the growth medium.
Ecology
The main difference between Bartonella henselae and Bartonella quintana is its reservoir ecology. Bartonella quintana, termed a specialist, only infects humans whereas Bartonella henselae is found primarily in cats and humans (4).
A unique feature of Bartonella henselae is the manner in which it deals with stress in a new host environment. The stress comes from temperature and pH changes and other invading cells from the host organism. Both Bartonella quintana and Bartonella henselae produce three major heat shock proteins of 70, 60, and 10 kDa. These proteins help the bacteria find an intracellular shelter inside the mammalian host (13).
The bacterium’s complete interaction with the host cell is yet to be determined.
Pathology
Bartonella henselae is a promiscuous bacterium that infects both cats and humans. When cats are infected, they show no symptoms, thus making it difficult for researchers to identify the disease (3). It is estimated that there are at least 24,000 cases of Cat Scratch Disease per year in the U.S. in humans.
The bacterium has different effects on immunocompromised individuals, such as AIDS patients, and immunocompetent individuals. In both types of people, bacteremia occurs, which is the presence of bacteria in the blood.
In immunocompromised individuals, Bartonella henselae is known to be the agent of bacillary angiomatosis, peliosis hepatitis, septicemia, endocarditis, recurring fever, and neurological disorders (2). The latter symptoms are fatal when misdiagnosed and improperly treated. Bartonella quintana is also known to produce bacillary angiomatosis, along with Bartonella henselae. Bacillary angiomatosis are aggregates of capillaries that produce excessive endothelial cells. It was first described in HIV-infected patients, but it is now determined to cause problems for immunocompetent patients as well. It involves the bone marrow, spleen, liver, brain, lymph nodes, or the bowel (12). The most striking pathological effect of Bartonella species is the ability to produce angiogenesis, the formation of new blood vessels from existing ones. These usually involve the capillaries and endothelial cells. Bartonella are the only bacteria able to produce angiogenic tumors in humans, very much like the Agrobacterium species that produce tumors in plants (12).
In immunocompetent individuals, Bartonella henselae can cause Cat Scratch Disease (CSD). It is a self-limited infection that occurs in immunocompetent individuals who have been bitten or scratched by a cat (3). Studies show that the vectors can be either the cat or the cat flea that transmits it to the cat. It lasts six to twelve weeks without antibiotic therapy. The main symptom that arises from this disease is lymphadenopathy, which is characterized by the swelling of the lymph nodes. Skin lesions become visible 3-10 days after the cat scratch or bite and lymphadenopathy occurs approximately one to two weeks later. The affected lymph nodes are surrounded by histiocytes, lymphocytes, and cells with giant nuclei. Fifty percent of the patients report that they had headaches, r anorexia, weight loss, vomiting, and occassionally, a sore throat (5). This disease does not respond to antibiotic therapy. The two genotypes, I and II of Bartonella henselare are both involved in Cat Scratch Disease, however, clinically, there are no essential differences between the two (6).
Bartonella henselae has two invasion mechanisms into endothelial cells (8). One mechanism is the invasome-mediated uptake. First, the bacterium elicits a massive rearrangement of the actin cytoskeleton. This causes aggregation and bacteria is engulfed by host cell membranes and eventually enters the endothelial cells. This structure is called the invasome. The infection elicits a proinflammatory response, which activates NF-kB, a transcription factor that regulates the innate immune response and adaptive immune response. This mechanism requires the use of Type IV secretion systems. The formation of an invasome usually takes 24 hours. When invasion is successful, numerous actin stress fibers will be found in a twisted morphology at the basal part of the invasome. The invasome is thought to cause a revolving locomotion of cell bodies, which would stimulate twisting forces and contribute to the evenly rounded structure of the cell. It is important to note that the aggregation step in invasome formation is a host cell-driven process; the invasome are not clumping together by themselves. Although, clumping has been observed with the expression of type IV pili which causes the twitching motility. However, it is not present in all Bartonella henselae strains. Once the cell responds to inflammation, it elicits an angiogenic response. Cytokines that promote inflammation such as tumor necrosis factor (TNF-alpha) are released and leads to angiogenesis. This is why patients with bacillary angiomatosis or peliosis hepatitis experience swelling(15). The other mechanism was discovered when working with adhesion to endothelial cells by the bacterium with human umbilical vein endothelial cells (HUVECs). Infection by the bacterium drives HUVEC into the cell. Vasoproliferative lesions caused by Bartonella species are usually surrounded by neutrophils. These neutrophils contribute to an inflammatory response which involves endothelial cell activiation. Neutrophils then attach to vessel walls and activate CD11/CD18, which are receptors to endothelial molecules such as intracellular adhesion molecule 1 (ICAM-1). Next, the bacterium enters the tissues and involves other adhesion molecules (20). This mechanism depends on small GTPases, Rho, Rac, and CDC42, which all regulate actin formation and reorganization.
The invasion mechanisms into erythrocytes is not yet known. The bacterium is so successful in invading because it is able to inhibit cell death of endothelial cells. This is done by controlling the early and late events in apoptosis such as caspase activation and DNA fragmentation. The translocation of BepA, a type IV secretion substrate, is important in inhibiting cell death. The mechanism is based on raising the level of cyclic adenosine monophosphate (cAMP) carried out by the plasma membrane (23). There is still much more research that needs to be done for the complete understanding of virulent factors.
Application to Biotechnology
There is still much to discover about the pathogenicity of this organism. Because it is mostly found in human and cats, there have not been much discovery of biotechnology advantage with this organism.
Current Research
Enter summaries of the most recent research here--at least three required
References
1. Bartonella henselae NCBI classification reference: http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=38323&lvl=3&lin=f&keep=1&srchmode=1&unlock
Edited by student of Rachel Larsen
