Brachyspira aalborgi
1. Classification
a. Higher order taxa
Domain: Bacteria Phylum: Spirochaetota Class: Spirochaetia Order: Brachyspirales Family: Brachyspiraceae
2. Description and significance
Brachyspira aalborgi is a Gram-negative, spiral-shaped bacteria that can colonize the intestinal lining of humans (2). B. aalborgi can cause Human Intestinal Spirochetosis (HIS), as well as other intestinal diseases (3). Infected individuals experience a range of gastrointestinal symptoms such as chronic diarrhea, bloating, rectal bleeding, and many more (3,4). There is an observed trend of increased incidence rate of HIS in homosexual males and HIV+ patients worldwide (5,6,7,8). Despite its global presence, B. aalborgi is both historically and currently, very difficult to isolate, diagnose, and treat within patients (8,9,10). Due to challenges in culturing B. aalborgi, the mechanisms behind its pathogenicity are still incompletely characterized.
3. Genome structure
B. aalborgi strains have an average genome size of 2,504,147 base pairs (2.5 Mbp) and a total of 2,289 genes (11). Among these genes, 2,204 are coding genes, with 41 coding for RNA (11). B. aalborgi have 35 tRNA and 3 rRNA genes including the 5S, 16S, and 23S (11). The GC content in the genome varies between 27.6% and 28.3% (11).
The genome of B. aalborgi contains genes that are known to be associated with pathogenicity. Studies on the pathology mechanism of B. aalborgi discovered B. aalborgi contained genes for sialidases, which are thought to play a role in the evasion of immune cells in pathogenic bacteria (11). Also, B. aalborgi possesses a gene that produces collagenase PrtC, a protease that multiple other pathogens (H. pylori, S. enterica, E. coli, and B. subtilis) contain (11). Collagenase PrtC aids the bacteria in the invasion of the body by cleaving extracellular matrix components (11).
Genomic and phylogenetic analysis indicates that B. aalborgi is distinct compared to other taxa in the Brachyspira genus (12). In a phylogenetic comparison of 16S rRNA gene and the NADH oxidase gene (nox gene) among the Brachyspira genus, B. aalborgi was evolutionarily distinct (12). Comparison of whole genome sequences among the Brachyspira taxa also indicates a higher diversity among B. aalborgi strains than that seen in other Brachyspira taxa (11).
4. Cell structure
B. aalborgi are Gram-negative spirochete bacteria, containing a thin peptidoglycan cell wall between two membranes, with the outer membrane containing lipopolysaccharides (2,13). It is characterized as unicellular, motile, and helicoidal, and does not possess cytoplasmic tubes (2,4). The cell size is 0.2 μm by 1.7-6 μm and has tapered ends (9) with four flagella attached at each end of the cell (2). The core of its flagellum, measuring 12 nm in width and comprising of microtubule bundles, is made up of flagellin protein (2,8). The flagella are covered by a membrane (sheath) and the sheathed flagella are 18 nm wide, composed of a hook and basal body complex (2). The basal body, acting as a rotary motor, is located in the cell envelope and connected to a curved structure, called the hook, at the base of the flagella; together the hook and basal body help move and propel the flagella’s long, narrow filament. Additionally, between the hook and basal body is a shaft that is 40 nm long and 14 nm wide (2). The shaft is encompassed by a pair of disks, of 33 nm, and is linked to the hook by a thin rod (2).
5. Metabolic processes
B. aalborgi can be grown anaerobically on solid tryptose blood soy agar at 38 °C but is capable of surviving oxygen-rich environments for a few hours (2). If properly maintained, B. aalborgi can remain viable for more than three months (9,14). In laboratory conditions, B. aalborgi is observed to possess the ability to weakly damage red blood cells (2,11,14). B. aalborgi can ferment a variety of sugars, such as fructose, galactose, glucose, lactose, maltose, mannose, and trehalose. However, B. aalborgi is differentiated from other members of the Brachyspira genus by its inability to ferment adonitol, inositol, raffinose, rhamnose, and sorbitol (14).
B. aalborgi use a variety of different enzymes to accelerate chemical reactions. Enzymes such as β-galactosidase, esterase, acid phosphatase, and phosphoamidase are used in metabolic processes and are actively found in B. aalborgi (2). B. aalborgi lacks the redox enzyme oxidase and is catalase negative, which makes the bacteria incapable of breaking down hydrogen peroxide (2). Additionally, tryptophanase is absent within B. aalborgi and is thus unable to produce indole (14). B. aalborgi, however, does contain the proteins necessary to hydrolyze esculin and hippurate (9,14).B. aalborgi can thus be categorized as a chemoorganoheterotroph.
6. Ecology
B. aalborgi is commonly found in the large intestines of humans, with similar strains found in the bowels of various other animals (15). Unlike other Brachyspira species, B. aalborgi is strictly found in humans (16). In humans who have been diagnosed with Human Intestinal Spirochetosis (HIS), colonization by B. aalborgi can occur in one local area of the intestines, multiple areas of the intestines, or along the entire colon (12). B. aalborgi in HIS cases have been reported in multiple regions of the world, such as Europe, Africa, Australia, North and South America, and multiple other countries in Asia such as Japan and India (5,7). The different regions exhibit different prevalence rates, with data collected from feces and intestinal biopsies (5). HIV-positive patients, homosexual males, and developing countries are reported to exhibit the highest prevalence of HIS cases (5,7). Compared to other populations, HIS cases in Western populations are less prevalent (5).
The anoxic environment in the colon provides an ideal environment for B. aalborgi (16). Regulation of colonic pH, water absorption, and nutrient flow are all factors that affect bacterial growth (17). B. aalborgi grew best at 38.5°C in a mixture of 95% H2 and 5% CO2 (2). Additionally, the spirochetes grow optimally at a pH of 7.2 (2).
Due to the challenges in isolating and culturing B. aalborgi strains, there have been failed attempts to experimentally infect animals, such as chickens, so no experimental infected animal model is currently available to study (18).
7. Pathology
Brachyspira aalborgi has been identified as one of the two organisms responsible for causing Human Intestinal Spirochetosis, or HIS (19). The symptoms of HIS include abdominal pain, diarrhea, and an inflamed colon (2). Additionally, B. aalborgi plays a role in diseases such as colorectal spirochetosis (CS) and irritable bowel syndrome (IBS) (8). There is an increased risk for individuals in third-world countries, those who are HIV positive, and homosexual males (5, 6, 7).
Limited research on the pathogenicity of B. aalborgi results in an incomplete understanding of how it causes disease. However, research has shown that the spirochetes can cause tissue damage, allowing B. aalborgi to reproduce inside tissue cells and cause symptoms such as diarrhea (20). Additionally, B. aalborgi studies have shown the presence of mucus metabolism may help the bacteria to invade the mucus layers of the intestine, causing damage such as the loss of microvilli on the surface of epithelial cells (11, 5). The current gold standard of diagnosis includes examining biopsy samples of the epithelial cells that line the infected location, with a dense coating of spirochetes as a diagnostic indicator of infection (8,19, 21). Histological examinations of biopsies are used because the traditional method of genome sequencing is unable to detect the Brachyspira genus (19). However, this method has limitations due to the misinterpretation of sample results, as non-HIS bacteria can resemble the false brush border attachment of spirochetes along the epithelium wall in the histological staining (23). Another method for diagnosis involves 16S Ribosomal DNA Sequencing using colonic biopsies (8,9). The antibiotic metronidazole is currently the standard treatment and is successful in most patients (21).
8. Challenges in B. aalborgi Cultures
The incomplete understanding of the pathogenic mechanism of B. aalborgi is primarily due to the challenges involved in culturing the spirochete (8). B. aalborgi is extremely difficult to grow and isolate due to its finicky growing requirements and slow growth rate (22, 23). Nearly all cultured B. aalborgi originate from the colonic biopsies of patients with Human Intestinal Spirochetosis (HIS), with very few successful cultures from fecal samples (8,10). Efforts to determine the most effective media to culture B. aalborgi indicated that brain heart infusion agar (BHIA) containing 10% bovine blood (BB) was the best media (10).
9. Current Research
B. aalborgi is one of two spirochetes associated with causing Human Intestinal Spirochetosis (HIS) (3). A recent study sought to determine variations in Human Intestinal Spirochetosis (HIS) symptoms between Brachyspira aalborgi and Brachyspira pilosicoli (3). Researchers observed that B. aalborgi caused abdominal pain, diarrhea, and an inflamed colon. Patients infected with B. aalborgi experienced milder symptoms compared to those infected with B. pilosicoli (3). Differences in diagnosis suggest these two infections are clinically different (3). Comparison of B. aalborgi to other species will help future understanding of this spirochete.
Another recent paper focused on characterizing HIS infections and comparing infections caused by either B. aalborgi or B. pilosicoli. In the study, fluorescence in situ hybridization (FISH) was used to visualize the attachment of spirochetes to the intestinal wall (23). Results indicated that B. pilosicoli formed a denser border along the intestinal wall than B. aalborgi (23). This study also used PCR analysis of the 16s rRNA gene to determine which bacteria were present in different HIS patients. They found that of the 76 patients with Brachyspira infections, 53 were due to B. aalborgi and 23 matched the sequence for B. pilosicoli (23). Many current diagnostic measures for HIS do not give information on the specific location of the bacteria along the intestine and the distribution of attachment. Due to the successful localization of bacteria in the HIS samples by FISH, this paper proposes that FISH might be an effective diagnostic and follow-up tool for HIS (23).
Additionally, more recent B. aalborgi research has focused on genetic information and variation among strains. One study focused on whole genome sequencing of bacteria cultured from individuals with HIS (11). The study successfully isolated spirochetes from 14 patients, of which 13 were strains of B. aalborgi (11). This study represented the first reported whole-genome sequencing of spirochetes isolated from patients with HIS, revealing high genetic diversity among B. aalborgi strains (8,11). Additionally, the study looked for genes that could contribute to the pathogenicity of B. aalborgi. It was found that all but one of the B. aalborgi strains contained genes encoding for sialidases, which are suspected to be used by pathogenic bacteria to evade immune cells (11). B. aalborgi also contains the gene for collagenase PrtC, which is found in other pathogens and helps bacteria invade hosts (11).
10. Authorship Statement
S.Y.(Sophia Yuan) wrote Classification, Introduction, and Metabolic Processes. K.N.(Kristina Nicolas) and S.Y. wrote Genome Structure. A.H.(Allison Huang) wrote Cell Structure. K.J.(Kiki Jiang) wrote Ecology. J.Z.(Jamie Zheng) wrote Pathology. K.N. wrote Challenges in B. aalborgi Cultures. J.Z. and K.N. wrote Current Research. A.H. and K.J. incorporated the ChatGPT edits. The final draft was edited by A.H., J.Z., K.J., K.N., and S.Y.
11. References
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