Bacillus anthracis: Difference between revisions

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The single chromosome found in the ''Bacillus anthracis'' genome is a circular, 5,227,293 bp DNA molecule.  The main virulent factors are encoded on two plasmids, pXO1 (189 kb, anthrax toxin) and pXO2 (96 kb, capsule genes).  The plasmids are circular, extrachromosomal, double-stranded DNA molecules.  Three components make up the anthrax toxin: a protective antigen (PA), lethal factor (LF), and edema factor (EF). "PA/LF and PA/EF complexes are internalized by host cells where the LF (metalloprotease) and EF (calmodulin-dependent adenylate cyclase) components act. At high levels LF induces cell death and release of the bacterium while EF increases host susceptibility to infection and promotes fluid accumulation in the cells."
The single chromosome found in the ''Bacillus anthracis'' genome is a circular, 5,227,293 bp DNA molecule.  The main virulent factors are encoded on two plasmids, pXO1 (189 kb, anthrax toxin) and pXO2 (96 kb, capsule genes).  The plasmids are circular, extrachromosomal, double-stranded DNA molecules.  The toxin is a complex of three plasmid-encoded proteins.  Two of the proteins are directly toxic, including LF (lethal factor) and EF (edema factor).  High LF levels destroys white blood cells and release the bacterium, while EF increases cyclic AMP levels.  The host is more susceptible to infection with EF.  Energy and water balance is impaired by the increase in cyclic AMP, resulting in the accumulation of fluid in cells.  The other plasmid-encoded protein, named PA (protective antigen), ushers the two toxic proteins in cells.  PA forms a multimeric ring, which inserts into the cell membranes of the host. PA is not toxic alone, but if it is inactivated, the two toxic proteins would not cause harm.  This is because PA allows the toxic components to pass through the membrane via a special toxin delivery system.  Capsule production depends on the pX02 plasmid.  Capsule formation is important because it allows the organism to resist phagocytosis.
 
 
The toxin is a complex of three plasmid-encoded proteins.  Two of the proteins are directly toxic, including LF (lethal factor) and EF (edema factor).  LF destroys white blood cells while EF increases cyclic AMP levels.  Energy and water balance is impaired by the increase in cyclic AMP.  The other plasmid-encoded protein, named PA (protective antigen), ushers the two toxic proteins in cells.  PA forms a multimeric ring, which inserts into the cell membranes of the host. PA is not toxic alone, but if it is inactivated, the two toxic proteins would not cause harm.  This is because PA allows the toxic components to pass through the membrane via a special toxin delivery system.   
 
Capsule production depends on the pX02 plasmid (60 megadalton).  Capsule formation is important because it allows the organism to resist phagocytosis.


==Cell structure and metabolism==
==Cell structure and metabolism==

Revision as of 09:47, 3 May 2007

A Microbial Biorealm page on the genus Bacillus anthracis

Classification

Higher order taxa

cellular organisms; Bacteria (domain); Firmicutes (phylum); Bacilli (class); Bacillales (order); Bacillaceae (family); Bacillus (genus); Bacillus cereus group

Genus

Bacillus anthracis


NCBI: Taxonomy

Description and significance

Bacillus anthracis is a Gram-positive, rod-shaped bacterium, 1 - 1.2µm in width and 3 - 5µm in length. It lives in soils worldwide at mesophilic temperatures. It can be grown in aerobic or anaerobic conditons (facultative anaerobe) in a medium with ordinary nutrients. In 1877, this organism was the first to be shown to cause disease by Dr. Robert Koch and verified by Dr. Louis Pasteur. The organism was isolated from sick animals and grown in the laboratory to study endospore formation. It is similar to Bacillus cereus and Bacillus thuringiensis in cellular size, morphology, and spore formation.


Bacillus anthracis is an important organism to study genome sequence because its use as a biological weapon creates concern. Genome sequencing can also be useful for the development of vaccines. The interactions between the host's immune system cells and the spores are an important area of research that will give us a better understanding of the anthrax disease. Development of better spore detectors will also be helpful.


Other names for this organisms include Bacteridium anthracis and Bacillus cereus var. anthracis. Common names include "anthrax" and "anthrax bacterium."

Genome structure

The single chromosome found in the Bacillus anthracis genome is a circular, 5,227,293 bp DNA molecule. The main virulent factors are encoded on two plasmids, pXO1 (189 kb, anthrax toxin) and pXO2 (96 kb, capsule genes). The plasmids are circular, extrachromosomal, double-stranded DNA molecules. The toxin is a complex of three plasmid-encoded proteins. Two of the proteins are directly toxic, including LF (lethal factor) and EF (edema factor). High LF levels destroys white blood cells and release the bacterium, while EF increases cyclic AMP levels. The host is more susceptible to infection with EF. Energy and water balance is impaired by the increase in cyclic AMP, resulting in the accumulation of fluid in cells. The other plasmid-encoded protein, named PA (protective antigen), ushers the two toxic proteins in cells. PA forms a multimeric ring, which inserts into the cell membranes of the host. PA is not toxic alone, but if it is inactivated, the two toxic proteins would not cause harm. This is because PA allows the toxic components to pass through the membrane via a special toxin delivery system. Capsule production depends on the pX02 plasmid. Capsule formation is important because it allows the organism to resist phagocytosis.

Cell structure and metabolism

The vegetative Bacillus anthracis cells are Gram-positive, therefore they contain an extensive peptidoglycan layer, lipoteichoic acids, and crystalline cell surface proteins (S-layer proteins). Bacillus anthracis differs from other Gram-positive bacteria in that it does not contain teichoic acids and the S-layer proteins are not glycosylated. Cell wall polysaccharides function in anchoring the protective S-layer to the cell wall. The cell wall polysaccharides are composed of galactose (Gal), N-acetylglucosamine (Glc-NAc), and N-acetylmannose (ManNAc)in a 3:2:1 ratio.


The capsule (slime layer) is a polymer of amino acids (D-glutamate), unlike most other bacteria which have polysaccharide capsules. The cells excrete the capsule for protection and virulence. The capsule and the S-layer are compatible, but they can both be formed independently (without the presence of the other). A characteristic mucoid or "smooth" colony variant is correlated with capsule production ability. Virulent strains all form the capsule, and "rough" colony capsules are avirulent. Growth in atmospheric CO2 cause the antiphagocytic capsule and anthrax toxin proteins to be synthesized. The nontoxic capsule has an important role in infection establishment, while the end disease phases are mediated by the toxin.


The genome of Bacillus anthracis contains one flagellin gene, however four essential proteins contain point mutations and frameshifts. Therefore, the flagellum are nonfunctional and the organism lacks motility. This is what distinguishes Bacillus anthracis from other Bacillus cereus group members.


When vegetative cells are deprived of certain nutrients, endospores are formed. Oxygen is necessary for spore formation. Initially, the septum forms asymmetrically in the nutrient deprived cells that produce large (mother cell) and small (forespore) genome containing compartments. The forespore is engulfed by the mother cell and surrounded with three layers (cortex, coat, and exosporium), which are simultaneously formed. The thickest and innermost layer is the cortex. It is made up of peptidoglycan. The coat, consisting of a large number of different proteins, tightly covers the cortex. The exosporium is a loose-fitting, balloon-like structure that encloses the spore and serves as a source of surface antigens. It is composed of an external hair-like nap and a paracrystalline basal layer. The hair-like nap has filaments that are mostly formed by a single collagen-like glycoprotein (called BclA), and the basal layer consists of a dozen different proteins. One of the proteins, BxpB (also called ExsF), is required for the attachment of the hair-like nap to the basal layer. Suppressing spore germination is another one of its roles. Large molecules that are a potential harm are excluded by the exosporium, which also serves as a semipermeable barrier.


The mother cell lyses and the spore is released when spore formation is finished. Spores can live in the soil and other inhospitable environments for many years because, once spores have matured, they are resistant to physical and chemical damage. They are highly resistant to heat, cold, dessication, radiation, and disinfectants. Spores germinate and grow as vegetative cells when they find an aqueous environment with the proper nutrients. Small-molecule germinants, including inosine and L-alanine, are recognized by spore receptors and activate germination. The receptors are found within the membrane of the spore that is under the cortex. Spores that enter a host germinate and grow, producing a fatal toxin.


Defense mechanisms are necessary for a bacteria to survive antimicrobial responses in the macrophage. Some of the antibacterial killing mechanisms include superoxide production by NADPH oxidase, hydrogen peroxide formation, generation of nitric oxide by nitric oxide synthase (NOS 2), defensin synthesis, and cationic protein activation. Superoxide dismutase (SOD) is the enzyme that regulates superoxide levels. Arginase, another protein, catalyzes the formation of L-ornithine and urea from L-arginine. Arginase regulates the production of nitric oxide by competing with NOS 2 for L-arginine. It is also involved in metabolite formation, including glutamic acid.

Ecology

Pathology

Bacillus anthracis causes the anthrax disease, which represents a complex interaction between the host and parasite. The particles of anthrax that are infectious are the Bacillus anthracis endospores. The organism penetrates into the blood stream and harms the host by producing toxins within the body. The slimy capsule layer that surrounds the organism allows it to resist phagocytosis by white cells.


The common disease forms are cutaneous, pulmonary, and gastrointestinal. The cutaneous form is caused by handling contaminated materials, and the pulmonary form is caused by inhalation. Skin abrasions allow spores to enter and cause local lesions by germinating there and developing gelatinous edema. Patients with a cutaneous anthrax disease mostly recover within 10 days, although a few progress to a life-threatening disease. Gastrointestinal anthrax is similar to cutaneous, but occurring on the intestinal mucosa. It is rare and has an extremely high mortality rate. The pulmonary form of the disease results in a higher mortality rate because the organism spreads through circulation. Macrophages in the lung's alveoli take up the spores and permit entry into the body. The infected macrophage lyses and bacteria is released into the blood stream, spreading though circulatory and lymphatic system. This results in septic shock, respiratory distress, and organ failure. Herbivorous animals become infected when they ingest spores from the soil. Experiments show that only about 3 x 106 cells/ml are needed to cause death in an animal. When humans contact infected animals (including flesh, bones, hides, hair and excrement), they become infected as well. Anthrax is almost never transmitted between people.


Until the 20th century, anthrax was a prevalent disease in humans and cattle. It is still an important pathogen in some countries today. Some scholars believe that the Egyptian plagues in the Bible may have been caused by Anthrax. However, most people had not heard of anthrax until the recent 2001 scare in the United States. Robert Koch and Louis Pasteur developed a vaccine against anthrax, which was the first infectious disease they studied. The vaccines today are not fully effective. However, if the disease is diagnosed soon enough after infection, antibiotic treatment is effective. Methods to detect the organism quickly and new vaccines are under development. Because Bacillus anthracis lives in many soils, outbreaks are still reported. In fact, in the upper Midwest of the United States, many farms are under quarantine due to anthrax.


During the 20th century anthrax was used as a weapon in many countries. It has also been directed toward farm animals for warfare. The significance of anthrax as a terror weapon was realized in 2001. Although small outbreaks can result in a strong response, some people argue that anthrax is not an ideal biological weapon because the organism is not particularly pathogenic. To infect people, a large number of spores are needed. The most effective form of anthrax is a very fine powder. Therefore, to make anthrax a weapon, the preparation needs to be grinded into a fine powder. Anticaking agents are necessary as well to prevent clumping of the spores. Bacillus anthracis can be grown easily, but it is important to have special containment facilities and to be careful when working with them. They can be engineered to be resistant to antibiotics even though they are usually sensitive to antibiotics including penicillin and ciprofloxacin.


The incidence (1-2 cases of cutaneous disease per year) of naturally acquired anthrax is rare in the United States. In fall 2001, intentional contamination of mail resulted in 22 cases of anthrax, of which 11 were inhalation and 11 cutaneous.

Application to Biotechnology

Current Research

References

[1] Biswa Choudhury, Christine Leoff, Elke Saile, Patricia Wilkins, Conrad P. Quinn, Elmar L. Kannenberg, and Russell W. Carlson The Structure of the Major Cell Wall Polysaccharide of Bacillus anthracis Is Species-specific J. Biol. Chem., Sep 2006; 281: 27932 - 27941.


[2] Hongbin Liu, Nicholas H. Bergman, Brendan Thomason, Shamira Shallom, Alyson Hazen, Joseph Crossno, David A. Rasko, Jacques Ravel, Timothy D. Read, Scott N. Peterson, John Yates III, and Philip C. Hanna. Formation and Composition of the Bacillus anthracis Endospore. J Bacteriol. 2004 January; 186(1): 164–178. doi: 10.1128/JB.186.1.164-178.2004.


[3] Jeremy A. Boydston, Ling Yue, John F. Kearney, and Charles L. Turnbough, Jr. "The ExsY Protein Is Required for Complete Formation of the Exosporium of Bacillus anthracis." J Bacteriol. 2006 November; 188(21): 7440–7448.


[4] Kimberly W. Raines, Tae Jin Kang, Stephen Hibbs, Guan-Liang Cao, John Weaver, Pei Tsai, Les Baillie, Alan S. Cross, and Gerald M. Rosen "Importance of Nitric Oxide Synthase in the Control of Infection by Bacillus anthracis." Infect Immun. 2006 April; 74(4): 2268–2276. doi: 10.1128/IAI.74.4.2268-2276.2006.


[5] NCBI Entrez Genome Project


[6] NCBI Entrez Nucleotide


[7] Rasko DA, et al. "The genome sequence of Bacillus cereus ATCC 10987 reveals metabolic adaptations and a large plasmid related to Bacillus anthracis pXO1." Nucleic Acids Res. 2004; 32(3): 977–988.


[8] Read TD, et al. "The genome sequence of Bacillus anthracis Ames and comparison to closely related bacteria." Nature. 2003 May 1;423(6935):81-6.


[9] Todar's Online Textbook of bacteriology.


[10] Wheeler DL, Chappey C, Lash AE, Leipe DD, Madden TL, Schuler GD, Tatusova TA, Rapp BA (2000). Database resources of the National Center for Biotechnology Information. Nucleic Acids Res 2000 Jan 1;28(1):10-4 [PubMed]


Schaechter, M., J.L. Ingraham, F.C. Neidhardt. Microbe. (ASM Press, Washington, DC, 2006).


Edited by Grace Ucar, student of Rachel Larsen and Kit Pogliano