Mycobacterium tuberculosis: Difference between revisions
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{{ | {{Tuberculosis}} | ||
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==Overview== | ==Overview== | ||
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[[Category:Gram positive bacteria]] | [[Category:Gram positive bacteria]] | ||
[[Category:Infectious disease]] | [[Category:Infectious disease]] | ||
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==Overview== | |||
The primary cause of tuberculosis is ''[[Mycobacterium tuberculosis]]'', an [[aerobic organism|aerobic]] [[bacterium]]. | |||
==Causes== | |||
[[Image:Mycobacterium tuberculosis.jpg|300px|left|thumb|Scanning electron micrograph of ''[[Mycobacterium tuberculosis]]'']] | |||
The primary cause of TB, ''[[Mycobacterium tuberculosis]]'' (M. TB), is an [[aerobic organism|aerobic]] [[bacterium]] that [[cell division|divides]] every 16 to 20 hours, an extremely slow rate compared with other bacteria, which usually divide in less than an hour.<ref name=Cox_2004>{{cite journal |author=Cox R |title=Quantitative relationships for specific growth rates and macromolecular compositions of ''Mycobacterium tuberculosis'', ''Streptomyces coelicolor'' A3(2) and ''Escherichia coli'' B/r: an integrative theoretical approach |journal=Microbiology |volume=150 |issue=Pt 5 |pages=1413–26 |year=2004 |url=http://mic.sgmjournals.org/cgi/content/full/150/5/1413?view=long&pmid=15133103#R35 | pmid = 15133103}}</ref> (For example, one of the fastest-growing bacteria is a strain of ''[[E. coli]]'' that can divide roughly every 20 minutes.) Since MTB has a cell wall but lacks a [[phospholipid]] [[Bacterial cell structure|outer membrane]], it is [[Tuberculosis classification|classified]] as a [[Gram-positive]] bacterium. However, if a [[Gram stain]] is performed, MTB either stains very weakly Gram-positive or does not retain dye due to the high lipid & [[mycolic acid]] content of its cell wall.<ref name=Madison_2001>{{cite journal |author=Madison B |title=Application of stains in clinical microbiology. |journal=Biotech Histochem |volume=76 |issue=3 |pages=119-25 |year=2001 |pmid=11475314}}</ref> MTB is a small rod-like [[bacillus]] that can withstand weak [[disinfectant]]s and survive in a [[Endospore|dry state]] for weeks. In nature, the bacterium can grow only within the cells of a [[host (biology)|host]] organism, but ''M. tuberculosis'' can be cultured ''[[in vitro]]''.<ref name=Parish_1999>{{cite journal |author=Parish T, Stoker N |title=Mycobacteria: bugs and bugbears (two steps forward and one step back) |journal=Mol Biotechnol |volume=13 |issue=3 |pages=191–200 |year=1999 | pmid = 10934532}}</ref> | |||
Using certain [[histology|histological]] techniques on expectorate samples from [[phlegm]] (also called sputum), scientists can identify MTB under a regular microscope. Since MTB retains certain stains after being treated with acidic solution, it is classified as an [[acid-fast bacillus]] (AFB).<ref name=Madison_2001>{{cite journal |author=Madison B |title=Application of stains in clinical microbiology |journal=Biotech Histochem |volume=76 |issue=3 |pages=119-25 |year=2001 | pmid = 11475314}}</ref> The most common staining technique, the [[Ziehl-Neelsen stain]], dyes AFBs a bright red that stands out clearly against a blue background. Other ways to visualize AFBs include an [[auramine-rhodamine stain]] and [[Fluorescence microscope|fluorescent microscopy]]. | |||
The ''M. tuberculosis'' complex includes 3 other TB - causing [[mycobacterium|mycobacteria]]: ''[[Mycobacterium bovis|M. bovis]]'', ''[[Mycobacterium africanum|M. africanum]]'' and ''[[Mycobacterium microti|M. microti]]''. The first two only very rarely cause disease in [[immunocompetent]] people. On the other hand, although ''M. microti'' is not usually [[pathogen]]ic, it is possible that the [[prevalence]] of ''M. microti'' infections has been underestimated.<ref name=Niemann_2000>{{cite journal |author=Niemann S, Richter E, Dalügge-Tamm H, Schlesinger H, Graupner D, Königstein B, Gurath G, Greinert U, Rüsch-Gerdes S |title=Two cases of ''Mycobacterium microti'' derived tuberculosis in HIV-negative immunocompetent patients |journal=Emerg Infect Dis |volume=6 |issue=5 |pages=539-42 |year=2000 |pmid = 10998387}}</ref> | |||
Other known pathogenic [[Mycobacterium|mycobacteria]] include ''[[Mycobacterium leprae]]'', [[Mycobacterium avium complex|''Mycobacterium avium'']] and ''M. kansasii''. The last two are part of the [[nontuberculous mycobacteria]] (NTM) group. Nontuberculous mycobacteria cause neither TB nor [[leprosy]], but they do cause pulmonary diseases resembling TB.<ref name=ALA_1997>{{cite journal |author= |title=Diagnosis and treatment of disease caused by nontuberculous mycobacteria. This official statement of the American Thoracic Society was approved by the Board of Directors, March 1997. Medical Section of the American Lung Association |journal=Am J Respir Crit Care Med |volume=156 |issue=2 Pt 2 |pages=S1–25 |year=1997 |pmid = 9279284}}</ref> | |||
===Evolution=== | |||
During its evolution, ''M. tuberculosis'' has lost numerous [[Coding region|coding]] and non-coding regions in its [[genome]], losses that can be used to distinguish between strains of the bacteria. The implication is that ''M. tuberculosis'' strains differ geographically, so their genetic differences can be used to track the origins and movement of each strain.<ref name=Rao_2005>{{cite journal |author=Rao K, Kauser F, Srinivas S, Zanetti S, Sechi L, Ahmed N, Hasnain S |title=Analysis of genomic downsizing on the basis of region-of-difference polymorphism profiling of Mycobacterium tuberculosis patient isolates reveals geographic partitioning |journal=J Clin Microbiol |volume=43 |issue=12 |pages=5978–82 |year=2005 | pmid = 16333085}}</ref> | |||
===Morphology=== | |||
Slender, straight or slightly curved bacillus with rounded ends, occuring singly, in pairs or in small clumps. | |||
These bacilli are acid fast, non sporing, non capsulated and non motile. | |||
===Physiology=== | |||
''M. tuberculosis'' is an [[obligate aerobe]] ( weakly [[Gram-positive]] [[mycobacterium]], hence Z.N. staining is used). While mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint (i.e. they do not retain the crystal violet stain), they are classified as an [[acid-fast]] Gram positive bacterium due to their lack of an outer cell membrane. ''M. tuberculosis'' divides every 15 to 20 hours—extremely slowly compared to other bacteria, which tend to have division times measured in minutes (for example, ''E. coli'' can divide roughly every 20 minutes). It is a small, rod-like [[bacillus]] that can withstand weak [[disinfectant]]s and can survive in a dry state for weeks. | |||
===Resistance=== | |||
Mycobacteria are killed at 60 degree celsius in 15-20 minutes. | |||
They are sensitive to UV rays and sunlight. | |||
They are relatively resistant to 5% phenol, 15% sulphuric acid, 5% oxalic acid, 4% sodium hydroxide. | |||
The bacillus are destroyed by tincture of sodium in five minutes and by 80% ethanol in 2-10 minutes. | |||
===Diagnosis=== | |||
Sputum is taken in 3 successive mornings as the number of organisms could be low, and the specimen is treated with 3% [[Potassium_hydroxide|KOH]] or [[Sodium_hydroxide|NaOH]] for liquefaction and decontamination. [[Gram stain]] should never be performed as the organism is an "[[acid-fast]] bacillus" (AFB), meaning that it retains certain stains after being treated with acidic solution. In the most common staining technique, the [[Ziehl-Neelsen stain]], AFB are stained a bright red, which stands out clearly against a blue background; therefore, the bacteria are sometimes called ''red snappers''.<ref>{{cite journal |author=Flowers T |title=Quarantining the noncompliant TB patient: catching the "Red Snapper" |journal=Journal of health and hospital law : a publication of the American Academy of Hospital Attorneys of the American Hospital Association |volume=28 |issue=2 |pages=95-105 |year=1995 |pmid=10141473 |doi=}}</ref> The reason for the acid-fast staining is because of its thick waxy cell wall.<ref name=Brock>{{cite book | author = Madigan, Michael; Martinko, John (editors) | title = Brock Biology of Microorganisms | edition = 11th ed. | publisher = Prentice Hall | year = 2005 | id = ISBN 0-13-144329-1 }}</ref> The waxy quality of the cell wall is mainly due to the presence of [[Mycolic acid|mycolic acids]]. This waxy cell wall also is responsible for the typical [[Caseous necrosis|caseous]] [[granuloma]] formation in tuberculosis. The component responsible, trehalose dimycolate, is called the cord factor. A grading system exists for interpretation of the microscopic findings based on the number of organisms obsereved in each field. Acid-fast bacilli can also be visualized by fluorescent microscopy using auramine-rhodamine stain which makes them appear somewhat golden in color. Also, ''M. tuberculosis'' is grown on a selective medium known as Lowenstein-Jensen medium which have traditionally been used for this purpose. However, this method is quite slow; as this organism requires 6-8 months to grow which certainly delays reporting of results. A faster results can now be obtained using Middlebrook medium. | |||
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Soumya Sachdeva; João André Alves Silva, M.D. [2]
Overview
Mycobacterium tuberculosis is the bacterium that causes most cases of tuberculosis.[1] It was first described on March 24, 1882 by Robert Koch, who subsequently received the Nobel Prize in physiology or medicine for this discovery in 1905; the bacterium is also known as Koch's bacillus. The M. tuberculosis genome was sequenced in 1998.[2][3]
Taxonomy
Bacteria; Actinobacteria; Actinobacteria; Actinobacteridae; Actinomycetales; Corynebacterineae; Mycobacteriaceae; Mycobacterium; M. tuberculosis
Biology
M. tuberculosis is an obligate aerobe ( weakly Gram-positive mycobacterium, hence Z.N. staining is used). While mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint (i.e. they do not retain the crystal violet stain), they are classified as an acid-fast Gram positive bacterium due to their lack of an outer cell membrane.[1] M. tuberculosis divides every 15 to 20 hours—extremely slowly compared to other bacteria, which tend to have division times measured in minutes (for example, E. coli can divide roughly every 20 minutes). It is a small, rod-like bacillus that can withstand weak disinfectants and can survive in a dry state for weeks.
Structure
Slender, straight or slightly curved bacillus with rounded ends, occuring singly, in pairs or in small clumps. These bacilli are acid fast, non sporing, non capsulated and non motile.
Resistance
Mycobacteria are killed at 60 degree celsius in 15-20 minutes. They are sensitive to UV rays and sunlight. They are relatively resistant to 5% phenol, 15% sulphuric acid, 5% oxalic acid, 4% sodium hydroxide. The bacillus are destroyed by tincture of sodium in five minutes and by 80% ethanol in 2-10 minutes.
Diagnosis
Sputum is taken in 3 successive mornings as the number of organisms could be low, and the specimen is treated with 3% KOH or NaOH for liquefaction and decontamination. Gram stain should never be performed as the organism is an "acid-fast bacillus" (AFB), meaning that it retains certain stains after being treated with acidic solution. In the most common staining technique, the Ziehl-Neelsen stain, AFB are stained a bright red, which stands out clearly against a blue background; therefore, the bacteria are sometimes called red snappers.[4] The reason for the acid-fast staining is because of its thick waxy cell wall.[5] The waxy quality of the cell wall is mainly due to the presence of mycolic acids. This waxy cell wall also is responsible for the typical caseous granuloma formation in tuberculosis. The component responsible, trehalose dimycolate, is called the cord factor. A grading system exists for interpretation of the microscopic findings based on the number of organisms obsereved in each field. Acid-fast bacilli can also be visualized by fluorescent microscopy using auramine-rhodamine stain which makes them appear somewhat golden in color. Also, M. tuberculosis is grown on a selective medium known as Lowenstein-Jensen medium which have traditionally been used for this purpose. However, this method is quite slow; as this organism requires 6-8 months to grow which certainly delays reporting of results. A faster results can now be obtained using Middlebrook medium.
References
- ↑ 1.0 1.1 Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0-8385-8529-9.
- ↑ Cole ST; Brosch R; Parkhill J; et al. (1998). "Deciphering the biology of Mycobacterium tuberculosis from the complete genome sequence". Nature. 393: 537&ndash, 544.
- ↑ Camus JC; Pryor MJ; Medigue C; Cole ST. (148). "Re-annotation of the genome sequence of Mycobacterium tuberculosis H37Rv". Microbiology. 2002: 2967&ndash, 2973.
- ↑ Flowers T (1995). "Quarantining the noncompliant TB patient: catching the "Red Snapper"". Journal of health and hospital law : a publication of the American Academy of Hospital Attorneys of the American Hospital Association. 28 (2): 95–105. PMID 10141473.
- ↑ Madigan, Michael; Martinko, John (editors) (2005). Brock Biology of Microorganisms (11th ed. ed.). Prentice Hall. ISBN 0-13-144329-1.