Tuberculosis causes: Difference between revisions
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===Physiology=== | ===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'' 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=== | ===Resistance=== | ||
Revision as of 18:35, 19 May 2014
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Tuberculosis Microchapters |
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Diagnosis |
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Treatment |
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Case Studies |
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Tuberculosis causes On the Web |
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American Roentgen Ray Society Images of Tuberculosis causes |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
The primary cause of tuberculosis is Mycobacterium tuberculosis, an aerobic bacterium.
Causes
The primary cause of TB, Mycobacterium tuberculosis (M. TB), is an aerobic bacterium that divides every 16 to 20 hours, an extremely slow rate compared with other bacteria, which usually divide in less than an hour.[1] (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 outer membrane, it is 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.[2] MTB is a small rod-like bacillus that can withstand weak disinfectants and survive in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but M. tuberculosis can be cultured in vitro.[3]
Using certain 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).[2] 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 fluorescent microscopy.
The M. tuberculosis complex includes 3 other TB - causing mycobacteria: M. bovis, M. africanum and M. microti. The first two only very rarely cause disease in immunocompetent people. On the other hand, although M. microti is not usually pathogenic, it is possible that the prevalence of M. microti infections has been underestimated.[4]
Other known pathogenic mycobacteria include Mycobacterium leprae, 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.[5]
Evolution
During its evolution, M. tuberculosis has lost numerous 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.[6]
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 disinfectants 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% 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.[7] The reason for the acid-fast staining is because of its thick waxy cell wall.[8] 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
- ↑ Cox R (2004). "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". Microbiology. 150 (Pt 5): 1413–26. PMID 15133103.
- ↑ 2.0 2.1 Madison B (2001). "Application of stains in clinical microbiology". Biotech Histochem. 76 (3): 119–25. PMID 11475314.
- ↑ Parish T, Stoker N (1999). "Mycobacteria: bugs and bugbears (two steps forward and one step back)". Mol Biotechnol. 13 (3): 191–200. PMID 10934532.
- ↑ Niemann S, Richter E, Dalügge-Tamm H, Schlesinger H, Graupner D, Königstein B, Gurath G, Greinert U, Rüsch-Gerdes S (2000). "Two cases of Mycobacterium microti derived tuberculosis in HIV-negative immunocompetent patients". Emerg Infect Dis. 6 (5): 539–42. PMID 10998387.
- ↑ "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". Am J Respir Crit Care Med. 156 (2 Pt 2): S1–25. 1997. PMID 9279284.
- ↑ Rao K, Kauser F, Srinivas S, Zanetti S, Sechi L, Ahmed N, Hasnain S (2005). "Analysis of genomic downsizing on the basis of region-of-difference polymorphism profiling of Mycobacterium tuberculosis patient isolates reveals geographic partitioning". J Clin Microbiol. 43 (12): 5978–82. PMID 16333085.
- ↑ 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.