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Mycobacterium

 
Medical Dictionary: Mycobacterium
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n.

A genus of aerobic, nonmotile bacteria containing gram-positive rods and including both parasitic and saprophytic species.

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Veterinary Dictionary: Mycobacterium
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The only genus in the family Mycobacteriaceae of bacteria; slender acid-fast rods which may be straight or slightly curved. They may produce filaments or cocci. The most serious disease caused by members of this genus is tuberculosis. M. fortuitum, M. chelonea, M. marinum are listed as causes of piscine tuberculosis. Other species, including M. aquae, M. kansasii and M. scrofulaceum, may occasionally cause disease in a number of different species.

  • M. avium — found mostly in birds but occasionally also in other animals and in humans. The tubercle bacillus of birds, it causes avian tuberculosis.
  • M. avium subsp. paratuberculosis — causes Johne's disease in cattle, sheep, goats, deer and camelids. Previously called M. johnei and M. paratuberculosis.
  • M. avium–intracellulare — complex see M. intracellulare (below).
  • M. bovis — the tubercle bacillus of the bovine, it causes tuberculosis in many animal species and humans.
  • M. chelonei, M. fortuitum, M. phlei, M. smegmatis, M. thermoresistible — cause disease in a number of animal species, including mastitis in cattle and cutaneous mycobacterial granuloma in cats and dogs. See also opportunist (atypical) mycobacteria.
  • M. farcinogenes, M. senegalense — associated with bovine farcy.
  • M. genovense — causes mycobacteriosis in birds.
  • M. intracellulare — found in tuberculin-positive cattle and causes limited lymph node lesions in pigs. Closely related to M. avium and also described as M. avium–intracellulare complex.
  • M. johnei — see M. avium subsp. paratuberculosis (above).
  • M. kansasii — causes tuberculosis-like disease in pigs, deer and cattle.
  • M. leprae — the cause of leprosy in humans.
  • M. lepraemurium — causes murine and feline leprosy.
  • M. marinum — found in water, it causes tuberculosis in fish and skin ulcers in humans.
  • M. microti — the vole bacillus; lesions sometimes occur in other species.
  • M. paratuberculosis — previously called M. johnei. See M. avium subsp. paratuberculosis (above).
  • M. tuberculosis — the tubercle bacillus of humans, but found also in monkeys and pigs, and rarely in cattle, dogs and parrots.
  • M. ulcerans — causes skin ulcers in humans and cats.
  • M. xenopi — causes mycobacterial granuloma in cats and lymph node lesions in pigs.
Wikipedia: Mycobacterium
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Mycobacterium
TEM micrograph of M. tuberculosis.
Scientific classification
Kingdom: Bacteria
Phylum: Actinobacteria
Order: Actinomycetales
Suborder: Corynebacterineae
Family: Mycobacteriaceae
Genus: Mycobacterium
Lehmann & Neumann 1896
Species

See below.

Mycobacterium is a genus of Actinobacteria, given its own family, the Mycobacteriaceae. The genus includes pathogens known to cause serious diseases in mammals, including tuberculosis and leprosy.[1] The Latin prefix "myco—" means both fungus and wax; its use here relates to the "waxy" compounds in the cell wall.

Contents

Microbiologic characteristics

Mycobacterial cell wall: 1-outer lipids, 2-mycolic acid, 3-polysaccharides (arabinogalactan), 4-peptidoglycan, 5-plasma membrane, 6-lipoarabinomannan (LAM), 7-phosphatidylinositol mannoside, 8-cell wall skeleton

Mycobacteria are aerobic and nonmotile bacteria (except for the species Mycobacterium marinum, which has been shown to be motile within macrophages) that are characteristically acid-alcohol fast.[1] Mycobacteria do not contain endospores or capsules, and are usually considered Gram-positive. A recent paper in PNAS showed sporulation in Mycobacterium marinum and perhaps in M. bovis [2]. 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. All Mycobacterium species share a characteristic cell wall, thicker than in many other bacteria, which is hydrophobic, waxy, and rich in mycolic acids/mycolates. The cell wall consists of the hydrophobic mycolate layer and a peptidoglycan layer held together by a polysaccharide, arabinogalactan. The cell wall makes a substantial contribution to the hardiness of this genus. The biosynthetic pathways of cell wall components are potential targets for new drugs for tuberculosis.[3]

Many Mycobacterium species adapt readily to growth on very simple substrates, using ammonia or amino acids as nitrogen sources and glycerol as a carbon source in the presence of mineral salts. Optimum growth temperatures vary widely according to the species and range from 25 °C to over 50 °C.

Some species can be very difficult to culture (i.e. they are fastidious), sometimes taking over two years to develop in culture.[citation needed] Further, some species also have extremely long reproductive cycles — M. leprae, may take more than 20 days to proceed through one division cycle (for comparison, some E. coli strains take only 20 minutes), making laboratory culture a slow process.[1] In addition, the availability of genetic manipulation techniques still lags far behind that of other bacterial species.[4]

A natural division occurs between slowly– and rapidly–growing species. Mycobacteria that form colonies clearly visible to the naked eye within seven days on subculture are termed rapid growers, while those requiring longer periods are termed slow growers. Mycobacteria are straight or slightly curved rods between 0.2-0.6 µm wide by 1.0-10 µm long.

Pigmentation

Some mycobacteria produce carotenoid pigments without light. Others require photoactivation for pigment production.

Photochromogens (Group I)
Produce nonpigmented colonies when grown in the dark and pigmented colonies only after exposure to light and reincubation.
  • Ex: M. kansasii, M. marinum, M. simiae.
Scotochromogens (Group II)
Produce deep yellow to orange colonies when grown in either the light or dark.
  • Ex: M. scrofulaceum, M. gordonae, M. xenopi, M. szulgai.
Non-chromogens (Groups III & IV)
Nonpigmented in the light and dark or have only a pale yellow, buff or tan pigment that does not intensify after light exposure.
  • Ex: M. tuberculosis, M. avium-intra-cellulare, M. bovis, M. ulcerans
  • Ex: M. fortuitum, M. chelonae

Staining characteristics

Mycobacteria are classical acid-fast organisms.[5] Stains used in evaluation of tissue specimens or microbiological specimens include Fite's stain, Ziehl-Neelsen stain, and Kinyoun stain.

Mycobacteria appear phenotypically most closely related to members of Nocardia, Rhodococcus and Corynebacterium.

Ecological characteristics

Mycobacteria are widespread organisms, typically living in water (including tap water treated with chlorine) and food sources. Some, however, including the tuberculosis and the leprosy organisms, appear to be obligate parasites and are not found as free-living members of the genus.

Pathogenicity

Mycobacteria can colonize their hosts without the hosts showing any adverse signs. For example, billions of people around the world are infected with M. tuberculosis but will never know it because they will not develop symptoms.

Mycobacterial infections are notoriously difficult to treat. The organisms are hardy due to their cell wall, which is neither truly Gram negative nor positive, and unique to the family, they are naturally resistant to a number of antibiotics that work by destroying cell walls, such as penicillin. Also, because of this cell wall, they can survive long exposure to acids, alkalis, detergents, oxidative bursts, lysis by complement, and antibiotics, which naturally leads to antibiotic resistance. Most mycobacteria are susceptible to the antibiotics clarithromycin and rifamycin, but antibiotic-resistant strains are known to exist.

As with other bacterial pathogens, surface and secreted proteins of M. tuberculosis contribute significantly to the virulence of this organism. There is an increasing list of extracytoplasmic proteins proven to have a function in the virulence of M. tuberculosis.[6]

Medical classification

Mycobacteria can be classified into several major groups for purpose of diagnosis and treatment: M. tuberculosis complex, which can cause tuberculosis: M. tuberculosis, M. bovis, M. africanum, and M. microti; M. leprae, which causes Hansen's disease or leprosy; Nontuberculous mycobacteria (NTM) are all the other mycobacteria, which can cause pulmonary disease resembling tuberculosis, lymphadenitis, skin disease, or disseminated disease.

Phenotypic testing

Various phenotypic tests can be used to identify and distinguish different Mycobacteria species and strains.

Phenotypic testing of Mycobacteria

Mycosides

Mycosides are phenolic alcohols (such as phenolphthiocerol) that were shown to be components of mycobacterium glycolipids that are termed glycosides of phenolphthiocerol dimycocerosate (Smith DW et al., Nature 1960, 186, 887) There are 18 and 20 carbon atoms in mycosides A, and B, respectively.[7]

Species

Phylogenetic Position of the Tubercle Bacilli within the Genus Mycobacterium The blue triangle corresponds to tubercle bacilli sequences that are identical or differing by a single nucleotide. The sequences of the genus Mycobacterium that matched most closely to those of M. tuberculosis were retrieved from the BIBI database (http://pbil.univ-lyon.fr/bibi/) and aligned with those obtained for 17 smooth and MTBC strains. The unrooted neighbor-joining tree is based on 1,325 aligned nucleotide positions of the 16S rRNA gene. The scale gives the pairwise distances after Jukes-Cantor correction. Bootstrap support values higher than 90% are indicated at the nodes.

In older systems, mycobacteria are grouped based upon their appearance and rate of growth. However, these are symplesiomorphies, and more recent classification is based upon cladistics.

Slowly growing

Mycobacterium tuberculosis complex

Mycobacterium avium complex

Mycobacterium gordonae clade

Mycobacterium kansasii clade

Mycobacterium nonchromogenicum/terrae clade

Mycolactone-producing mycobacteria

Mycobacterium simiae clade

Ungrouped

Intermediate growth rate

Rapidly growing

Mycobacterium chelonae clade

Mycobacterium fortuitum clade

Mycobacterium parafortuitum clade

Mycobacterium vaccae clade

CF

Ungrouped

Ungrouped

References

  1. ^ a b c Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9. 
  2. ^ Ghosh, Jaydip, Pontus Larsson, Bhupender Singh, B M Fredrik Pettersson, Nurul M Islam, Sailendra Nath Sarkar, Santanu Dasgupta, y Leif A Kirsebom. 2009. Sporulation in mycobacteria. Proceedings of the National Academy of Sciences of the United States of America 106, no. 26 (Junio 30): 10781-10786. http://www.ncbi.nlm.nih.gov/pubmed/19541637
  3. ^ Bhamidi S (2009). "Mycobacterial Cell Wall Arabinogalactan". Bacterial Polysaccharides: Current Innovations and Future Trends. Caister Academic Press. ISBN 978-1-904455-45-5. 
  4. ^ Parish T, Brown A (editors) (2009). Mycobacterium: Genomics and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-40-0. 
  5. ^ McMurray DN (1996). "Mycobacteria and Nocardia". in Baron S et al. (eds.). Baron's Medical Microbiology (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1. 
  6. ^ McCann et al. (2009). "Secreted and Exported Proteins Important to Mycobacterium tuberculosis Pathogenesis". Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis. Caister Academic Press. ISBN 978-1-904455-42-4. 
  7. ^ fatty alcohols and aldehydes

Further reading

External links


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Medical Dictionary. The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company.  Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Mycobacterium".  Read more