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Mycobacteria)
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Mycobacterium is the a genus of actinobacteria, given its own family, the Mycobacteriaceae. It includes many pathogens known to cause serious diseases in mammals, including tuberculosis and leprosy. Most mycobacteria are classified into two categories, the fast-growing kind and the slow-growing kind, and most mycobacteria share some common characteristics:
- They are widespread organisms, typically living in water (including tap water treated with chlorine) and food sources.
- They can colonize their hosts without the hosts showing any adverse signs. For example, millions of people around the world are infected with M. tuberculosis but will never know it because they will not develop symptoms.
- All mycobacteria are aerobic and acid fast. As a genus, they share a characteristic cell wall, thicker than in many other bacteria, hydrophobic, waxy and rich in mycolic acids /mycolates . The mycobacterial cell wall makes a substantial contribution to the hardiness of this genus.
- Mycobacterial infections are notoriously difficult to treat. The organisms are hardy and can survive long exposure to 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.
- Mycobacteria tend to be fastidious (difficult to culture), sometimes taking over two years to develop in culture. As well as being fastidious, some species also have extremely long reproductive cycles ( M. leprae, for example, may take more than 20 days to proceed through one division cycle; E. coli, for comparison, takes only half an hour ), making laboratory culture a slow process.
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. micoti
- 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.
Species
- M. abscessus , which is also a common water contaminant and was until recently thought to be a subspecies of M. chelonae.
- M. africanum
- M. asiaticum
- M. avium complex (MAC), which is a significant cause of death in AIDS patients. This complex also includes M. avium paratuberculosis, which has been implicated in Crohn's disease in humans and Johne's disease in sheep.
- M. bovis
- M. chelonae , which is a common water contaminant and can also infect wounds.
- M. fortuitum
- M. gordonae
- M. haemophilum
- M. intracellulare
- M. kansasii , which can cause life-threatening infections in people with compromised immune systems
- M. leprae, which causes leprosy
- M. malmoense
- M. marinum
- M. microti
- M. scrofulaceum
- M. smegmatis
- M. tuberculosis, which causes tuberculosis
- M. ulcerans
- M. uvium
- M. xenopi
Staining
Reference
- Diagnosis and Treatment of Disease Caused by Nontuberculous Mycobacteria. American Thoracic Society. Am J Respiratory and Critical Care Medicine. Aug 1997 156(2) Part 2 Supplement PDF format
See also