Download Cycle 29 Organism 2 - Mycobacteriaceae

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MICROBIOLOGY LEGEND
CYCLE 29 – ORGANISM 2
Mycobacteriaceae
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 (Mycobacterium tuberculosis) and leprosy
(Mycobacterium leprae). The Latin prefix "myco—" means both fungus and wax; its use here reflects the "waxy"
compounds that compose parts of the cell wall.
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. Mycobacteria do not contain endospores
or capsules and are usually considered Gram-positive. However, this has been strongly argued by other scientists. While
Mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint (i.e. they generally do not
retain the crystal violet stain well), 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.
Mycobacterium tuberculosis bacteria using
Mycobacterium tuberculosis
acid-fast Ziehl-Neelsen stain
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 sometimes
taking over two years to develop in culture. Furthermore, some species also have extremely long reproductive cycles —
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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. In addition, the availability of genetic manipulation
techniques still lags far behind that of other bacterial species.
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 cells are straight or slightly curved rods between 0.2-0.6 µm wide and 1.0-10 µm long.
Staining characteristics
Mycobacteria are classical acid-fast organisms. 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 have asymptomatic infections of M. tuberculosis. Mycobacterial infections are notoriously difficult to
treat. The organisms are hardy due to their cell wall, which is neither truly Gram negative nor positive. Additionally, they
are naturally resistant to a number of antibiotics that disrupt cell-wall biosynthesis, such as penicillin. Due to their
unique cell wall, they can survive long exposure to acids, alkalis, detergents, oxidative bursts, lysis by complement, and
many antibiotics. Most Mycobacteria are susceptible to the antibiotics, clarithromycin and rifamycin, but antibioticresistant strains have emerged.
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.
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.
References
1. Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill.
2. http://en.wikipedia.org/wiki/Mycobacterium
Questions
1. Discuss the morphological characteristics of Mycobacteria.
2. How would you identify Mycobacteria in your laboratory?
3. Discuss the pathogenicity of Mycobacteria.
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