10 line above mycobacteria
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10 line above mycobacteria
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Mycobacteria are a type of germ. There are many different kinds. The most common one causes tuberculosis. Another one causes leprosy. Still others cause infections that are called atypical mycobacterial infections.Mycobacteria are a type of germ. There are many different kinds. The most common one causes tuberculosis. Another one causes leprosy. Still others cause infections that are called atypical mycobacterial infections.
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Mycobacteria may be a possible causative agent as mycobacterial tuberculosis catalase-peroxidase protein has been identified in sarcoidosis tissue samples
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Basic Microbiology
Mycobacteria are Gram-positive, catalase positive, non-motile, non-spore forming rod-shaped bacteria (0.2–0.6 μm wide and 1.0–10 μm long). The colony morphology of mycobacteria varies with some species growing as rough or smooth colonies. Colony colour ranges from white to orange or pink (Iivanainen, 1999). Most mycobacteria are aerobic organisms, although some species are microaerophilc (Falkinham, 1996).
The cell walls of mycobacteria are very thick and consist of four layers. The innermost layer is composed of peptidoglycan and the others of lipids. The presence of lipid provides the bacteria with resistance to acid and alkaline environments and renders the cells relatively impermeable to various basic dyes, which need to be combined with phenol to allow penetration of the cell wall. The cell wall composition renders mycobacteria hydrophobic, and as a result these bacteria tend to grow in aggregates that ‘float’ on the surface of liquid media. Detergents, such as Tween® 80 can be added to culture media to disperse the organisms. Certain mycobacteria, including Mycobacterium paratuberculosis (MAP), can shed their cell walls, forming spheroplasts which are not detected using the acid-fast stain test (Hines and Styer, 2003).
Clinically, the most important species is Mycobacterium tuberculosis, which causes tuberculosis in humans. Tuberculosis in cattle and humans is also caused by Mycobacterium bovis, whilst Mycobacterium africanum is a rare cause of human tuberculosis in central Africa. Whilst these pathogenic species exhibit some phenotypic differences, they are genetically very similar and are therefore often classed as the ‘Mycobacterium tuberculosis complex’. Other pathogenic mycobacteria include the Mycobacterium avium complex (MAC) and the non-tuberculous mycobacteria (NTM, atypical mycobacteria, or environmental mycobacteria). Both these groups are frequent opportunistic pathogens, particularly in immunosuppressed individuals (Holland, 2001).
Mycobacteria can be divided into two groups based on growth rate. Rapid growers form visible colonies on solid media often within seven days, while slow growers, e.g. MAC, take longer to produce visible colonies.
Of the 140 known Mycobacterium species, the majority are environmental opportunistic pathogens existing as saprophytes in soil and water. NTM typically occur in natural water (von Reyn et al., 1994), drinking water distribution systems (Covert et al., 1999; Le Dantec et al., 2002), soils (Iivanainen et al., 1997) and dust. The major route of dissemination is by way of aerosols (Parker et al., 1993) and this is significant considering the recent discovery of spore formation for a number of mycobacteria including NTM. Spore formation is one reason why many NTM persist in the environment and also in or on patients for long periods of time, thereby facilitating skin and pulmonary infection.
Characteristics of the Genus
Mycobacteria are members of the order Actinomycetales, and the only genus in the family Mycobacteriaceae. Currently, the genus Mycobacterium has more than 100 recognized or proposed species, including numerous pathogens and saprophytic organisms of warm-blooded animals. The distinguishing characteristics of this genus include acid-fastness and the presence of mycolic acids. Mycobacteria are slender, non spore-forming, rod-shaped, aerobic, slow-growing, and free-living in soil and water. These bacteria have a generation time of about 20 h, thus isolation and identification may take up to 6 weeks (although a few species may grow in only 5–7 days).
These bacteria are acid-alcohol-fast, which means that after staining they resist decolorization with acidified alcohol as well as strong mineral acids. The property of acid-fastness, resulting from waxy materials in the cell walls, is particularly important for recognizing mycobacteria. The staining procedures must be carefully performed because other Gram-positive bacteria (e.g., Nocardia, Corynebacterium, and Rhodococcus) are often partially acid-fast