MYCOBACTERIUM

84 plus species including phototrophic groups 1 - 5:

SPECIES

(Slow Growing, >7 Days)

(Rapidly Growing, <7 Days)

MICROSCOPIC APPEARANCE

Gram Stain: Positive, but stain poorly.
Morphology: Slightly curved, straight bacilli; filamentous or mycelial like growth may occur. Pigmentation noticeable in some strains.
Size: 0.2-0.8 micrometers by 1.0-10.0 micrometers.
Motility: Non-motile.
Capsules: None.
Spores: None.
Other: Growth is usually very slow and colonies tend to appear in 2-60 days at optimum temperature.

MACROSCOPIC APPEARANCE

Group I Photochromogens Produce a light yellow color when grown in the presence of light.
Group II Scotochromogens Produce a yellow pigment even when grown in the dark; when grown in the light, the pigment is orange.
Group III Nonchromagens Appear colorless or slowly produce a light yellow pigment when grown in the light.
Group IV Non-pigmented,
rapid growers
Appear colorless.

Most strains of mycobacteria form more than one kind of colony, but colonies of some species, as as M. tuberculosis , are regularly rough; some, as of M. intracellulare on primary culture from clinical specimens, are more commonly smooth. Cells of rough strains are usually compacted on curving strands; cells of smooth strains are not visibly oriented in any pattern. Some colonies, as of M. fortuitum and M. xenopi , in early growth may be mycelial, older ones exhibiting branching filamentous extensions on and into some media such as Cornmeal Glycerol Agar; fragmentation to bacilli usually occurs in smear preparation. Aerial filamentous extensions rare, never visible without magnification (x30-100). Pigment is not diffusible and the surface is generally dull or rough.

METABOLIC PROPERTIES

Aerobic, with increased CO 2 , acid-fast at least during early growth stages. Typically Chemoorganotrophic, having a respiratory type metabolism. Some species are fastidious, requiring special supplements while others are noncultivable. M. genavense requires human blood for optimum growth.

KEY BIOCHEMICAL REACTIONS

All species maintain the following biochemical reactions:

HABITAT

Most species are free living in soil and water but a major ecological niche for some species is the diseased tissue of warm-blooded hosts (obligate parasites, saprophytes and intermediate forms). With large enough inoculum all mycobacteria produce granulomatous lesions in experimental animals. However, only some of the species grow in host tissues and produce progressive or self-limiting disease. Individual species pathogenic for cold-blooded animals, mammals and/or birds. Depending on species, may show predilection for internalorgans, especially the lungs, skin, nerves, and intestinal tract.

PATHOGENICITY

Pathogenic to humans and animals. Causative agent of tuberculosis (in humans, fowl, and cattle), leprosy, and other chronic, more or less necrotizing, limited or extensive granulomas and infections in humans.

Mycobacteria should be handled in a biosafety cabinet to prevent dissemination in case the human pathogen Mycobacterium tuberculosis should occur among the cultures. (1,2)

RECOMMENDED MEDIA

For culture: Egg Medium, Lowenstein-Jensen Media, Middlebrook Media.
For selective isolation: Lowenstein-Jensen Media or Egg Medium.
For maintenance: Egg Medium, Middlebrook 7H10/7H11 Agar,
Tween ® 80 for short-term maintenance and lyophilization for long-term preservation.

INCUBATION

Temperature: a. 35 degrees C. (in closed tubes or bottles).
b. 30-33 degrees C. (material taken from superficial lesions).
Time: a. 8-10 weeks (in closed tubes or bottles).
b. Up to 12 weeks (material taken from superficial lesions).
Atmosphere: Aerobic with increased CO 2 (5-10%)

REFERENCES

1. Holt, J.G., et al. 1994. Bergey's Manual of Determinative Bacteriology , 9th ed. Williams & Wilkins, Baltimore, MD.

2. Holt, J.G., et al. 1986. Bergey's Manual of Systemic Bacteriology , Vol. I & II. Williams & Wilkins, Baltimore, MD.

3. The Oxoid Vade-Mecum of Microbiology . 1993. Unipath Ltd., Basingstoke, UK.

4. Murray, P.R., et al. 1995. Manual of Clinical Microbiology , 6th ed. American Society for Microbiology, Washington, D.C.

5. Internet: www.hardlink.com /Bacterial Database Search, February, 1998.

6. Hensyl, B.R., et al. 1990. Stedman's Medical Dictionary , 25th ed. Williams & Wilkins, Baltimore, MD.

7. Koneman, et al. 1997. Color Atlas and Textbook of Diagnostic Microbiology , 5th ed. Lippincott, Philadelphia, PA.


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