Mycobacteria spp.

BACTERIA

Overview

• Includes obligate pathogens, opportunistic pathogens and saprophytes

Characteristics

• Aerobic acid-fast rods
• Non-motile, non-spore forming
• Cell walls contain mycolic acid
• Require egg-based media for growth

• M. bovis and M. tuberculosis and M. avium in tuberculosis of cattle and tuberculosis of pigs and tuberculosis of dogs

Overview

• Mycobacterial infections are caused by bacteria belonging to the family Mycobacteriaceae, order Actinomycetales.
• Mycobacterium sp. are aerobic, weakly gram-positive, non-spore forming, non-motile bacilli with wide variations in host affinity.
• Mycobacteria stain with carbol dyes and resist subsequent decolorization with inorganic acids. This characteristic which is due to the spatial arrangement of mycolic acids within the cell wall makes them acid fast.

• The ability of mycobacteria to survive and multiply within macrophages determines whether disease will occur within the host.
• Mycobacteria sp. utilize several virulence factors including cord factor or trehalose dimycolate, surface glycolipid, sulfatides, lipoarabinomannan, heteropolysaccharide, heat shock protein, complement, and tubuloprotein.
• The types of immune responses that are critical in responding to mycobacterial infection are cell-mediated immunity and the delayed hypersensitivity response.
• Pathogenicity of mycobacteria depends on their ability to escape phagocytic killing.
  • Mostly imparted by the cell wall consitiutents…
    • Cord factor (trehalose dimycolate) – surface glycolipid responsible for serpentine growth in vitro
    • Suphatides – surface glycolipid containing sulphur which prevents fusion of phagosome with lysosome. cAMP secreted by the bacteria may also facilitate this. Also something about the cholesterol content of the phagosome….. nature article
    • LAM – heteropolysaccharide which inhibits macrophage activation by IFNγ and induces macrophages to secrete TNFα -induces fever, etc and IL-10 which suppresses mycobacteria-induced T cell proliferation.
    • The wax of the cell wall, peptidoglycans and other glycolipids are responsible for the adjuvant activity – attracts APCs.
    • Tubuloprotein – important Ag, purified tubuloprotein is the basis of the tuberculin test.

Four major disease groups are recognised:

• The 'classical' tuberculosis lesions are caused by the Mycobacterium tuberculosis complex
• The Johne's type lesions and lesions caused by the MAC (M.avium complex) are Mycobacterium avium complex
  • Cause of Johne's Disease/ paratuberculosis.
• Mycobacteria causing leprosy (human and feline/murine) Mycobacterium leprae and M.lepraemurium
• Atypical mycobacteriosis is a localized opportunistic skin and subcutaneous infection caused by saprophytic and rapidly growing mycobacteria Atypical Mycobacterium
• Granulomatous lesions in muscle
• Granulomatous lesions in skin