The Acid Fast Cell Wall

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The Acid Fast Cell Wall

• Dr. Jennifer Coetzee

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Definition

• Cell wall that retains carbol fuchsin dye in the
  presence of acid alcohol
• Stains use in lab to detect acid fastness
• Ziehl-Neelsen
• Kinyoun

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Organisms with Acid Fast Cell Wall

• Mycobacteria
• Mycobacterium tuberculosis
• Mycobacterium leprae
• Mycobacterium bovis
• 57 other mycobacterial species!
• Nocardia species
• Nocardia asteroides
• And many more

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Structure of an Acid Fast Cell Wall

• 3 major molecules
• Peptidoglycans
• Contains N-glycolylmuramic acid instead of
  N-acetylmuramic acid found in other bacteria
• Prevents osmotic lysis
• Arabinogalactans
• 2 lipopolysaccharides, lipoarabinomannan (LAM)
  and lipomannan
• Mycolic acids

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Mycolic Acids

• Almost exclusive to mycobacteria
• Essential to acid fast staining properties
• Up to 60 of composition of mycobacterial cell
  wall
• Branched fatty acids with a short and long branch
  (22-24 and 40-64 carbons)
• MTB genome 4000 genes
• Mostly coding for enzymes involved in lipolysis
  and lipogenesis

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Lipoarabinomannan

• Directs macrophage function
• Inhibits the processing of mycobacterial
  peptides/proteins by antigen presenting cells
• Induces tumor necrosis factor (TNF-?)
• Considered a mycobacterial virulence factor

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Permeability of Cell Wall

• Membrane proteins called porins control diffusion
  of hydropilic molecules
• Relatively impermeable to hydrophilic solutes
• Permeability of lipophilic molecules
• Inner leaflet low fluidity, nearly crystalline
  structure
• Diffusion through lipid bilayer requires fluid
  interior
• Bilayer uncommonly thick

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Clinical Implications

• High degree of intrinsic resistance to most
  antibiotics
• Low permeability
• Unusual structure
• Slow growth rate
• Clinicians awaiting culture results!

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• Relatively resistant to drying, alkali, and many
  more chemical disinfectants
• Very difficult to prevent transmission of M.
  tuberculosis in institutions
• Infection control implications
• Target for design of new drugs
• Vaccine development
• Better understanding of pathogenesis, immunity

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Summary

• Cell wall structure totally different from other
  bacteria
• Mycolic acids major component, responsible for
  acid fast staining characteristic
• Also peptidoglycan, arabinogalactans
• Clinical implications of the above

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References

• Rodrigo Gay Ducati et al. The resumption of
  consumption. Mem Inst Oswaldo Cruz, 101(7), 2006
• Brennan PJ, Nikaido H. The envelope of
  mycobacteria. Annu Rev Biochem. 19956429-63
• Barry CE, Mdluli K. Drug sensitivity and
  environmental adaptation of mycobacterial cell
  wall components. Trends Microbiol.
  19964(7)275-81