Acid Fast Stain

Aka: Acid Fast Stain, Ziehl–Neelsen Stain, Acid Fast Bacteria, Acid Fast Stain Positive, Acid Fast Bacillus

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II. Background

  1. Lipid rich cell walls (Mycobacterium, Nocardia) retain red stain on Acid Fast Staining

III. Technique

  1. Carbolfuchsin (red stain) is applied to a sample smear and then heated
  2. Acid Alcohol (HCL 3% and 95% Ethanol) is applied to smear to remove red stain
    1. Despite the acid Alcohol application, lipid rich cell walls retain the red stain
  3. Methylene blue stain is applied as a counter stain

IV. Interpretation

  1. Acid Fast Positive
    1. Lipid-rich cell walls retain red stain
    2. Acid Fast Bacilli
      1. Mycobacterium species
      2. Nocardia species (partially acid-fast, also considered Gram Positive)
    3. Non-Bacteria may also be acid-fast (e.g. Bacterial endospores, Cryptosporidium parvum)
  2. Acid Fast Negative
    1. Non-Acid Fast Bacteria instead stain blue

V. Resources

  1. Acid Fast Bacteria (StatPearls)
    1. https://www.ncbi.nlm.nih.gov/books/NBK537121/
  2. Acid Fast Stain Protocol (American Society of Microbiology)
    1. https://asm.org/ASM/media/Protocol-Images/Acid-Fast-Stain-Protocols.pdf?ext=.pdf

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