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Acid-fast bacilli

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Title: Acid-fast bacilli  
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Subject: AFB, Tuberculous meningitis
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Acid-fast bacilli

Acid-fastness is a physical property of certain bacteria (and, less commonly, protozoa), specifically their resistance to decolorization by acids during staining procedures.[1][2]

Acid-fast organisms are difficult to characterize using standard microbiological techniques (e.g. Gram stain - if an acid-fast bacillus (AFB) was gram stained, the result would be an abnormal gram positive organism, which would indicate that further testing was necessary), though they can be stained using concentrated dyes, particularly when the staining process is combined with heat. Once stained, these organisms resist the dilute acid and/or ethanol-based de-colorization procedures common in many staining protocols, hence the name acid-fast.[2]

The high mycolic acid content of certain bacterial cell walls, like those of Mycobacteria, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the Ziehl-Neelsen stain, in which the acid fast bacilli are stained bright red and stand out clearly against a blue background. Another method is the Kinyoun method, in which the bacteria are stained bright red and stand out clearly against a green background. Acid-fast bacteria can also be visualized by fluorescence microscopy using specific fluorescent dyes (auramine-rhodamine stain, for example).[3] Some bacteria may also be partially acid-fast. The eggs of the parasitic lung fluke Paragonimus westermani are actually destroyed by the stain, which can hinder diagnosis in patients who present with TB-like symptoms.

Some Acid Fast Staining techniques

  1. Ziehl–Neelsen stain (Classic and modified bleach types)[4]
  2. Life team
  3. For color blind people (or in backgrounds where detecting red bacteria is difficult) Victoria Blue can be substituted for carbol fuchsin and Picric acid can be used as the counter stain instead of methylene blue, and rest of Kinyoun technique can be used.[5]
    1. Various bacterial spore staining techniques using Kenyon e.g.
      1. Moeller's method
      2. Dorners method[6] (acid alcohol decolorizer) without the Schaeffer–Fulton[7] modification (decolorize by water)[8]
      3. Detergent method, using Tergitol 7, nonionic polyglycol ether surfactants type NP-7 [9]
  4. Fite stain[10]
    1. Fite-Faraco Staining[11][12]
    2. Wade Fite staining[13]
  5. Ellis and Zabrowarny stain[14][15] (no Phenol/carbolic acid)
  6. Auramine-rhodamine stain
  7. Auramine phenol stain

Notable acid-fast structures

Very few structures are acid fast; this makes staining for acid-fastness particularly useful in diagnosis. The following are notable examples of structures which are acid fast:

  • All Mycobacteria - M. tuberculosis, M. leprae, M. smegmatis and atypical Mycobacterium
  • Actinomyces (especially some aerobic ones) with Mycolic acid in their cell wall (note Streptomyces do NOT have) e.g.
  • Head of sperm
  • Bacterial spores, see Endospore
  • Certain cellular inclusions e.g.
    • Cytoplasmic inclusion bodies seen in
      • Neurons in layer 5 of cerebral cortex neuronal ceroid-lipofuscinoses (Batten disease).
    • Nuclear inclusion bodies seen in
      • Lead poisoning
      • Bismuth poisoning.
  • Oocysts of some coccidian parasites in faecal matter, such as:
  • A few other parasites:
    • Taenia saginata eggs stain well but Taenia solium eggs don't (Can be used to distinguish)
    • Hydatid cysts, especially their "hooklets" stain irregularly with ZN stain but emanate bright red fluorescence under green light, and can aid detection in moderately heavy backgrounds or with scarce hooklets.[19]
  • Fungal yeast forms are inconsistently stained with Acid-fast stain which is considered a narrow spectrum stain for fungi.[20] In a study on acid-fastness of fungi,[21] 60% of blastomyces and 47% of histoplasma showed positive cytoplasmic staining of the yeast-like cells, and Cryptococcus or candida did not stain, and very rare staining was seen in Coccidioides endospores.


Online protocol examples

  • PDF format).
  • Alternate Ellis & Zabrowarny method for staining AFB.
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