Physical property of certain bacterial and eukaryotic cells
Acid-fastness is a physical property of certain
bacterial and
eukaryoticcells, as well as some
sub-cellular structures, specifically their resistance to decolorization by
acids during laboratory
staining procedures.[1][2] Once stained as part of a sample, these organisms can resist the acid and/or ethanol-based decolorization procedures common in many staining protocols, hence the name acid-fast.[2]
The mechanisms of acid-fastness vary by species although the most well-known example is in the genus Mycobacterium, which includes the species responsible for
tuberculosis and
leprosy. The acid-fastness of Mycobacteria is due to the high
mycolic acid content of their
cell walls, which is responsible for the staining pattern of poor absorption followed by high retention. Some bacteria may also be partially acid-fast, such as Nocardia.
Acid-fast organisms are difficult to characterize using standard microbiological techniques, though they can be stained using concentrated dyes, particularly when the staining process is combined with heat. Some, such as Mycobacteria, can be stained with the
Gram stain, but they do not take the crystal violet well and thus appear light purple, which can still potentially result in an incorrect gram negative identification.[3]
The most common staining technique used to identify acid-fast bacteria is the
Ziehl–Neelsen stain, in which the acid-fast species 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 Mycobacteria can also be visualized by
fluorescence microscopy using specific fluorescent dyes (
auramine-rhodamine stain, for example).[4] 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.[citation needed]
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 the rest of the Kinyoun technique can be used.[6]
Various bacterial spore staining techniques using Kenyon e.g.
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 or modified acid-fast:
All
mycobacteria – M. tuberculosis, M. leprae, M. smegmatis and atypical Mycobacterium
Actinomycetes (especially some aerobic ones) with mycolic acid in their cell wall (note Streptomyces do NOT have); not to be confused with Actinomyces, which is a non-acid-fast genus of
actinomycete
Nocardia (weakly acid-fast; resists decolorization with weaker acid concentrations)
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.[20]
Fungal yeast forms are inconsistently stained with Acid-fast stain which is considered a narrow spectrum stain for fungi.[21] In a study on acid-fastness of fungi,[22] 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.
References
^Madison B (2001). "Application of stains in clinical microbiology". Biotech Histochem. 76 (3): 119–25.
doi:
10.1080/714028138.
PMID11475314.
^
abRyan KJ; Ray CG, eds. (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill.
ISBN0-8385-8529-9.