Acid-fastness is a physical property of certain and Eukaryote cells, as well as some Sub-cellular, specifically their resistance to decolorization by during laboratory staining procedures.
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 , 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.
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 stain, 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).
Some acid-fast staining techniques
-
Ziehl–Neelsen stain (classic and modified bleach types)
-
Kinyoun stain, a development of ZN that requires no heating; variants:
-
Alternative dyes (Victoria blue instead of fuchsin, picric acid instead of methylene blue), which is useful to color-blind people and materials where the classical ZN/Kinyoun dyes provide insufficient legibility.
[Theory and Practice of Histological Techniques, John D Bancroft, 6th ed, p314]
-
Moeller stain
-
Dorner's method
[Dorner, W. 1926. Un procédé simple pour la colouration des spores. Le Lait 6:8–12.] (acid alcohol decolorizer) without the Schaeffer–Fulton
-
Detergent method, using Tergitol 7, nonionic polyglycol ether surfactants type NP-7 for decolorizing
-
Fite stain
-
Fite-Faraco stain
-
Wade Fite stain
-
Ellis and Zabrowarny stain
-
Auramine-rhodamine stain
-
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 or modified acid-fast:
-
All Mycobacteria – M. tuberculosis, M. leprae, M. smegmatis and atypical mycobacteria .
-
Certain Actinomycetota (especially aerobic ones in the order Mycobacteriales) with mycolic acid in their cell wall; not to be confused with Actinomyces, which is a non-acid-fast genus of actinomycete. Note that Streptomyces do not contain mycolic acid.
-
Head of sperm
-
Bacterial spores, see Endospore
-
Legionella micdadei
-
Certain cellular inclusions e.g.
-
Cytoplasmic inclusion bodies seen in
-
Neurons in layer 5 of cerebral cortex neuronal ceroid lipofuscinosis (Batten disease).
-
Nuclear inclusion bodies seen in
-
Oocysts of some coccidian parasites in faecal matter, such as:
-
A few other parasites:
-
Sarcocystis
-
Taenia saginata eggs stain well but Taenia solium eggs don't (can be used to distinguish)
-
Echinococcosis, 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.
-
Fungal yeast forms are inconsistently stained with Acid-fast stain which is considered a narrow spectrum stain for fungi.
[Wages ds, Wear dJ. acid-fastness of fungi in blastomycosis and histoplasmosis. Arch Pathol Lab Med 1982; 106:440-41.] 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
