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[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 ...
The results of Ziehl–Neelsen staining is variable because many fungal cell walls are not acid fast. [11] An example of a common type of acid-fast fungus that is usually stained with Ziehl–Neelsen staining is called Histoplasma (HP). [12] Histoplasma is found in soil and the feces of birds and bats. [13]
Acid-fast organisms display a reddish-yellow fluorescence. [2] Although the auramine–rhodamine stain is not as specific for acid-fast organisms (e.g. Mycobacterium tuberculosis or Nocardia ) as the Ziehl–Neelsen stain , it is more affordable and more sensitive, therefore it is often utilized as a screening tool.
The Kinyoun method can be modified as a weak acid fast stain, which uses 0.5–1.0% sulfuric acid instead of hydrochloric acid.The weak acid fast stain, in addition to staining Mycobacteria, will also stain organisms that are not able to maintain the carbol fuchsin after decolorizing with HCl, such as Nocardia species and Cryptosporidium.
This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear weakly Gram-positive. [3] Acid-fast stains such as Ziehl–Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope.
Mycobacterium smegmatis is an acid-fast bacterial species in the phylum Actinomycetota and the genus Mycobacterium. It is 3.0 to 5.0 μm long with a bacillus shape and can be stained by Ziehl–Neelsen method and the auramine-rhodamine fluorescent method.
Sputum smears and cultures should be done for acid-fast bacilli if the patient is producing sputum. [1] The preferred method for this is fluorescence microscopy (auramine-rhodamine staining), which is more sensitive than conventional Ziehl–Neelsen staining. [4]
[2] [3] Carbol fuchsin is used as the primary stain dye to detect acid-fast bacteria because it is more soluble in the cells' wall lipids than in the acid alcohol. If the bacteria is acid-fast the bacteria will retain the initial red color of the dye because they are able to resist the destaining by acid alcohol (0.4–1% HCl in 70% EtOH). [4]