Please describe how an acid-fast stain works. How does it get its name?
An acid-fast stain is a laboratory technique used to identify certain bacteria that have a unique cell wall structure called mycolic acid. This staining method is commonly used to identify the bacteria of the genus Mycobacterium, including the species that cause tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae).
The acid-fast stain is named based on its distinctive staining properties. It involves the use of a primary stain called carbol hsin, which contains a red dye and a lipid solvent (usually phenol). When this stain is applied to a bacterial smear on a slide, it penetrates the bacterial cells, including those with mycolic acid in their cell walls. The lipid solvent in the stain helps the dye to penetrate the waxy cell wall of acid-fast bacteria.
After applying the carbol hsin stain, the slide is heated gently to help the dye penetrate further into the cells. This step is called heat fixing. Then, the slide is rinsed with an acidic alcohol solution like acid alcohol or acidified ethanol. This acid-alcohol wash acts as a decolorizing agent, removing the stain from non-acid-fast bacteria.
However, acid-fast bacteria retain the red stain even after the acid-alcohol wash because their mycolic acid cell walls are resistant to the decolorizing effect. This property is due to the high lipid content of the cell walls, which prevents the stain from being easily washed out.
Finally, a counterstain, usually methylene blue or brilliant green, is applied to the slide. This counterstain provides contrast and helps visualize the non-acid-fast bacteria that have been decolorized.
Under a microscope, acid-fast bacteria appear as bright red rods or cocci, while non-acid-fast bacteria appear as blue or green. This staining technique is essential in the identification and diagnosis of acid-fast bacteria, particularly those causing tuberculosis and leprosy.
To summarize, an acid-fast stain works by using a primary stain called carbol hsin, which penetrates the mycolic acid-containing cell walls of acid-fast bacteria due to the solvent effect of the lipid solvent. The stain is then retained in acid-fast bacteria even after decolorizing with an acidic alcohol solution, while non-acid-fast bacteria are decolorized. The name "acid-fast" is derived from its ability to retain the stain despite exposure to acid-alcohol wash.