Another commonly used differential stain is the acid fast stain. This stain is used to identify bacilli, such as Mycobacterium tuberculosis, that have long-chain fatty acids (mycolic acids) in their cell walls that make these organisms extremely difficult to stain with the Gram stain. The acid fast stain uses carbolfuchsin (a red dye) as the primary stain, an acid alcohol decolorizer, and methylene blue (a blue dye) as the counterstain.
An organism is “acid fast” because it resists decolorizing the carbolfuchsin dye by acid alcohol and therefore remains red. Methylene blue is added for contrast, staining non-acid fast bacteria, yeast, white blood cells, red blood cells, and epithelial cells blue.
Similar to Gram stain reports, acid fast stain reports state the presence (and quantity) or absence of acid fast bacilli. On the other hand, the presence or absence of non-acid fast organisms and tissue cells are not reported. The reason is that the presence of acid fast bacilli is usually clinically significant, but the presence of non-acid fast organisms or tissue cells is not.
The acid fast stain can be modified using a weaker acid alcohol decolorizer with carbolfuchsin and methylene blue dyes to stain organisms like Nocardia that are not fully acid fast but do not stain well with the Gram stain.
This slide is an acid fast stain of an expectorated sputum specimen from a 41-year-old woman. She is a homeless person with a chronic cough, night sweats, and apparent 20-pound weight loss.
Look at the slide (This will open a new browser window that you can refer to while answering the questions below. Close when finished.)
What color are the acid fast orgnaisms?
What color are the tissue cells and non-acid fast organisms?