Many bacteria of the genera Mycobacterium and Nocardia are medically significant, causing infectious diseases such as tuberculosis, leprosy and other lung and skin infections. Therefore, it is clinically important to be able to accurately identify members of these genera.
Article Summary: Acid-fast staining is used to identify bacteria of the genera Mycobacteria & Nocardia, which have mycolic acid in their cell wall. Here's how it works.
Acid-fast Ziehl-Neelsen Stain Reaction
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The acid-fast stain is a first step in identification of these types of bacteria.
Cell Wall of Mycobacteria and Nocardia
The Gram stain is a differential stain reaction that is used to categorize most bacteria as either Gram+ or Gram-. This distinction is due to differences in the bacterial cell wall structure of these two categories of bacteria and has significance with respect to which antibiotics can be used to kill or control bacteria.
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Because the cell wall is resistant to water-based stains, acid-fast organisms require a special staining technique involving heat to drive stain into their waxy cell wall.
Bacteria of the genera Mycobacterium
and Nocardia have unusual cell walls that are waxy and nearly impermeable due to the presence of the waxy molecule mycolic acid. Bacteria that produce mycolic acid are highly resistant to disinfectants, desiccation and are difficult to stain with water-based stains such as the Gram.
Prior to staining bacteria, a bacterial smear must be heat fixed onto a microscope slide. A smear is a sample of bacteria suspended in a small amount of water on a slide. That sample is then dried using heat. The heat kills the bacteria and attaches the sample to the slide so that it does not easily wash away.