Acid-fastness

Acid-fastness is a physical property of certain bacteria and protozoa, which makes them resistant to decolorization by acids during laboratory staining procedures. This characteristic is used to identify and classify these microorganisms, particularly Mycobacterium species, including Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium leprae, which causes leprosy.

Overview

Acid-fast organisms are difficult to characterize using standard microbiological techniques, as they do not readily take up stains due to the high lipid content in their cell walls. However, once stained, they resist decolorization with acid-alcohol, hence the term "acid-fast". The most commonly used staining technique is the Ziehl-Neelsen stain, which uses carbolfuchsin, a phenolic compound, as the primary stain.

Mechanism

The acid-fastness property is due to the presence of mycolic acids in the cell wall of acid-fast organisms. Mycolic acids are long-chain fatty acids that confer a waxy character to the cell wall, making it impermeable to most stains. During the Ziehl-Neelsen staining procedure, the application of heat and carbolfuchsin allows the stain to penetrate this waxy barrier. Once inside, the stain is retained even when the smear is washed with an acid-alcohol solution.

Clinical significance

Acid-fastness is a critical diagnostic feature in clinical microbiology. It allows for the identification and differentiation of Mycobacterium species, which are responsible for significant global health problems such as tuberculosis and leprosy. Acid-fast staining is also used to detect Nocardia species, which can cause a variety of infections in humans, and Cryptosporidium, a protozoan parasite that causes cryptosporidiosis, a diarrheal disease.

See also

• Gram stain
• Mycobacterium
• Ziehl-Neelsen stain

Acid-fastness Resources
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