Acid-fastness

Acid-fastness is the ability of certain bacteria to retain primary stains despite decolorization with acid-alcohol. It is associated with lipid-rich, waxy cell envelopes.

Mycolic acid: Long-chain fatty acids that form a major part of the waxy cell envelope in mycobacteria. These envelope lipids help resist acid-alcohol decolorization.
Mycobacterium tuberculosis: A slow-growing bacterium that causes tuberculosis and has a mycolic acid-rich cell envelope. Its acid-fast bacilli can be sought in respiratory specimens.
Gram staining: A differential staining method that classifies bacteria by cell-envelope properties as Gram-positive or Gram-negative. Its classification depends on a different staining response than acid-fastness.
Non-acid-fast bacteria: Bacteria that lose the primary dye when treated with acid-alcohol during acid-fast staining. Their decolorization is the defining contrast to acid-fast organisms.
Ziehl–Neelsen stain: A hot carbol fuchsin staining method that detects acid-fast organisms by retaining red dye after acid-alcohol treatment. It uses heat to drive primary stain through the waxy envelope.
Mycobacterium leprae: An obligate intracellular bacterium that causes leprosy and is strongly acid-fast. Its staining properties help characterize organisms in affected tissue.
Bacterial cell wall: A structural layer outside the bacterial cytoplasmic membrane that helps maintain cell shape and resist osmotic pressure. Acid-fastness reflects unusual envelope chemistry, not simply the presence of a cell wall.
Partially acid-fast bacteria: Bacteria that retain acid-fast stains inconsistently or weakly, often because their envelope contains less mycolic acid. Their intermediate staining behavior complicates a simple positive-or-negative distinction.
Kinyoun stain: A carbol fuchsin acid-fast staining method that omits heating by using a more concentrated phenol-dye solution. It detects acid-fastness without the heating step used in Ziehl–Neelsen staining.
Nocardia: A genus of aerobic, branching bacteria; many species are partially acid-fast because they contain mycolic acids. Partial acid-fastness distinguishes Nocardia from fully acid-fast mycobacteria.

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MycobacteriumRelated: Mycobacteria resist decolorization because their cell walls contain abundant mycolic acids.