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Mycobacterium Leprae : Wade-Fite Stain

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Acid-fast bacilli: The cell walls of mycobacteria and related pathogens contain mycolic acids which are waxy substances composed of complex branched hydrocarbons. They make the bacterial cell wall resistant to many harsh chemicals such as detergents and strong acids. These lipid substances allow the bacteria to retain stains like carbolfuchsin even when they are decolorized with strong acid. Hence, they are labeled acid-fast bacilli.

Ziehl-Neelsen Stain: Briefly, the bacteria are first treated with carbolfuchsin (primary stain) and heated on a steam bath (mordant) which allows the stain to penetrate the cell wall. This is followed by application of acid alcohol (decolorizer) and finally staining with methylene blue (counterstain). The acid-fast bacteria appear red; other structures take on blue-green color of the counterstain.

Modified Ziehl-Neelsen Stain (Wade-Fite Stain): Mycobacterium leprae are much less acid- and alcohol-fast as compared to mycobacterium tuberculosis. The mycolic acid coat of leprosy bacilli is less strong and is easily decolorized by the standard Ziehl-Neelsen technique. A modification is used (Wade-Fite technique) in which peanut oil is used with the deparaffinizing solvent (xylene) which minimizes the exposure of the bacterial cell wall to organic solvents and preserves their acid-fastness.

The image shows numerous mycobacterium leprae singly and in clusters (globi) with Wade-Fite stain in skin biopsy of a patient with lepromatous leprosy.

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