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Acid-fastness is a physical property of certain bacterial and eukaryotic cells, as well as some sub-cellular structures, specifically their resistance to decolorization by acids during laboratory staining procedures.
However, the mycolic acid in the cell wall of M. tuberculosis does not absorb the stain. Instead, acid-fast stains such as Ziehl–Neelsen stain, or fluorescent stains such as auramine are used. [4] Cells are curved rod-shaped and are often seen wrapped together, due to the presence of fatty acids in the cell wall that stick together. [11]
The two most common methods for visualizing these acid-fast bacilli as bright red against a blue background are the Ziehl-Neelsen stain and modified Kinyoun stain. Fite's stain is used to color M. leprae cells as pink against a blue background. Rapid Modified Auramine O Fluorescent staining has specific binding to slowly-growing mycobacteria ...
When looking at the smears for TB, it is stained using an acid-fast stain. These acid-fast organisms like Mycobacterium contain large amounts of lipid substances within their cell walls called mycolic acids. These acids resist staining by ordinary methods such as a Gram stain. [9] It can also be used to stain a few other bacteria, such as Nocardia.
Mycobacterium leprae is an intracellular, pleomorphic, non-sporing, non-motile, acid-fast, pathogenic bacterium. [3] It is an aerobic bacillus (rod-shaped bacterium) with parallel sides and round ends, surrounded by the characteristic waxy coating of mycolic acid unique to mycobacteria.
The genus is acid-fast to some degree, it stains only weakly Gram positive. The most common form of human nocardial disease is a slowly progressive pneumonia, the common symptoms of which include cough, dyspnea (shortness of breath), and fever. It is not uncommon for this infection to spread to the pleura or chest wall.
The fatty acid synthase-I and fatty acid synthase-II pathways producing mycolic acids are linked by the beta-ketoacyl-(acyl-carrier-protein) synthase III enzyme, often designated as mtFabH. Novel inhibitors of this enzyme could potentially be used as therapeutic agents. The mycolic acids show interesting inflammation controlling properties.
The Kinyoun method can be modified as a weak acid fast stain, which uses 0.5–1.0% sulfuric acid instead of hydrochloric acid.The weak acid fast stain, in addition to staining Mycobacteria, will also stain organisms that are not able to maintain the carbol fuchsin after decolorizing with HCl, such as Nocardia species and Cryptosporidium.