Because of the waxy substance (mycolic acids) present on the cell walls, cells of species of Mycobacterium do not stain readily with ordinary dyes. However, treatment with cold carbol fuchsin for several hours or at high temperatures for five minutes will dye the cells. Once the cells have been stained, subsequent treatment with a dilute hydrochloric acid solution or ethyl alcohol containing 3% HCl (acid-alcohol) will not decolorize them. Such cells are thus termed acid-fast in that the cell will hold the stain fast in the presence of the acidic decolorizing agent. This property is possessed by few bacteria other than Mycobacterium.
Microscopic examination of tissues or of sputum stained by the acid-fast staining procedure is an aid in the diagnosis of tuberculosis. If an individual has pulmonary tuberculosis, and if the tubercles in the lungs are open, the bacteria (Mycobacterium tuberculosis) will be present in the sputum. The bacteria which cause leprosy (Hansen's disease; caused by M. leprae) can also be detected with this staining procedure. The finding of acid-fast cells in milk, on the skin, or in feces is of no great signifi-cance, because these bacteria may be commonly-found saprophytic species of Mycobacterium.
The above video demonstrates the acid-fast stain protocol.
Below is pictured an example of an acid-fast stain
Figure 3.17. The acid fast stain. A photomicrograph of Mycobacterium smegmatis (pink) and Micrococcus luteus (blue) at 1000x magnification. M. smegmatis is acid-fast, retaining the carbol fuchsin dye, thus appearing pink. M. luteus is not acid-fast, loses the carbol fuchsin during decolorizaiton, and is counter-stained with methylene blue.