Acid fastness is a phenomenon that is found in certain types of bacteria where they resist the process of de-colorization that occurs when acid is used to wash a sample that contains these bacteria. These bacteria also resist staining and it may require heat and concentrated staining to colorize them. Once this colorization has taken place, it becomes difficult to decolorize using acid, or stain them with another color unless the heat and concentration technique is used. This is why the term given to them is 'acid fast' just like one would use the term 'color fast' for cloth that does not leak color when washed.
The acid fast staining procedure is used to identify acid fast bacteria. The most common type of acid fast bacteria is mycobacteria. A strain of mycobacteria is responsible for the disease tuberculosis. In diagnostic medicine, the acid fast test may be used on a person in order to detect the existence of the bacteria that can cause tuberculosis. It is therefore used as a diagnosis of the disease because other symptoms of tuberculosis are not useful as they overlap with other conditions. Acid fast stain results can confirm the presence of the bacteria known as mycobacteria tuberculosis which is the bacteria responsible for causing tuberculosis.
Acid fast stain testing can be done on any part of the body where the bacteria are suspected to be located. It could be done on the blood, the stool, and even the bone marrow. In case there is a tissue sample being tested, the procedure to slice the tissue off could be painful. Once the sample is collected, it is sent to a laboratory to test for acid fast stain bacteria. The substance that has been collected is mixed with a stain and heated so that the stain catches on. The resulting stained substance is then washed with acid.
This is where the acid fastness property of the bacteria comes into the picture. The sample is then re-stained with another color. When the results are viewed, the substance will appear in the second stain color whereas the bacteria will appear colored by the first stain. This difference in coloration confirms the existence of the bacteria that is being tested for. A modified acid fast stain test is used for the nocardia bacterium. The results of the test allow a diagnosis to be made, following which treatment, if necessary could be undertaken.