Why Are Acid-Fast Stains Useful?

Scientists use microscopes and staining techniques to detect and differentiate microorganisms, including bacteria, in patient specimens. Most bacteria stain well using conventional staining methods. However, a few are impermeable to conventional dyes and require the use of a special acid-fast stain.

Procedure

• According to the American Society for Microbiology, acid-fast staining involves flooding a slide containing a small amount of patient specimen with red carbol fuchsin dye, rinsing it with an acid-alcohol solution, and restaining it with methylene blue dye. All structures, except acid-fast bacteria, release the red pigment when washed with acid and absorb the blue stain instead. Acid-fast bacteria retain the red dye and appear as red rod-shaped structures against a blue background.

Tuberculosis

• Acid-fast bacteria include the genus Mycobacterium, a group of bacteria that cause tuberculosis and other related diseases. Tuberculosis is a highly infectious disease that affects one-third of the world's population each year and is the leading cause of death from bacterial infectious disease, according to Dr. Kenneth Todar, professor of microbiology at the University of Wisconsin-Madison.

Importance of Staining

• Culturing and identifying bacteria requires time. According to Dr. Todar, it takes four to six weeks to grow visible colonies of Mycobacterium tuberculosis. The Centers for Disease Control notes that acid-fast staining provides physicians with a rapid and cost-effective means of making a preliminary tuberculosis diagnosis.