Acid-fast staining is used to detect which group of organisms?

• A. Fungi
• B. Mycobacteria such as M. tuberculosis and M. leprae
• C. Gram-positive cocci
• D. Gram-negative rods

Answer: (B)

Acid-fast staining highlights organisms with a waxy, mycolic acid–rich cell wall. That lipid content makes the cell wall resistant to decolorization by acid-alcohol after staining, so these bacteria keep the red dye and appear red or pink under the microscope. The classic examples are mycobacteria, including Mycobacterium tuberculosis and Mycobacterium leprae, which is why this stain is used to detect them. Other organisms, like fungi, Gram-positive cocci, or Gram-negative rods, do not have that waxy wall and lose the dye during the acid wash, so they do not appear acid-fast.