UAMS Medical Laboratory Technician–Medical Laboratory Scientist (MLT-MLS) Practice Exam

Mycobacteria have a waxy, mycolic-acid–rich cell wall that makes them acid-fast. Stains designed for acid-fast organisms—such as Ziehl-Neelsen (with heat and carbol fuchsin), Kinyoun (cold method), and Auramine-rhodamine (a fluorescent stain)—are used because their cell walls retain the dye even after acid-alcohol decolorization or glow under fluorescence, making them easy to detect in clinical samples. The Gram stain isn’t suitable for these bacteria because the lipid-rich wall resists consistent coloration and decolorization, leading to unreliable and often misleading results. So, while Gram staining works well for many bacteria, it is not used for identifying mycobacteria; acid-fast stains are preferred for accurate detection.