Using histological stains on expectorated samples from phlegm (also called "sputum"), scientists can identify MTB under a microscope. Since MTB retains certain stains even after being treated with acidic solution, it is classified as an <answer>acid-fast</answer> bacillus. The most common <answer>acid-fast</answer> staining techniques are the Ziehl–Neelsen stain and the Kinyoun stain, which dye <answer>acid-fast</answer> bacilli a bright red that stands out against a blue background. Auramine-rhodamine staining and fluorescence microscopy are also used.
What's the term for bacillus that can be exposed to acidic solutions without losing their stains?
acid-fast