Which stain is used to identify bacteria that are not decolorized by acid-alcohol?

The acid-fast stain is specifically designed to identify bacteria that resist decolorization by acid-alcohol. This characteristic is critical for differentiating organisms like Mycobacterium species (including Mycobacterium tuberculosis) which possess a waxy cell wall containing mycolic acids that prevent the uptake of conventional stains and enable them to withstand harsh staining and decolorization processes.

The methodology of the acid-fast stain involves applying carbol fuchsin, which penetrates and stains the bacteria. After this initial staining, the bacteria are subjected to an acid-alcohol solution. Acid-fast bacteria retain the red color of the carbol fuchsin due to their cell wall structure, while non-acid-fast bacteria lose the color and can subsequently be counter-stained with a different dye (usually methylene blue), allowing for a clear distinction.

Other staining techniques listed, such as the Gram stain, identify bacteria based on their cell wall composition, particularly differentiating between Gram-positive and Gram-negative bacteria. The Giemsa-Wright stain is typically employed for blood smears and to identify certain parasites. The India ink stain is primarily used to visualize the capsule of encapsulated organisms, especially Cryptococcus neoformans, and is not related to identifying acid-fast bacteria.