Final answer:
The counterstain used in the acid-fast stain is methylene blue, which colors non-acid-fast bacteria blue after decolorization with acid-alcohol.
Step-by-step explanation:
The counterstain used in the acid-fast stain method is methylene blue. This differential staining technique utilizes carbol fuchsin as the primary stain, and acid-alcohol as the decolorizer. After decolorization, non-acid-fast bacteria are counterstained with methylene blue, which turns them blue, while acid-fast bacteria retain the red or pink color of the primary stain (carbol fuchsin). Two methods of acid-fast staining are the Ziehl-Neelsen and the Kinyoun techniques, which differentiate between gram-positive cells based on the presence of waxy mycolic acids in their cell walls.