Which of the following stains can be used as a counterstain in acid-fast staining protocols?

In acid-fast staining protocols, safranin is used as a counterstain after the primary stain, which is usually carbol fuchsin. The purpose of the counterstain is to color the non-acid-fast cells, allowing for differentiation between acid-fast bacteria, such as Mycobacterium species, which retain the red color of the carbol fuchsin, and non-acid-fast bacteria, which will take up the safranin and appear pink or red against a clearer background.

Safranin, being a basic dye, effectively stains the non-acid-fast cells a contrasting color, aiding in the visualization of the acid-fast cells under a microscope. This layered staining technique is crucial for diagnosing infections caused by acid-fast bacteria, such as tuberculosis.

Other options, like crystal violet, methylene blue, and India ink, have different staining properties or applications that do not serve as effective counterstains in this specific protocol. Crystal violet is typically used in Gram staining, while methylene blue is a simple stain and India ink is used for negative staining, particularly to visualize encapsulated bacteria. Thus, while they are important in various staining processes, they do not fulfill the role of a counterstain in the acid-fast staining procedure