What is the principle of the Kinyoun-cold method?

Kinyoun-cold method was developed to differentiate between acid-fast and non-acid-fast bacteria. The technique is used for detecting acid-fast microorganisms like Mycobacterium and Nocardia. There is no heating involved in the procedure, and thus can also be called a “cold method” of staining. The primary stain (carbol fuchsin) is applied to the cells and phenol is utilized to allow for the stain to enter the surface of acid-fast microorganisms. The excess stain is then removed by application of sulfuric acid. The secondary stain, methylene blue is then applied to the cells. Acid fast bacteria will appear pink, while non-acid fast bacteria will appear blue.