This staining kit is based on the Ziehl-Neelsen stain (a carbofulfuchsin solution) recommended by the WHO. Acid-fast bacteria such as Mycobacterium tuberculosis and Mycobacterium leprae are difficult to stain because of their lipoid capsule in the cell wall. However, once the lipoid capsule is stained with an enhanced dye such as carbolfuchsin, the newly formed compound will resist decolorization from acid-alcohol and retain the original color stained (red), which can be easily differentiated from other microorganisms that still adsorb the counter blue stain.