Many bacterial cells are easily stained by using simple dyes or Gram stain. However, a few bacterial strains, such as Mycobacteria and Nocardia cannot be stained using simple dyes (or, if successfully stained, the results may vary significantly). Cellular wall of the Mycobacteria strain contains waxy substance – mycolic acid. Those are beta-hydroxy carboxylic acids with chains containing up to 90 carbon atoms. Its resistance to acidity is associated with mycolic acid chain length. In order to stain such strains, a higher concentration of dye or a longer period of heating is required. However, once stained, the dye is even more difficult to remove from the cells. Those bacteria are called acid fast because they maintain their primary color even after decolorization using acid alcohol (TB Fuchsin reagent, phenol-free). Early laboratory diagnosis of tuberculosis is based on the interpretation of stained smears, and one of the best diagnostic methods is analyzing sputum sample under microscope. The most common and renowned method used for detecting the tuberculosis bacteria is staining according to Ziehl-Neelsen. This kit uses modified Ziehl-Neelsen method that contains TB Fuchsin reagent, phenol-free, acid alcohol as decolorizing agent (two packages of TB Decolorizer) and Methylene Blue solution as counterstain (Methylene Blue Loeffler reagent).