Explain the Primary Stain - Ziehl-Neelsen Method?
Ordinary aqueous stains like methylene blue, crystal violet, etc. are not able to stain most of the species of Mycobacteria. Carbol fuchsin, a red mixture of basic fuchsin and phenol can be used for staining these bacteria. Basic fuchsin, binds to carboxylic acid group of mycolic acid and phenol enhances penetration of basic fuchsin into lipid. Penetration is further enhanced by slow heating to steaming for 2-3 minutes. This drives the stain inside the cytoplasm. In modified Ziehl-Neelsen method, instead of heating, a wetting agent - Turgitol - can be added to the stain. This agent reduces surface tension between the cell wall of the mycobacteria and the stain. After staining, all cells become red.