Acid fast staining

Acid fast stain is the differential stain. It was developed by Paul Ehrlich in 1882 which was later on modified by Zeihl- Neelson and is being used by the present day microbiologists. Bacteria are classified as acid-fast if they retain the primary stain (carbolfuchsin) after washing with strong acid and appear red or as non-acid-fast if they lose their colour on washing with acid and counter stained by methylene blue. The property of acid fastness appears to be due to the presence of high contents of a lipid called mycolic acid in the cell wall, that makes penetration of stains extremely difficult. In the acid fast procedure, bacterial smears is treated with carbon fuchsia followed by heat fixing and treatment with acid alcohol and methylene blue.

Acid fast staining procedure is useful for the identification of members of mycobacterium specially in staining of sputum that have mycobacterium tuberculosis, the cause of tuberculosis, because this bacillus is the only acid-fast organism commonly found in the sputum. M.smegmatis is non-pathogenic but normal inhabitant of the genitals and may be mistaken for M.tuberculosis in the urine.