Index >>Prokaryotes Microbes >> Acid Fastness

Acid Fastness - To stain tubercle bacillus, in smears or in tissues, the commonly used method is the Ziehl Neelsen method. The smeared specimen is heated to steaming for 2-3 min. in carbolfuchsin ( a mixture of triphenyl methane dyes rosaniline and oararosaniline in aqueous 5% phenol) Subsequent washing in 95% ethanol - 3% HCI (acid alcohol) decolorises most bacteria in a few seconds, whereas acid fast organisms retail) the stain much longer.

This property is attributed to high lipid content in their cell wall. Mycobacteria appear to be Gram positive, but they take up the stain weakly and irregularly and without requiring iodine treatment to retain it