In addition to peptidoglycan,
the acid-fast cell wall of Mycobacterium contains a large amount of
glycolipids, especially mycolic acids. The peptidoglycan layer is linked to
arabinogalactan (D-arabinose and D-galactose) which is then linked to high-molecular
weight mycolic acids. The arabinogalactan/mycolic acid layer is overlaid with
a layer of polypeptides and mycolic acids consisting of free lipids, glycolipids,
and peptidoglycolipids. Other glycolipids include lipoarabinomannan and phosphatidyinositol
mannosides (PIM). Because
of its unique cell wall, when it is stained by the acid-fast procedure, it
will resist decolorization with acid-alcohol and stain red, the color of the
initial stain, carbol fuchsin. With the exception of a very few other acid-fast
bacteria such as Nocardia, all other bacteria will be decolorized and
stain blue, the color of the methylene blue counterstain.
