Cell Walls of Acid-Fast Bacteria

Mycobacteriá (e.g., Mycobacterium tuberculosis) have an unusual

cell wall, resulting in their inability to be Gram-stained (Figure

2-4B). These bacteria are said to be acid-fast because they

resist decolorization with acid-alcohol after being stained with

carbolfuchsin. This property is related to the high concentration

of lipids, called mycolic acids, in the cell wall of Mycobacteria.

Note that Nocardia asteroides,\ a bacterium that can cause

lung and brain infections in immunocompromised individu-

als, is weakly acid-fast. The meaning of the term "weakly" is

that if the acid-fast staining process uses a weaker solution of

hydrochloric acid to decolorize than that used in the stain for

Mycobacteria, then N. astero1des will not decolorize. However

if the regular-strength hydrochloric acid is used, N. asteroides

will decolorize.