| Slender red rods of Mycobacterium tuberculosis
are shown here in an acid fast stain at 1000 X. These pathogenic
bacteria are present in a sputum specimen from a patient with tuberculosis.
Other non-acid fast bacteria are also visible. Mycobacterium
are said to be "acid-fast" because they retain the red primary dye (carbolfuchsin)
even when exposed to acid-alcohol. Most other bacteria lose the carbolfuchsin
dye and then absorb the methylene blue counterstain, resulting in blue
non-acid-fast organisms and red acid-fast