Tuberculosis is caused by a slender rod known as Mycobacterium tuberculosis. This slow- growing rod is neither Gram- positive nor Gram- negative; rather, it is acid- fast. When stained with carbolfuchsin, steam must be used to force the stain through the bacterial cell wall. Once stained, however, the organism cannot be decolorized even if dilute acid- alcohol is used. This acid- fast characteristic is due to the presence of a very thick cell wall containing mycolic acid.