USMLE – Mycobacteria
Mycobacteria are aerobic, acid-fast bacilli (rods). They are neither gram-positive nor gram negative; ie, they are stained poorly by the dyes used in Gram stain. They are virtually the only bacteria that are acid-fast. (One exception is Nocardia asteroides, the major cause of nocardiosis, which is also acid-fast.) The term “acid-fast” refers to an organism’s ability to retain the carbolfuchsin stain despite subsequent treatment with an ethanol-hydrochloric acid mixture. The high lipid content (approximately 60%) of their cell wall makes mycobacteria acid-fast.
The major pathogens are Mycobacterium tuberculosis, the cause of tuberculosis and Mycobacterium leprae, the cause of leprosy. Atypical mycobacteria, such as Mycobacterium avium intracellulare complex and Mycobacterium kansasii, can cause tuberculosis like disease but are less frequent pathogens. Rapidly growing mycobacteria, such as Mycobacterium chelonei, are saprophytes that occasionally cause human disease in immunocompromised hosts.