Mycobacteria are aerobic, non–spore-forming, nonmotile gram-positive bacilli that are essentially defined by their distinctive acid-fast staining property (1). Owing to their thick, lipid-rich cell walls, mycobacteria are relatively impermeable to various basic dyes; however, once stained, they retain the stain with tenacity. Thus, they resist decolorization with acidified organic solvents and are therefore considered to be acid-fast. The genus Mycobacterium comprises over 20 species which range from organisms that are highly pathogenic for humans, such as Mycobacterium tuberculosis and Mycobacterium leprae, to a variety of innocuous organisms widely spread in soil and water that can become pathogenic under conditions of immunodeficiency.