TB is a chronic granulomatous disease with worldwide prevalence caused by M. tuberculosis (1) and less often by M. bovis (both referred to as the tubercle bacilli). Historically present since the early evolutionary stages of our species, mycobacteria have evolved to effectively infect and establish latency in almost all mammalian and other species. M. tuberculosis is an aerobic, non-spore-forming, slow-growing bacillus with a lipid-laden thick cell wall that tends to grow in parallel groups, exhibiting serpentine cording morphology. M. tuberculosis complex (MTB complex) refers to a group of genetically related mycobacteria and includes M. tuberculosis, M. bovis, M. microti, M. canetti, and M. africanum. Mycobacteria appear faintly positive or colorless on Gram stain. The term "acid-fast bacilli" (AFB) is almost synonymous with mycobacteria, although Nocardia species and other organisms can also be variably acid fast. The classic Ziehl-Neelsen stain, the modified Kinyoun stain and the auramine-rhodamine fluorochrome stains are commonly used for AFB. Multidrug-resistant tuberculosis (MDR-TB) is defined as being resistant to INH and rifamycin (RMP or rifabutin).
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