ABSTRACT
All 71 members of the genus are acid-alcohol-fast by virtue of the prodigious lipid content of their cell wall which includes mycolic acid. When stained by the Kinyoun (cold) or Ziehl-Neelsen (hot) method, carbol fuchsin is taken up by mycolic acid and resists decoloration with 3% hydrochloric acid in ethanol. Mycobacteria can also be stained a bright yellow by the auramine-rhodoamine fluorochrome stain. Although mycobacteria are classified as Gram-positive, most species, with the exception of the rapid growers (M. fortuitum, M. chelonei), stain poorly by the Gram method, usually appearing as highly refractile, slightly curved, beaded bacilli. Mycobacteria are aerobic, non-spore-forming, and non-motile. Rapid growers may show branching and ‘diphtheroid’-like bacilli. Many species produce yellow-pigmented colonies, either upon exposure to light (photochromogens) or in the dark (scotochromogens). Mycobacterium species produce a wide spectrum of human and animal infections, both localized and disseminated. For ease of discussion and epidemiologic correlates, mycobacteria can be divided into the M. tuberculosis complex (M. tuberculosis, M. bovis, M. africanum) and the non-tuberculous mycobacteria, which are not transmissible person to person, and which may colonize healthy individuals. Mycobacterium leprae, the causative agent of leprosy, does not grow on bacteriologic media.
