The genus Mycobacterium is characterized by slender, non-motile rods with complex, lipid-rich cell walls resisting de-staining by acid alcohol (hence "acid fast"). Mycobacteria are strictly aerobic and grow on fairly simple solid or fluid media. The many members of this genus are conventionally differentiated by rate and optimal temperature of growth, production of pigments and biochemical tests. Some mycobacterial species as Mtb, M. africanum, M. ulcerans and M. bovis share growth characteristics and biochemical reactions and are classified together within the M. tuberculosis complex. The BCG vaccine is derived from M. bovis. More than 55 species of environmental mycobacteria are known, half of which may cause disease in humans. The prevalence of environmental mycobacteria is higher in hot than in cold climates. The M. avium-intracellulare complex is the most ubiquitous of the environmental mycobacteria.
M. tuberculosis has a long generation time (8-24 hours), and growth on solid media such as the Lowenstein-Jensen medium is detectable only after 2-6 weeks. With fluid media and automated detection systems, growth may be detected within 1-2 weeks. Under field conditions, the diagnosis of TB is usually based on microscopy, demonstrating acid-fast bacilli in sputa or other clinical specimens using the Ziehl-Neelsen staining technique or a fluorescent acid-fast dye such as autamine. Isolation of the organism is required for a definitive species diagnosis and determination of antibiotic sensitivity. Modern molecular techniques based on nucleic acid amplification and genetic probes may provide rapid species information directly on clinical material and are now routinely used in modern laboratory settings as supplements to more conventional diagnostic methods. In recent years, DNA probes have become available that allow detailed epidemiological studies using DNA fingerprint techniques.