Mycobacterium

Mycobacterium is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis (M. tuberculosis) and leprosy (M. leprae) in humans. The Greek prefix myco- means 'fungus', alluding to this genus' mold-like colony surfaces. Since this genus has cell walls with a waxy lipid-rich outer layer that contains high concentrations of mycolic acid, acid-fast staining is used to emphasize their resistance to acids, compared to other cell types. Mycobacterial species are generally aerobic, non-motile, and capable of growing with minimal nutrients. The genus is divided based on each species' pigment production and growth rate. While most Mycobacterium species are non-pathogenic, the genus' characteristic complex cell wall contributes to evasion from host defenses. Mycobacteria are aerobic with 0.2-0.6 μm wide and 1.0-10 μm long rod shapes. They are generally non-motile, except for the species Mycobacterium marinum, which has been shown to be motile within macrophages. Mycobacteria possess capsules and most do not form endospores. M. marinum and perhaps M. bovis have been shown to sporulate; however, this has been contested by further research. The distinguishing characteristic of all Mycobacterium species is a thick, hydrophobic, and mycolic acid-rich cell wall made of peptidoglycan and arabinogalactan, with these unique components offering targets for new tuberculosis drugs. Many Mycobacterium species readily grow with minimal nutrients, using ammonia and/or amino acids as nitrogen sources and glycerol as a carbon source in the presence of mineral salts. Temperatures for optimal growth vary between species and media conditions, ranging from 25-45 °C. Most Mycobacterium species, including most clinically relevant species, can be cultured in blood agar. However, some species grow very slowly due to extremely long reproductive cycles, such as M. leprae requiring 12 days per division cycle compared to 20 minutes for some E.