M. tuberculosis is a slim, strongly acid – alcohol – fast rod. It frequently shows irregularbeading in its staining, appearing as connected series of acid-fast granules (Fig 28 – 1). It grows at 37°C but not at room temperature, and it requires enriched or complex media for primary growth. Growth is enhanced by 5 to 10% carbon dioxide but is still very slow, with a mean generation time of 12 to 24 hours. The classic medium, Löwenstein – Jensen, contains homogenized egg in nutrient base with dyes to inhibit the growth of nonmycobacterial contaminants. The dry, rough, buff-colored colonies usually appear after 3 to 6 weeks of incubation. Mycobacterial growth is more rapid in two semisynthetic oleic acid – albumin media. Virulent strains grown in the latter demon-strate “cording” in which multiplying organisms remain attached in parallel bundles to form long intertwining cords or ropes. The major phenotypic tests for identification of M. tuberculosis are summarized in Table 28 – 1. Of particular importance is the ability of M. tuberculosis to produce large quantities of niacin, which is uncommon in othermycobacteria.
Due to its hydrophobic lipid surface, M. tuberculosis is unusually resistant to drying, to most common disinfectants, and to acids and alkalis. Tubercle bacilli are sensitive to heat, including pasteurization, and individual organisms in droplet nuclei are susceptible to inactivation by ultraviolet light.
As with other mycobacteria, the M. tuberculosis cell wall structure is dominated by mycolic acids and LAM. Its antigenic makeup includes many protein and polysaccharide antigens of which tuberculin is the most studied. It consists of heat-stable proteins liber-ated into liquid culture media. A purified protein derivative (PPD) of tuberculin is used for skin testing for hypersensitivity and is standardized in tuberculin units according to skin test activity.