Mycobacterium tuberculosis is a slow-growing, rod-shaped, acid-fast bacterium that causes tuberculosis in humans.
Characteristics and pathogenesis
Mycobacterium tuberculosis belongs to the Mycobacteriaceae family and has a lipid-rich cell wall containing mycolic acids and arabinogalactan. This unique envelope makes the organism acid-fast when stained by the Ziehl-Neelsen or auramine-rhodamine techniques and contributes to resistance against desiccation and many disinfectants. M. tuberculosis is an obligate aerobe that grows best in the lungs, where oxygen tension is high. It replicates slowly, with a generation time of around 24 hours, and forms rough, buff-coloured colonies on egg-based media. Infection occurs via inhalation of aerosolized droplets containing bacilli. The bacteria are phagocytosed by alveolar macrophages but can survive and replicate within phagosomes by inhibiting phagolysosome fusion. Cellular immune responses control bacterial multiplication, leading to the formation of granulomas composed of macrophages, lymphocytes and fibroblasts. These structures contain the infection but may harbour latent bacilli for years. Factors such as malnutrition, HIV infection or immunosuppression can disrupt host defences and allow reactivation of dormant bacteria.
Disease manifestations and control
Tuberculosis primarily affects the lungs but can spread to lymph nodes, bones, the central nervous system and other organs. Pulmonary disease presents with cough, weight loss, fever and night sweats. Extrapulmonary forms include miliary tuberculosis, tuberculous meningitis and osteomyelitis. Diagnosis relies on acid-fast staining, culture, nucleic acid amplification tests and immunological assays such as the tuberculin skin test or interferon-gamma release assays. Treatment requires prolonged multidrug regimens, typically including isoniazid, rifampicin, pyrazinamide and ethambutol. Emergence of multidrug-resistant and extensively drug-resistant strains complicates therapy and underscores the need for adherence and drug susceptibility testing. The Bacillus Calmette-Guerin vaccine provides partial protection against severe childhood disease but is less effective against adult pulmonary tuberculosis. Public health measures, including prompt detection, directly observed therapy and infection control practices, are essential for controlling transmission.
Mycobacterium tuberculosis remains one of the leading causes of infectious mortality worldwide. Understanding its unique biology and implementing effective diagnostic, therapeutic and preventive strategies are critical for reducing the burden of tuberculosis.
Related Terms: Acid-fast, Latent infection, Granuloma, Tuberculosis, Mycobacteria