Mycobacterium tuberculosis

Mycobacterium tuberculosis
Name	Mycobacterium tuberculosis
Caption	Scanning electron micrograph of Mycobacterium tuberculosis
Field	Infectious disease, Medical microbiology
Symptoms	Chronic cough, fever, night sweats, weight loss
Complications	Miliary tuberculosis, Tuberculous meningitis, Pott disease
Causes	Infection by Mycobacterium tuberculosis
Risks	HIV/AIDS, Diabetes mellitus, Malnutrition, Smoking
Diagnosis	Sputum culture, Chest radiograph, Tuberculin skin test, Interferon-gamma release assay
Treatment	Antibiotics (Isoniazid, Rifampicin, Pyrazinamide, Ethambutol)
Prevention	BCG vaccine, Infection control
Frequency	~10 million new cases annually (global)
Deaths	~1.5 million annually (global)

Mycobacterium tuberculosis is the primary causative agent of the infectious disease tuberculosis in humans. It is an obligate pathogenic bacterial species belonging to the family Mycobacteriaceae and is characterized by its complex, lipid-rich cell wall. First identified by Robert Koch in 1882, this slow-growing acid-fast bacillus remains a major global public health challenge, responsible for significant morbidity and mortality worldwide.

Description and classification

The bacterium is a small, rod-shaped, non-motile aerobic organism that stains weakly Gram-positive due to its unique cell envelope. Its most distinctive feature is a waxy, hydrophobic cell wall rich in mycolic acid, which confers resistance to many common stains and disinfectants. This complex structure is the basis for the Ziehl–Neelsen stain, a cornerstone of microscopic identification. Taxonomically, it is the type species of the Mycobacterium tuberculosis complex, a group that includes other pathogenic members like Mycobacterium bovis and Mycobacterium africanum. The organism's genome was fully sequenced in 1998 by a team at the Sanger Institute, revealing a high proportion of genes involved in lipid metabolism.

Pathogenesis and transmission

Infection typically occurs via inhalation of aerosolized droplets containing the bacilli, expelled by individuals with active pulmonary disease. Upon reaching the alveoli in the lungs, the bacteria are phagocytosed by alveolar macrophages but can survive within these immune cells by inhibiting phagosome-lysosome fusion. This initial infection can lead to a contained, asymptomatic state known as latent tuberculosis infection. Progression to active disease often involves the formation of characteristic granulomatous lesions, or tubercles, which can caseate and liquefy, allowing bacterial proliferation and potential dissemination to other sites like the meninges, kidneys, or spine.

Clinical manifestations

Pulmonary tuberculosis is the most common form, presenting with a persistent cough, hemoptysis, chest pain, and systemic symptoms like fever, night sweats, and weight loss. Extrapulmonary disease can affect virtually any organ system. Notable forms include miliary tuberculosis, a disseminated infection; tuberculous meningitis, a severe infection of the central nervous system; and Pott disease, which involves the vertebral column. The risk of active disease is dramatically increased in immunocompromised individuals, particularly those co-infected with HIV.

Diagnosis

Definitive diagnosis relies on microbiological confirmation. Sputum smear microscopy using acid-fast stains is a rapid, low-cost method used widely in high-burden settings. Culture on specialized media like Lowenstein–Jensen medium remains the gold standard, though it can take several weeks. Molecular techniques, such as the Xpert MTB/RIF assay endorsed by the World Health Organization, allow for rapid detection and identification of rifampicin resistance. Immunological tests include the century-old tuberculin skin test and modern interferon-gamma release assays. Radiographic findings on a chest radiograph, such as upper lobe infiltrates and cavities, are also highly suggestive.

Treatment and prevention

Standard treatment for drug-susceptible cases involves a multi-drug regimen administered over six months, typically including first-line agents isoniazid, rifampicin, pyrazinamide, and ethambutol. The emergence of multidrug-resistant tuberculosis and extensively drug-resistant tuberculosis necessitates longer, more complex, and toxic regimens with second-line drugs like fluoroquinolones and aminoglycosides. Prevention strategies include vaccination with the Bacillus Calmette–Guérin vaccine, which provides variable protection against severe childhood forms, and public health measures like active case-finding and directly observed therapy, short-course.

Epidemiology

It is estimated that nearly one-quarter of the global population has a latent infection. In 2022, the World Health Organization reported approximately 10.6 million new cases of active tuberculosis and 1.3 million deaths, making it one of the top infectious disease killers worldwide, surpassing HIV/AIDS. The burden is disproportionately high in regions of Southeast Asia, Africa, and the Western Pacific. Major risk factors driving the epidemic include poverty, HIV co-infection, diabetes, and malnutrition. Global control efforts are coordinated by entities like the Stop TB Partnership and the Global Fund to Fight AIDS, Tuberculosis and Malaria.

Category:Mycobacterium Category:Bacterial diseases Category:Infectious diseases