Mycobacterium tuberculosis
Introduction
Mycobacterium tuberculosis (Mtb) is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, it is classified as an acid-fast bacillus due to its resistance to acid-fast staining, a characteristic shared with other members of the Mycobacterium genus.
Morphology
Mycobacterium tuberculosis is a small, aerobic, nonmotile bacillus. The bacterium is a Gram-positive organism, but it does not retain the crystal violet dye used in the Gram staining process due to the high lipid content in its cell wall. Instead, it is classified as acid-fast and can be visualized using special staining techniques such as the Ziehl-Neelsen stain.
Genetics
The genome of M. tuberculosis is a single circular chromosome that contains approximately 4,400 genes. The genome is highly conserved, with little variation observed between different strains of the bacterium. This is thought to be due to the bacterium's slow growth rate and the absence of horizontal gene transfer.
Pathogenesis
Mycobacterium tuberculosis is transmitted from person to person via airborne droplets that are produced when individuals with active tuberculosis cough or sneeze. Once inhaled, the bacteria are engulfed by alveolar macrophages in the lungs but are able to resist destruction by these immune cells. Instead, they multiply within the macrophages and eventually cause cell death, leading to the formation of a granuloma.
Immune Response
The immune response to M. tuberculosis is complex and involves both the innate and adaptive immune systems. The initial response is mediated by alveolar macrophages and dendritic cells, which engulf the bacteria and present antigens to T cells, initiating the adaptive immune response. The formation of granulomas is a key aspect of the immune response to M. tuberculosis, serving to contain the infection and prevent its spread.
Clinical Manifestations
The primary infection with M. tuberculosis is often asymptomatic, but can sometimes cause a mild, flu-like illness. If the immune system is unable to contain the infection, active tuberculosis can develop, with symptoms including chronic cough, fever, weight loss, and night sweats. In some cases, the bacteria can disseminate to other parts of the body, causing extrapulmonary tuberculosis.
Diagnosis
Diagnosis of tuberculosis is based on clinical symptoms, imaging studies, and microbiological testing. The tuberculin skin test and interferon-gamma release assays can be used to detect latent tuberculosis infection, while sputum culture and molecular tests are used to diagnose active disease.
Treatment
Treatment of tuberculosis involves a combination of several antibiotics, typically isoniazid, rifampicin, pyrazinamide, and ethambutol. The treatment is long, often requiring at least six months of therapy, and drug resistance is a growing problem.
Epidemiology
Tuberculosis is a major global health problem, with an estimated 10 million new cases and 1.5 million deaths in 2018. The disease is most common in low- and middle-income countries, particularly in sub-Saharan Africa and Southeast Asia.
Prevention
Prevention strategies for tuberculosis include vaccination with the BCG vaccine, treatment of latent tuberculosis infection, and infection control measures in healthcare settings.