Mycobacterium tuberculosis (MTB) is the bacterium responsible for tuberculosis (TB), a serious infectious disease. TB primarily affects the lungs, but it can also impact other parts of the body. It has a significant global health impact, with millions of new cases and deaths reported annually.
Understanding Mycobacterium Tuberculosis and Tuberculosis
MTB is a slow-growing, rod-shaped bacterium that requires oxygen. Its unique cell wall, rich in lipids like mycolic acid, contributes to its resistance to many common antibiotics and allows it to survive within host immune cells. In laboratory settings, visible colonies can take several weeks to form.
Tuberculosis manifests in two forms: latent TB infection (LTBI) and active TB disease. In LTBI, the bacteria are present but dormant, causing no symptoms and preventing transmission. The immune system contains the infection.
Active TB disease occurs when the bacteria multiply and overcome the immune system, leading to symptoms and the ability to spread the infection. This form can be categorized as pulmonary TB, which affects the lungs and is the most common and often contagious type, or extrapulmonary TB, which develops outside the lungs. Extrapulmonary TB can involve organs such as lymph nodes, bones, joints, kidneys, or the brain.
How Tuberculosis Spreads
Tuberculosis is an airborne disease, spreading through the air from person to person. When individuals with active pulmonary TB cough, sneeze, speak, or sing, they release tiny airborne particles called droplet nuclei containing the bacteria. These infectious droplets can remain suspended in the air for several hours, particularly in enclosed spaces with poor ventilation.
Transmission occurs when a susceptible person inhales these droplet nuclei, which travel into the respiratory tract and settle in the lungs. Close and prolonged contact with an infected person is generally required. For instance, people living in the same household or close workplace colleagues are at a higher risk. Latent TB infection is not contagious, as individuals with LTBI do not expel the bacteria.
Recognizing Tuberculosis Symptoms
Active pulmonary TB presents with symptoms that develop gradually over several weeks. A persistent cough lasting three weeks or longer is common, and this cough may produce sputum, sometimes mixed with blood. Individuals may also experience chest pain, particularly when breathing or coughing.
Beyond respiratory symptoms, active TB disease often causes general signs affecting the whole body. These include weakness, unexplained weight loss, and loss of appetite. Fever, chills, and night sweats are also reported.
Symptoms of extrapulmonary TB vary significantly depending on the organ affected. For instance, TB of the spine can cause back pain, while TB of the kidney might lead to blood in the urine. TB meningitis, affecting the brain, can result in headaches or confusion, and TB of the lymph nodes may cause firm, swollen glands. Latent TB infection has no physical signs or symptoms because the bacteria are inactive.
Diagnosing and Treating Tuberculosis
Diagnosing tuberculosis involves tests to identify both latent infection and active disease. For latent TB infection (LTBI), common methods include the Tuberculin Skin Test (TST) and Interferon-Gamma Release Assays (IGRAs). TST involves injecting tuberculin under the skin to measure an immune response, while IGRAs are blood tests that measure interferon-gamma release in response to TB antigens. IGRAs require only one patient visit and are not affected by prior BCG vaccination, which can cause false positives in TST.
For active TB disease, a chest X-ray is often performed to check for lung abnormalities, such as white spots or cavities, that may indicate active infection. However, a chest X-ray alone is not sufficient for a definitive diagnosis and cannot detect latent TB. Sputum smear microscopy involves examining a sputum sample under a microscope for acid-fast bacilli, which suggests active pulmonary TB. Sputum culture involves growing the Mycobacterium tuberculosis bacteria from a specimen, though this process can take several weeks.
Rapid molecular tests, such as the GeneXpert MTB/RIF assay, detect Mycobacterium tuberculosis and mutations associated with rifampicin resistance directly from sputum specimens in less than two hours. These tests allow for faster diagnosis and detection of drug resistance, which is important for managing multidrug-resistant TB (MDR-TB).
Tuberculosis is treatable with multiple antibiotics, typically administered for 6 to 9 months. The initial phase for active TB usually involves four drugs, such as isoniazid, rifampicin, pyrazinamide, and ethambutol, for about two months, followed by a continuation phase with fewer drugs. For latent TB, treatment regimens are shorter, ranging from 3 to 9 months, and may involve a single drug like isoniazid or a combination of rifampicin and isoniazid.
Completing the full course of treatment is important to kill all TB germs and prevent drug resistance and relapse. Drug-resistant TB (DR-TB) presents a significant challenge, occurring when bacteria become resistant to standard anti-TB medications. Multidrug-resistant TB (MDR-TB) is resistant to at least isoniazid and rifampicin, the two most powerful first-line drugs. Extensively drug-resistant TB (XDR-TB) is an even more severe form, resistant to isoniazid, rifampicin, any fluoroquinolone, and at least one additional second-line injectable drug. Treating DR-TB requires longer, more complex regimens, often lasting 18 to 24 months, utilizing second-line drugs that can be more toxic and less effective than first-line treatments.
Preventing Tuberculosis
Preventing tuberculosis involves vaccination, infection control, and proactive treatment of latent infections. The Bacillus Calmette-Guérin (BCG) vaccine is widely used globally, particularly in countries with a high burden of TB. This vaccine is 70-80% effective in preventing severe forms of TB in children, such as TB meningitis, but its effectiveness against pulmonary TB in adults is more variable. For adults over 35, the vaccine’s effectiveness is limited.
In healthcare settings, infection control measures are crucial to minimize the spread of Mycobacterium tuberculosis. These include ensuring proper ventilation, as TB bacteria can remain suspended in the air for hours. Implementing respiratory hygiene, such as covering coughs and sneezes with a tissue, helps reduce the dispersal of infectious droplets. Isolation of patients with active pulmonary TB in special rooms with negative pressure or designated units also helps prevent transmission.
Treating latent TB infection in high-risk individuals is a significant prevention strategy, aiming to stop the dormant bacteria from progressing to active disease. This preventive treatment can involve medications like isoniazid, sometimes combined with rifapentine, taken for several months. Public health efforts further support prevention through systematic screening, contact tracing to identify individuals exposed to active cases, and ensuring widespread access to timely diagnosis and appropriate treatment for both latent and active TB.