Mycobacterium tuberculosis- An Overview


Mycobacterium tuberculosis (M. tb) is a type of bacteria that causes tuberculosis (TB), a chronic infectious disease that mainly affects the lungs, but can also involve other organs and tissues. TB is one of the oldest and deadliest diseases in human history, and remains a major global health problem. According to the World Health Organization (WHO), TB killed 1.6 million people in 2021, making it the second leading infectious killer after COVID-19.

M. tb was first discovered in 1882 by Robert Koch, a German physician and microbiologist who isolated the bacteria from the sputum of a patient with pulmonary TB. He also demonstrated that M. tb could be transmitted from one person to another through the air, and that it could be cultured on solid media. For his groundbreaking work, Koch was awarded the Nobel Prize in Physiology or Medicine in 1905.

M. tb belongs to the genus Mycobacterium, which comprises more than 200 species of bacteria that share some common features, such as a high content of mycolic acid in their cell walls, which makes them acid-fast, meaning that they retain certain dyes even after being washed with acid. This property helps to identify M. tb under the microscope using special stains, such as the Ziehl-Neelsen stain, which gives them a bright red color against a blue background.

M. tb is an obligate aerobe, meaning that it requires oxygen to grow and survive. It is also a slow-growing bacterium, with a generation time of about 15 to 24 hours, compared to other bacteria that can divide every 20 minutes or less. This slow growth rate makes M. tb difficult to culture and treat with antibiotics, as it can take several weeks to months to obtain visible colonies on solid media or to determine drug susceptibility.

M. tb is also an intracellular pathogen, meaning that it can invade and multiply within host cells, especially macrophages, which are immune cells that normally engulf and destroy foreign particles and microbes. M. tb has evolved various strategies to evade the killing mechanisms of macrophages, such as preventing the fusion of phagosomes with lysosomes, inhibiting the acidification of phagosomes, and modulating the host immune response.

M. tb has a complex genome that consists of about 4.4 million base pairs of DNA and encodes about 4000 genes. The genome contains many repetitive sequences and large regions of difference (RDs) that are deleted or absent in some strains of M. tb compared to others. These RDs can be used as molecular markers to differentiate M. tb strains and to trace their evolutionary history and geographic origin.

M. tb is transmitted from person to person through respiratory droplets that are released when an infected person coughs, sneezes, speaks, or sings. The droplets can remain suspended in the air for several hours and can be inhaled by susceptible individuals who share the same environment. Not everyone who is exposed to M. tb becomes infected or develops TB disease, as this depends on various factors, such as the dose and duration of exposure, the immune status of the host, and the virulence of the strain.

There are two main stages of TB infection: latent TB infection (LTBI) and active TB disease (ATBD). LTBI occurs when M. tb enters the body and is contained by the host immune system, forming granulomas (small nodules) in the lungs or other organs where the bacteria remain dormant but alive for years or decades without causing any symptoms or signs of disease. People with LTBI are not contagious and do not need treatment unless they have a high risk of developing ATBD.

ATBD occurs when M. tb reactivates from latency and starts multiplying and destroying tissue, causing symptoms such as cough, fever, weight loss, night sweats, chest pain, and hemoptysis (coughing up blood). ATBD can also spread from the lungs to other parts of the body through the bloodstream or lymphatic system, causing extrapulmonary TB that can affect organs such as the brain, spine, kidneys, lymph nodes, bones, joints, skin, or eyes.

The diagnosis of TB infection or disease is based on clinical history, physical examination, laboratory tests, and imaging studies. The most common tests for TB infection are the tuberculin skin test (TST) and the interferon-gamma release assay (IGRA), which measure the immune response to specific antigens of M. tb in the blood or skin. The most common tests for TB disease are sputum smear microscopy and culture for M. tb , which detect the presence of acid-fast bacilli in respiratory specimens.

The treatment of TB infection or disease involves a combination of antibiotics that are taken for several months to kill all the bacteria and prevent relapse or resistance. The standard regimen for LTBI consists of isoniazid (INH) for 6 to 9 months or rifampin (RIF) for 4 months. The standard regimen for ATBD consists of four drugs: INH , RIF , pyrazinamide (PZA), and ethambutol (EMB) for 2 months followed by INH and RIF for 4 months.

The prevention of TB infection or disease includes various measures such as improving living conditions and ventilation; reducing overcrowding and contact with infectious cases; implementing infection control practices in health care settings; screening high-risk groups for LTBI and offering preventive treatment; providing early diagnosis and effective treatment for ATBD cases; ensuring adherence and completion of therapy; monitoring drug resistance and adverse effects; promoting health education and awareness; and supporting research and development for new drugs , vaccines ,and diagnostics.