Infectious diseases

Mycobacterium tuberculosis

Tuberculosis, also known as TB, is an airbone infectious disease caused by the bacteriumMycobacterium tuberculosis that is preventable and curable. TB usually affects the lungs, but can also affect other parts of the body, such as the brain, the kidneys or the spine. Only people who are sick with TB in their lungs are infectious. 

Mycobacteria are a group of microorganisms that represent one of the most serious health problems worldwide. Three groups can be defined within the Mycobacterium genus: 1) M. tuberculosis complex (MTB) that causes tuberculosis and includes M.tuberculosis, M.bovis(including BCG), M.africanum and M.microti; 2) M. leprae which is the causal agent of leprosy; 3) Mycobacteria other than Tuberculosis (MOTT) which includes other less pathogenic mycobacteria with non-tuberculosis syndromes. MOTT are being increasingly isolated and its differentiation from MTB is of great clinical and public health importance, both for the isolation of patients in special rooms and the beginning of contact investigations.

TB is still a leading killer of young adults worldwide. According to World Health Organisation, each year 8 million people worldwide develop active TB and nearly 2 million die. Prevalence and severity of the disease are greater in patients coinfected with the human immunodeficiency virus (HIV).

Clinical features: Most infections in humans result in an asymptomatic latent infection. Overall, about 5 to 10% of infected people who do not receive treatment for latent TB infection eventually progresses to active disease which, if left untreated, will infect between 10 and 15 people a year. 

Symptoms of active TB depend on where in the body the TB bacteria are growing. The general symptoms of TB disease include feelings of sickness of weakness, weight loss, fever and night sweats. Active Pulmonary TB causes symptoms such as coughing, difficulties for breathing, chest pain and coughing up of the blood. Symptoms of TB disease involving areas other than the lungs vary depending upon the organ or area affected. 

Diagnosis: An early diagnosis of tuberculosis cases is essential for the control of the disease, in order to interrupt the transmission chain. The microbiological diagnosis of the tuberculosis infection has been traditionally based in direct microscopic examination (smear microscopy) together with culture. However, smear microscopy has a moderate sensitivity and around 5,000-10,000 bacilli per millilitre of sample are required for a positive detection. Moreover, since a negative result is not reliable to discard the disease, samples must be processed for culture and confirmation of the diagnosis. Culture enables an increase of the diagnostic sensitivity, as well as typing and drug susceptibility studies of the isolates. Nevertheless, the slow growth of mycobacteria is its major disadvantage, requiring up to 6-8 weeks for a result to score a sample as negative. After a positive culture, the identification of M. tuberculosis by molecular or classic methods is still necessary.Molecular methods based on the amplification of nucleic acids directly in samples provide an early detection with high sensitivity that show clear advantages over the traditional methods above mentioned. Furthermore, these methods can be used in grown-up cultures for the identification of the species, as well as the detection of genetic mutations related with the resistance to main antibiotics. 

Treatment: With appropriate antibiotic treatment, TB can be cured in most people. Treatment usually combines several different antibiotic drugs that are given for at least 6 months and up to 12 months. It is important that the patient take all the required medicines as prescribed. When the treatment is partial or inconsistent, the bacilli can become drug resistant and the patient could become sick again and spread TB to others. This is a particularly form of tuberculosis, known as multidrug-resistant TB (MDR-TB), which fails to respond to standard first-line drugs and becomes harder and more expensive to treat. Treatment for MDR TB often requires the use of special TB drugs, all of which can produce serious adverse drug reactions. Extensively drug-resistant TB (XDR-TB) takes place when resistance to second-line drugs develops on top of MDR-TB. Inappropriate management of these cases can have life-threatening results.

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