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Mycobacteria and Tuberculose

 

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The organism
Taxonomy
Clinical aspects
The NRC
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2010 [fr] [nl]

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Project leader: Dr. V.  Mathys

Mycobacteria are acid fastness bacilli of the genus Mycobacterium which includes obligate parasites, saprophytes and opportunistic pathogens. Most of the 150 different species are free living in soil and water, but the major ecological niche for others such as Mycobaterium tuberculosis and M. leprae is diseased tissue of humans and other warm-blooded animals.

 

The organism

Mycobacteria are slightly curved or straight bacilli considered as gram positive bacteria.
But the high lipid content of the cell wall (including long carbon-chain mycolic acids) renders them impermeable to the usual aniline dyes. They require special acid-fast staining procedures like Ziehl-Neelsen, where they are not decolourized by acid-alcohol after staining in red by Carbol fuchsin.
They can also be coloured by fluorochromes (fluorescent dyes like auramine-rhodamine).
The DNA of mycobacteria has a high G+C content (70%).
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Taxonomy

The genus Mycobacterium is the only genus of the family Mycobacteriaceae in the order of Actinomycetales.
  1. A natural division occurs between slowly and rapidly growing species of mycobacteria. Slow growers (generation time from 2 to more than 20 h) require more than 7 days to produce easily seen colonies on solid culture media from a diluted inoculum, under ideal culture conditions. Rapid growers require less than 7 days in comparable conditions.
  2. Within these 2 groups, the different species are either achromogen (without pigmentation), photochromogen (pigmented at light) or scotochromogen (always pigmented) (basis of the Runyon classification).
  3. The genus Mycobacterium includes the species of the Mycobacterium tuberculosis complex, non cultivable mycobacteria (M.leprae) and the large group of non tuberculous mycobacteria named NTM.

Clinical aspects

TUBERCULOSIS is caused by bacteria of the Mycobacterium tuberculosis complex.
The complex includes the species M.tuberculosis, M.africanum and M.canetti responsible for tuberculosis (TB) in humans, M.bovis, M.microti and M.pinnipedii responsible for TB in animals. These last ones are zoonotic agents and can be transmitted to humans.

Tuberculosis (TB) is a highly contagious airborne disease. The form of the disease is often pulmonary but it can also be extrapulmonary and cause cervical adenitis, skin infections, pericarditis, synovitis and meningitis.

The most common symptoms of pulmonary tuberculosis are persistent cough with or without sputum production for more than 2-3 weeks, low-grade fever, night sweats, weight loss, and sometimes dyspnoea and chest pain.
Tuberculosis in AIDS patients is much more rapidly progressive and causes disseminated disease.

Thanks to the host’s immune system, only 50 % of the airborne contaminated persons will remain infected by the tuberculosis bacilli. The infected patients have a latent tuberculosis infection or LTBI. They are not ill but their tuberculin skin test is positive (as well as the blood test interferon-gamma-release assay).
Only 5 to 10 % of the infected patients will develop active tuberculosis (= tuberculosis disease) during their live. The diagnosis of active tuberculosis is confirmed by the detection of M.tuberculosis in clinical specimens.

In conjunction with contact tracing, early diagnosis and effective treatment of infection and disease are the major strategies for limiting TB transmission.

Active tuberculosis is treated by a combination of 4 antibiotics (first-line antituberculosis drugs) during 2 months followed by a 4 months bitherapy (total 6 months). However the emergence of multidrug-resistant strains, and more recently of extensively drugresistant cases of tuberculosis, requires administration of second-line antibiotics and longer-term treatment.


MYCOBACTERIOSIS can be caused by different species of non-tuberculous mycobacteria (NTM). The disease has varied manifestations and is usually not transmitted from man to man.

Non-tuberculous mycobacteria have been reported to cause localized or disseminated disease depending on local predisposition and/or degree of immune deficit.
- In non-HIV patients, different NTM may cause localized pulmonary disease, adenitis, soft tissue infections, infections of joints/bones, bursae, skin ulcers and generalized disease in immunodeficient individuals like patients with leukaemia, transplant patients etc...
- In AIDS patients the manifestations may range from localized to disseminated disease.

Clinical features will include local organ specific signs and symptoms to persistent fever, night sweats, anaemia and weight loss in addition to nonspecific symptoms of malaise, anorexia, diarrhoea, myalgia and occasional painful adenopathy.

The diagnosis of an infection by NTM is confirmed by isolation and identification of the mycobacterial species in clinical specimens obtained from lesion/affected organ. Histopathology should not exclude microbiological diagnosis.

The National Reference Center (NRC)

The Belgian Reference Centre of Tuberculosis and Mycobacteria, is a laboratory specialized in the microbiological diagnosis of tuberculosis and mycobacterioses, and in the molecular surveillance of tuberculosis.
The objectives are:

  1. Identification of clinical strains of mycobacteria to the species level (molecular techniques)
  2. Drug susceptibility testing (bacteriological and molecular methods)
  3. Genetic fingerprinting (molecular methods)

Technical details concerning our activity are given in our compendium.

  • The species identification of mycobacteria isolated by culture from clinical specimens is important to determine if the isolated microorganism can be considered as the cause of the disease. Moreover the treatment is depending on the mycobacterial species and is totally different for tuberculosis and NTM infections.
  • Determination of susceptibility to drugs is essential to correctly treat tuberculosis. It is performed on M.tuberculosis and on pathogenic NTM only when they are really considered as the cause of an infection.
  • Fingerprinting is the determination of the DNA genotype of the isolated M.tuberculosis strain. It allows determining the degree of genetic similarity or polymorphism between isolates. On isolates from different patients it is used for contact tracing (to trace tuberculosis transmission in the population) and for outbreak investigations. On isolates from the same patient, it can differentiate a relapse from an exogenous re-infection. Fingerprinting can also confirm suspected false-positive laboratory results due to cross-contamination between specimens of different patients during the technical process.

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