Micobactérias

Mycobacterium tuberculosis complex

Acid-fast bacilli (AFB), not seen on Gram stain

Identification & Growth

Identification

Ziehl-Neelsen or auramine; confirmation by rapid molecular testing (Xpert MTB/RIF)
Niacin-positive, nitrate reduction positive, heat-labile catalase

Growth Conditions

Löwenstein-Jensen: rough, dry, buff colonies in 3–8 weeks at 37 °C
Automated liquid medium (MGIT) reduces the time to 1–3 weeks

Clinical Significance

Pulmonary and extrapulmonary tuberculosis; compulsory notification

Intrinsic Resistance

Resistant to most routine antibacterials; treated with the specific RIPE regimen

Bench Alerts

EUCAST/BrCAST do not apply: susceptibility by dedicated methods (proportion/MGIT) and WHO criteria
Handling requires a BSL-3 laboratory and a biological safety cabinet

Clinical Notes

Clinical presentations

Pulmonary tuberculosis: chronic cough, evening fever, night sweats, weight loss and occasional hemoptysis, with upper-lobe cavitation.
Extrapulmonary forms include lymph node, pleural, bone/vertebral (Pott's disease), meningeal and miliary (disseminated) tuberculosis.
Latent tuberculosis infection is asymptomatic, with lifelong reactivation risk, particularly under immunosuppression.

Specimens and collection

Sputum (ideally three specimens, including one early-morning sample) for smear microscopy (Ziehl-Neelsen or auramine), culture and rapid molecular testing (Xpert MTB/RIF or similar).
Culture on solid medium (Löwenstein-Jensen) and/or liquid medium (automated MGIT-type system), the gold standard for diagnostic confirmation and susceptibility testing.
Extrapulmonary specimens (CSF, pleural fluid, tissue biopsy, urine) according to the suspected clinical site, with a specific request for AFB smear and mycobacterial culture.

Epidemiology and at-risk populations

One of the leading infectious causes of death worldwide according to WHO; transmitted via airborne droplet nuclei (Wells droplets).
Increased risk in people living with HIV, household contacts of smear-positive cases, incarcerated populations, people experiencing homelessness and healthcare workers.
Multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis represent a major public health challenge, requiring laboratory surveillance and mandatory reporting.

Resistance and therapeutic implications

Standard first-line regimen combines rifampicin, isoniazid, pyrazinamide and ethambutol, with a minimum duration of 6 months for drug-susceptible cases.
Rapid molecular tests (Xpert MTB/RIF) detect rifampicin resistance as a surrogate marker for MDR-TB, guiding early initiation of an alternative regimen.
Phenotypic and/or genotypic susceptibility testing for all first- and second-line drugs is essential when resistance is suspected or confirmed, per WHO guidance.

Bench and reporting notes

Acid-fast bacillus (AFB) with slow growth (weeks), requiring biosafety level 3 (BSL-3) precautions for culture handling.
The M. tuberculosis complex includes M. tuberculosis, M. bovis, M. africanum and other closely related species, distinguishable by molecular methods.
Tuberculosis is a notifiable disease; a positive smear, culture or molecular test result should trigger immediate epidemiological surveillance workflow.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Mycobacterium tuberculosis complex in the lab?

Mycobacterium tuberculosis complex is identified through ziehl-neelsen or auramine; confirmation by rapid molecular testing (xpert mtb/rif), niacin-positive, nitrate reduction positive, heat-labile catalase.

What are the intrinsic resistances of Mycobacterium tuberculosis complex?

This organism is naturally resistant to resistant to most routine antibacterials; treated with the specific ripe regimen. These drugs should not be reported as susceptible.

Where is Mycobacterium tuberculosis complex commonly found?

It is typically associated with pulmonary and extrapulmonary tuberculosis; compulsory notification.

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