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Microbiology · Mycobacteria

Mycobacterium tuberculosis !

  • Bacterium
  • Acid-fast
  • Airborne
  • Exam priority: high

The tuberculosis bacterium: it travels in the air when people cough, mainly infects the lungs and is cured with several antibiotics over months.

Updated 2026-11-01

Also called: Koch's bacillus, TB bacillus

At a glance

Gram
Acid-fast
Shape
Thin straight rod; forms «cords» in culture
Oxygen
Strict aerobe (favours the lung apices)
Family
Mycobacteriaceae
Stain
Ziehl-Neelsen: red acid-fast bacilli; auramine-rhodamine for fluorescence
Size
About 2–4 µm long

Morphology

  • Thick wall with mycolic acids, arabinogalactan and lipoarabinomannan
  • No spores and no flagella
  • Rough, dry, cream-coloured colonies on Löwenstein-Jensen medium after several weeks

Transmission

  • Droplet nuclei (aerosols) expelled by a person with pulmonary or laryngeal TB when coughing, talking or sneezing
  • Aerosols can stay airborne for hours in closed, poorly ventilated rooms
  • Objects do not transmit it; closed extrapulmonary TB is barely contagious

Reservoir: Humans (M. bovis, of the same complex, in cattle)

Isolation precautions

What the care team adds to standard precautions.

Airborne
N95 respirator and a negative-pressure room.

Virulence factors

  • Cord factor (trehalose dimycolate): inhibits neutrophil migration and drives granuloma formation
  • Sulfatides and lipoarabinomannan: block phagosome–lysosome fusion so it survives inside macrophages
  • The Th1 response (interferon gamma, TNF) builds the caseating granuloma that contains infection; if immunity fails, it reactivates

Diseases it causes

Clinical picture

  • Cough for more than two weeks, evening fever, night sweats, weight loss and haemoptysis
  • Primary infection: Ghon complex (peripheral focus plus hilar node)
  • Reactivation: upper-lobe cavitation
  • Miliary and meningeal forms are more common in HIV and young children

Diagnosis

  • Sputum smear with Ziehl-Neelsen (AFB)
  • Rapid molecular tests (such as GeneXpert MTB/RIF): detect the bacillus and rifampicin resistance within hours
  • Culture on Löwenstein-Jensen or liquid medium (reference standard and susceptibility testing)
  • Latent infection: tuberculin skin test (PPD) or IGRA; IGRA is not affected by BCG
  • Biopsy: granuloma with caseous necrosis

Treatment

Combined first-line therapy with isoniazid, rifampicin, pyrazinamide and ethambutol, in an intensive and a continuation phase, with directly observed treatment. Multidrug-resistant TB (resistant to isoniazid and rifampicin) is treated with all-oral second-line regimens that include bedaquiline and linezolid.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Find and treat active cases early; screen contacts
  • Airborne isolation: single negative-pressure room, N95 respirator for staff and a surgical mask on the patient during transport
  • Preventive treatment of latent infection in contacts and people with HIV
  • BCG vaccine at birth

Vaccine: BCG (live attenuated M. bovis), given at birth in Colombia's PAI schedule. It protects children from meningeal and miliary TB but offers little protection against adult pulmonary TB.

Test yourself

Think of the answer, then tap the question to see it.

Which test detects latent infection without interference from BCG?

The IGRA (interferon-gamma release assay), because it uses antigens absent from BCG.

Which precaution category and respiratory protection does staff use for active pulmonary TB?

Airborne precautions, with an N95 respirator and a negative-pressure room.

Which TB drug turns urine and tears orange?

Rifampicin, which also induces liver enzymes and weakens other drugs.

What defines multidrug-resistant TB?

Resistance to at least isoniazid and rifampicin, the two most potent drugs of the regimen.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Tuberculosis (World Health Organization (WHO), 2025); About Tuberculosis | Tuberculosis (TB) | CDC (Centers for Disease Control and Prevention (CDC), 2025); Tuberculosis (Ministerio de Salud y Protección Social de Colombia, 2024); Disseminated tuberculosis: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2024).

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Representative model, for educational purposes only.