[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Mycobacteria](https://durgical.com/en/microbiology/groups/mycobacteria/) › Mycobacterium tuberculosis

[Microbiology](https://durgical.com/en/microbiology/) · [Mycobacteria](https://durgical.com/en/microbiology/groups/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

## Key points

- Airborne precautions: negative-pressure room and N95 respirator (a surgical mask is not enough)
- Ziehl-Neelsen: acid-fast bacilli; culture on Löwenstein-Jensen
- Isoniazid: peripheral neuropathy (prevented with pyridoxine) and hepatitis; rifampicin: orange body fluids and cytochrome P450 induction
- Ethambutol: optic neuritis (colour-vision loss); pyrazinamide: hyperuricaemia
- Latent TB is ruled out before starting anti-TNF drugs

## 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

- TB infection (latent) and active TB disease [Tuberculosis →](https://durgical.com/en/conditions/tuberculosis/)
- Primary and reactivation pulmonary TB (apical cavities) [Pulmonary tuberculosis →](https://durgical.com/en/conditions/pulmonary-tuberculosis/)
- Pott disease: tuberculous spondylitis [Spinal tuberculosis (Pott disease) →](https://durgical.com/en/conditions/spinal-tuberculosis/)
- Miliary, meningeal, pleural, lymph node (scrofula), renal and intestinal TB

## 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:

- [Isoniazid](https://durgical.com/en/pharmacology/isoniazid/)
- [Rifampicin](https://durgical.com/en/pharmacology/rifampicin/)
- [Pyrazinamide](https://durgical.com/en/pharmacology/pyrazinamide/)
- [Ethambutol](https://durgical.com/en/pharmacology/ethambutol/)
- [Bedaquiline](https://durgical.com/en/pharmacology/bedaquiline/)
- [Linezolid](https://durgical.com/en/pharmacology/linezolid/)

## 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.

## In Colombia

A public health priority: the National TB Prevention and Control Programme (MinSalud, Resolution 227 of 2020) sets the diagnostic algorithms and regimens, and the Ministry buys the drugs centrally. It is a mandatory notifiable event in SIVIGILA (INS). TB/HIV coinfection and prisons are focus areas.

## Memory aid

Think of «the locked-up bacillus»: it arrives by air, hides inside the macrophage and the body builds it a cheese prison (caseous granuloma); when the guards weaken (HIV, anti-TNF), it breaks out.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/mycobacterium-tuberculosis/)

## Related

- [Mycobacterium leprae](https://durgical.com/en/microbiology/mycobacterium-leprae/)
- [Mycobacterium avium complex and other non-tuberculous mycobacteria](https://durgical.com/en/microbiology/mycobacterium-avium-complex-and-other-non-tuberculous-mycobacteria/)

## Related conditions

- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)
- [Pulmonary tuberculosis](https://durgical.com/en/conditions/pulmonary-tuberculosis/)
- [Spinal tuberculosis (Pott disease)](https://durgical.com/en/conditions/spinal-tuberculosis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

**Contact**
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

**Droplet**
Surgical mask within about a metre; single room.

**Airborne**
N95 respirator and a negative-pressure room.

[← Back to the group: Mycobacteria](https://durgical.com/en/microbiology/groups/mycobacteria/)

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

Sources: [Tuberculosis](https://www.who.int/news-room/fact-sheets/detail/tuberculosis) (World Health Organization (WHO), 2025); [About Tuberculosis | Tuberculosis (TB) | CDC](https://www.cdc.gov/tb/about/index.html) (Centers for Disease Control and Prevention (CDC), 2025); [Tuberculosis](https://www.minsalud.gov.co/salud/publica/PET/Paginas/Tuberculosis.aspx) (Ministerio de Salud y Protección Social de Colombia, 2024); [Disseminated tuberculosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000624.htm) (MedlinePlus (U.S. National Library of Medicine), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/mycobacterium-tuberculosis/