Tuberculosis

Tuberculosis (TB) is the archetype of a chronic infectious disease: a slow-growing bacterium that can hide silently in the body for decades, then reactivate to cause a wasting, cavitating pneumonia that spreads through the air. It has killed more human beings across history than any other single infection, and even today it remains one of the top infectious killers worldwide, especially where poverty, crowding, HIV, and malnutrition intersect. Understanding TB means understanding a peculiar organism, a two-state disease (latent and active), a diagnostic puzzle, and a long, disciplined multidrug treatment that must be finished to the last dose.

For a student, TB is worth mastering because it teaches ideas that recur everywhere in medicine: how the immune system can contain but not eliminate a pathogen, why drug resistance emerges when treatment is short-changed, and why public-health thinking and individual patient care are inseparable.

Learning Objectives

• Describe the microbiology of Mycobacterium tuberculosis and why its cell wall shapes both diagnosis and treatment.
• Distinguish latent TB infection from active TB disease clinically and in terms of transmissibility.
• Select and interpret the main diagnostic tests: sputum smear, GeneXpert MTB/RIF, culture, chest radiography, and tests for latent infection (TST and IGRA).
• Explain the standard multidrug regimen (RIPE), why four drugs are used, and the principles behind treatment duration and adherence.
• Recognise drug-resistant TB (MDR/XDR) and the role of directly observed therapy and public-health control.

Quick Answer

Tuberculosis is caused by Mycobacterium tuberculosis, an aerobic, slow-growing, acid-fast bacillus spread by inhaled airborne droplet nuclei. After infection, most people develop latent TB — the bacteria are walled off in granulomas, the person feels well and is not infectious, but roughly 5-10% will reactivate over their lifetime. Active TB (classically a chronic productive cough, fever, night sweats, and weight loss) is diagnosed with sputum smear microscopy, rapid molecular testing (GeneXpert MTB/RIF), culture, and chest imaging. Standard treatment is a four-drug regimen — rifampicin, isoniazid, pyrazinamide, and ethambutol (RIPE) — for 2 months, then rifampicin and isoniazid for 4 more months. Multiple drugs are essential to prevent resistance, and completing the full course is the single most important determinant of cure.

Where It Came From

TB is ancient. Skeletal evidence of spinal TB (Pott disease) appears in Egyptian mummies, and the disease was described by Hippocrates as phthisis — a "wasting away" so common and so fatal that he warned physicians against visiting late-stage cases lest their own reputations suffer with the inevitable death. By the eighteenth and nineteenth centuries, TB — then called consumption — was killing perhaps one in seven people in industrialising Europe, romanticised in art even as it devastated crowded cities.

The decisive scientific moment came on 24 March 1882, when Robert Koch announced to the Berlin Physiological Society that he had identified the bacillus responsible. This was a monumental achievement: the organism is exceptionally hard to see and to grow. Koch developed special staining methods to make the bacillus visible, grew it in pure culture on coagulated serum, and reproduced the disease in animals — satisfying what became known as Koch's postulates, the logical criteria for proving that a specific microbe causes a specific disease. The motivation was urgent and practical: consumption was the leading cause of death, and no one knew whether it was hereditary, environmental, or contagious. Koch's proof that it was an infectious, transmissible disease reframed it as something that could, in principle, be prevented and controlled. (24 March is still observed as World TB Day.)

Everything that followed built on this foundation: the BCG vaccine (Calmette and Guerin, 1921), Wilhelm Roentgen's X-rays making the lungs visible, and — transformatively — the arrival of effective drugs. Streptomycin (1943) was the first antibiotic to treat TB, but the crucial lesson came fast: single-drug therapy quickly bred resistance, and patients relapsed. This hard-won insight — that TB must be treated with several drugs together — remains the governing principle of therapy today.

The Organism: Mycobacterium tuberculosis

M. tuberculosis is a rod-shaped, non-motile, non-spore-forming bacterium with several defining features that drive almost every clinical decision.

• Acid-fast. Its cell wall is thick with mycolic acids (long-chain fatty acids), making it waxy and impermeable. Ordinary Gram stain does not work well; instead the Ziehl-Neelsen stain uses heat and strong acid-alcohol washing, and the bacilli resist decolourisation, appearing as red rods against a blue background — hence "acid-fast bacilli" (AFB).
• Slow-growing and aerobic. It divides roughly every 15-20 hours (versus 20 minutes for E. coli), so cultures take weeks, not days, and the organism prefers the well-oxygenated lung apices — explaining why cavities form there.
• Intracellular survivor. It is engulfed by alveolar macrophages but can block the fusion of the phagosome with the lysosome, surviving inside the very cells meant to kill it. This is why cell-mediated (T-cell) immunity, not antibodies, controls TB.

That waxy wall is a double-edged sword: it makes the organism tough to stain, tough to kill, and tough for many antibiotics to penetrate — which is part of why treatment is so long.

Latent vs Active TB: The Two-State Disease

This distinction is the conceptual heart of TB and a favourite exam theme.

When an infectious person coughs, they release tiny droplet nuclei carrying bacilli. A susceptible person inhales them; the bacilli reach the alveoli and are taken up by macrophages. Over the next weeks, the immune system mounts a cell-mediated response, forming granulomas — organised balls of macrophages, lymphocytes, and giant cells that wall off the bacteria. In most people this succeeds in containment but not eradication.

Latent TB infection (LTBI):

• The bacteria are alive but dormant and contained.
• The person is asymptomatic, feels well, and is NOT infectious.
• Chest X-ray is usually normal; sputum tests are negative.
• Detected only by an immune-memory test: the tuberculin skin test (TST/Mantoux) or an interferon-gamma release assay (IGRA).
• Lifetime risk of progression to active disease is about 5-10%, highest in the first 2 years and dramatically higher with immunosuppression.

Active TB disease:

• Bacteria are multiplying and causing tissue damage.
• Symptoms: chronic cough more than 2-3 weeks, often productive or with blood (haemoptysis), plus fever, drenching night sweats, unexplained weight loss, and fatigue.
• Pulmonary TB (the lungs) is most common and is the infectious form. Extrapulmonary TB (lymph nodes, pleura, spine, meninges, kidneys, etc.) is generally not spread person-to-person.
• Confirmed by finding the organism in sputum or tissue.

Why does latent TB reactivate? Anything that weakens cell-mediated immunity tips the balance. The most powerful risk factor is HIV (which can raise annual reactivation risk to 5-10% per year rather than per lifetime). Others include diabetes, TNF-alpha inhibitors (e.g. for rheumatoid arthritis), chronic kidney disease, malnutrition, corticosteroids, and old age. This is why patients are screened and treated for LTBI before starting immunosuppressive biologics.

A worked case

A 34-year-old man presents with a 6-week cough, 5 kg weight loss, and night sweats. He is a recent migrant from a high-burden country. Chest X-ray shows a right upper-lobe cavity. Sputum GeneXpert is positive for M. tuberculosis with no rifampicin resistance detected. This is active pulmonary TB. Contrast with his well 30-year-old wife, who is asymptomatic with a normal chest X-ray but a positive IGRA — she has latent TB and is a candidate for preventive therapy, not full treatment.

Diagnosis

TB diagnosis combines clinical suspicion, imaging, and microbiological confirmation.

Sputum smear microscopy (Ziehl-Neelsen or fluorescent auramine). Cheap, fast, and identifies the most infectious patients, but it needs a high bacterial load (roughly 5,000-10,000 bacilli/mL) and cannot distinguish live from dead organisms or TB from other mycobacteria.

GeneXpert MTB/RIF (nucleic acid amplification). A game-changing rapid molecular test that detects M. tuberculosis DNA and rifampicin resistance in under 2 hours. It is far more sensitive than smear and is now the recommended initial test in most settings. A newer version (Ultra) is more sensitive still.

Culture. The gold standard — the most sensitive method and the only one that yields full drug-susceptibility testing. Liquid culture (e.g. MGIT) turns positive faster than solid Lowenstein-Jensen medium, but still takes 1-6 weeks because the organism grows so slowly.

Chest radiography. Classic active pulmonary TB shows upper-lobe infiltrates and cavitation; healed disease shows fibrosis and calcification. Imaging supports but does not confirm the diagnosis.

Tests for latent infection (used when there are no symptoms of active disease):

• Tuberculin skin test (Mantoux): intradermal injection of purified protein derivative; induration is read at 48-72 hours. It can be falsely positive after BCG vaccination or exposure to environmental mycobacteria.
• IGRA (e.g. QuantiFERON): a blood test measuring interferon-gamma released by the patient's T-cells in response to TB-specific antigens; it is not affected by prior BCG.

A crucial rule: before treating someone for latent TB, you must exclude active disease (history, exam, chest X-ray, and sputum if indicated) — otherwise you risk giving inadequate single-drug therapy to a person with active disease and breeding resistance.

Multidrug Therapy

Treatment rests on one principle learned the hard way: never treat active TB with a single drug. A large TB cavity contains billions of bacilli, and among them, by random mutation, a few are already resistant to any one drug. Give one drug and you kill the susceptible majority while selecting out the resistant survivors, guaranteeing relapse with a resistant strain. Using several drugs at once makes the chance that any single bacillus is resistant to all of them vanishingly small.

The standard first-line regimen for drug-susceptible TB is remembered as RIPE:

Drug	Key action	Signature adverse effect
R — Rifampicin	Powerful sterilising drug	Hepatotoxicity; harmless orange-red urine, tears, sweat; strong drug interactions (induces liver enzymes)
I — Isoniazid	Kills actively dividing bacilli	Hepatotoxicity; peripheral neuropathy (prevented with vitamin B6/pyridoxine)
P — Pyrazinamide	Works in acidic granuloma environment	Hepatotoxicity; raised uric acid (gout/arthralgia)
E — Ethambutol	Prevents resistance	Optic neuritis — check colour vision and acuity

The conventional schedule is an intensive phase of all four drugs for 2 months, followed by a continuation phase of rifampicin and isoniazid for 4 more months — a total of 6 months for most drug-susceptible pulmonary TB. Some sites (e.g. TB meningitis, bone) need longer.

Because three of the four drugs are hepatotoxic, baseline and monitored liver function is important, and patients are told to report jaundice, dark urine, or nausea. Adherence is supported by directly observed therapy (DOT), where a health worker watches the patient swallow the pills, and increasingly by fixed-dose combination tablets and digital adherence tools.

Latent TB is treated differently and more briefly — commonly isoniazid for a few months, or shorter rifampicin-based regimens (e.g. 3 months of weekly rifapentine plus isoniazid) — because the bacterial load is tiny and the goal is prevention.

Drug-resistant TB is the looming threat:

• MDR-TB = resistant to at least both rifampicin and isoniazid, the two most important drugs. Treatment is longer, uses more toxic and expensive second-line drugs, and increasingly newer agents such as bedaquiline.
• XDR-TB = MDR plus resistance to key additional drug classes, harder still to cure.

Resistance is almost always man-made — the product of interrupted, incomplete, or poorly chosen treatment. This is the enduring lesson from streptomycin in the 1940s, and it is why finishing every dose matters not just for the individual but for everyone.

Real-World Applications

• Clinical practice: Any patient with a cough lasting more than 2-3 weeks, especially with weight loss or night sweats or from a high-burden setting, should prompt TB testing. Before starting a biologic like a TNF-alpha inhibitor, screen for latent TB with an IGRA and treat it first.
• Public health: TB is a notifiable disease. Diagnosing one case triggers contact tracing — screening household and close contacts for both active and latent infection. Airborne isolation of infectious inpatients protects staff and other patients.
• Everyday relevance: BCG vaccination in childhood (used in high-burden countries) reduces severe childhood forms such as TB meningitis and miliary TB, though it is less reliable at preventing adult pulmonary disease. Good ventilation, nutrition, and reducing overcrowding remain powerful, low-tech defences.

Common Mistakes

1. "A positive skin test means the person has active TB." Wrong. TST/IGRA detect immune memory of infection, i.e. latent infection (or past exposure). They cannot diagnose active disease and can even be negative in severe disease or immunosuppression (anergy). Active TB is diagnosed by finding the organism, not by an immune test.

2. "If the patient feels better after a few weeks, they can stop the tablets." Wrong and dangerous. Symptoms improve long before the slow-growing, semi-dormant bacilli are eradicated. Stopping early leaves survivors that relapse — often as resistant disease. The full 6 months (or more) must be completed.

3. "One good drug should cure TB." Wrong. Monotherapy predictably selects for resistant mutants already present in a large bacterial population. This is the entire rationale for multidrug therapy, and it also means you must never add a single new drug to a failing regimen.

4. (Bonus) "Orange urine on rifampicin means kidney or liver damage." Wrong. Harmless discolouration of body fluids is an expected, benign effect of rifampicin; warn patients in advance so they are not alarmed.

Comparison and Connections

Feature	Latent TB	Active TB
Symptoms	None	Cough, fever, night sweats, weight loss
Infectious to others	No	Yes (pulmonary form)
Chest X-ray	Usually normal	Often abnormal (infiltrate/cavity)
Sputum smear/culture	Negative	May be positive
Best test	TST or IGRA	Smear, GeneXpert, culture
Treatment	Preventive (e.g. isoniazid, 1 drug)	Full regimen (RIPE, 4 drugs)

TB connects to immunology (granuloma formation and cell-mediated immunity — see ../../34._Immunology/index.md), to microbiology (acid-fast staining and mycobacteria — see ../../6._Microbiology/index.md), to pharmacology (antimycobacterial drugs and resistance — see ../../5._Pharmacology/index.md), and to community medicine (notification, DOT, contact tracing — see ../../8._Community_Medicine/index.md). It is also a common HIV-associated opportunistic infection, linking it to infectious diseases.

Contrast TB with community-acquired pneumonia: pneumonia is typically acute (days), responds to short antibiotic courses, and is not chronic; TB is indolent (weeks to months), needs months of multidrug therapy, and produces upper-lobe cavitation rather than lobar consolidation.

Practice Questions

Recall

Q: Which stain is used to demonstrate M. tuberculosis, and what colour do the bacilli appear? A: The Ziehl-Neelsen (acid-fast) stain; the bacilli appear red against a blue background, because their waxy mycolic-acid wall resists acid-alcohol decolourisation.

Understanding

Q: Why is active TB treated with four drugs rather than one? A: A large bacterial population contains, by spontaneous mutation, a few bacilli resistant to any single drug. Monotherapy kills the susceptible majority and selects the resistant survivors, causing relapse with resistant disease. Combining drugs makes the probability that any bacillus is simultaneously resistant to all of them extremely low, so the whole population is eliminated.

Application

Q: A patient about to start a TNF-alpha inhibitor for rheumatoid arthritis has a positive IGRA, no symptoms, and a normal chest X-ray. What is the diagnosis and management? A: Latent TB infection. Because the biologic will suppress cell-mediated immunity and risk reactivation, treat the latent infection first (e.g. isoniazid, or a shorter rifamycin-based regimen) after excluding active disease, then proceed with the biologic.

Analysis

Q: A patient on standard TB therapy improves initially, then deteriorates at month 4; repeat GeneXpert now shows rifampicin resistance. What likely happened, and what mistake must be avoided? A: This suggests emergence or unmasking of drug resistance (MDR-TB), commonly from non-adherence or inadequate initial therapy. The patient needs full drug-susceptibility testing and a tailored second-line regimen. The classic mistake to avoid is adding a single new drug to a failing regimen, which simply selects resistance to that drug too; the regimen must be rebuilt with multiple effective agents.

FAQ

Is TB always in the lungs? No. Pulmonary TB is most common and is the contagious form, but TB can affect lymph nodes, the pleura, spine (Pott disease), kidneys, brain and meninges, and more. Extrapulmonary TB is generally not spread person-to-person.

Can you catch TB from a person with latent TB? No. People with latent infection have no active, multiplying bacteria in their airways and are not infectious. Only people with active pulmonary (or laryngeal) TB spread the organism.

How infectious is someone with active TB once treated? Effective treatment rapidly reduces infectiousness, usually within about 2 weeks of starting the right drugs in drug-susceptible disease, though sputum tests and clinical response guide when isolation can end.

Does the BCG vaccine prevent TB completely? No. BCG substantially reduces severe childhood forms (TB meningitis, miliary TB) but is unreliable at preventing adult pulmonary TB. It is used routinely in high-burden countries and can complicate interpretation of the skin test (but not the IGRA).

Why does treatment take six months when other infections clear in a week? Because M. tuberculosis grows extremely slowly and includes semi-dormant, metabolically quiet bacilli that antibiotics kill only sluggishly. Its waxy wall also limits drug penetration. Short courses leave survivors and cause relapse, so a prolonged regimen is needed to sterilise the tissue.

Quick Revision

• Cause: Mycobacterium tuberculosis — aerobic, slow-growing, acid-fast bacillus; spread by airborne droplet nuclei.
• Koch identified the bacillus in 1882 (World TB Day, 24 March).
• Latent TB: contained, asymptomatic, not infectious; ~5-10% lifetime reactivation risk; detect with TST or IGRA.
• Active TB: cough more than 2-3 weeks, fever, night sweats, weight loss; upper-lobe cavitation; infectious if pulmonary.
• Diagnosis: sputum smear, GeneXpert MTB/RIF (rapid, detects rifampicin resistance), culture (gold standard + susceptibility), chest X-ray.
• Treatment: RIPE (Rifampicin, Isoniazid, Pyrazinamide, Ethambutol) for 2 months, then Rifampicin + Isoniazid for 4 months = 6 months.
• Never use a single drug; finish the full course to prevent MDR/XDR-TB.
• Key toxicities: hepatotoxicity (R, I, P), neuropathy (I — give B6), optic neuritis (E), orange fluids (R — harmless).
• Biggest reactivation risk factor: HIV.

Related Topics

Prerequisites

• ../../6._Microbiology/index.md — bacterial structure and acid-fast staining
• ../../34._Immunology/index.md — cell-mediated immunity and granuloma formation

Related Topics

• ../../5._Pharmacology/index.md — antimycobacterial drugs and resistance mechanisms
• ../../8._Community_Medicine/index.md — disease notification, DOT, and contact tracing
• ../index.md — Pulmonology branch overview

Next Topics

• Community-acquired and hospital-acquired pneumonia (Pulmonology)
• HIV and opportunistic infections (Infectious Diseases)
• Pleural effusion and empyema (Pulmonology)