What you need to know
- 6.6How M. tuberculosis infects human cells, causing symptoms that may result in death.
This is a mechanism page, not a prescribing guide, diagnosis tool or personal health questionnaire. HIV coinfection is linked in 6A 8. Antibiotics and resistance are 6B.
By the end, you should be able to…
- Name Mycobacterium tuberculosisThe bacterium that causes tuberculosis (TB). It can survive inside macrophages in the lungs. and the lung as the usual first organ.
- Describe uptake by macrophages and persistence in tubercleA mass of infected macrophages and other cells walled off in the lungs in tuberculosis..
- Link tissue destruction to reduced gas exchange and possible death.
Build your understanding
Inhaled, then hidden
Tuberculosis is caused by the bacterium Mycobacterium tuberculosisThe bacterium that causes tuberculosis (TB). It can survive inside macrophages in the lungs.. Infectious droplets from a person with active lung disease can be inhaled. The bacteria reach alveoli and are taken up by macrophages. Unlike many ingested microbes, M. tuberculosis can survive inside those cells instead of being digested.
Survival inside macrophages hides the bacterium from antibodyA Y-shaped protein made by plasma cells that binds specifically to one antigen. and lets infection persist. Aggregates of infected cells and host defences form tubercleA mass of infected macrophages and other cells walled off in the lungs in tuberculosis.. Bacteria can remain latent in tubercles for years without symptoms, which is why infection is not the same as active disease.
When the lung is damaged
If bacteria multiply and the immune control fails, tubercles destroy lung tissue. Gas exchange surface is lost, coughing may bring up blood, and the person can become wasted. Death can follow from respiratory failure or from spread of the bacteria. That chain is the 6.6 requirement: infection of named cells, then symptoms that may kill.
Active lung disease is the stage that can spread by droplets. Latent infection is not a droplet source. Those public-health facts are context from CDC pages, not extra named stages you must memorise as a mark scheme.
What this page is not
Do not write a drug regimen, a vaccine schedule or a travel-advice leaflet. Do not request or record anyone’s symptoms. 6B will treat antibioticA substance that kills bacteria or stops them growing, without harming human cells. Antibiotics do not work on viruses.. 6A 8 will explain why falling T helper cellA T cell that binds to an antigen-presenting cell and releases cytokines that activate B cells and T killer cells. numbers make TB more likely to reactivate.
M. tuberculosis is a bacterium, so 6.5 structure still applies: DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code., cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting., no capsidThe protein coat of a virus, made of capsomeres.. It is not a virus.
NOT: M. tuberculosis is not a virus, and latent infection does not spread by coughing droplets.
- Survival inside macrophages hides the bacteria from antibodies
- Active disease can destroy lung tissue and cause wasting
- Death can follow respiratory failure or spread of the bacteria
- This page is not a drug or vaccine schedule
- Falling T helper numbers, in untreated HIV, make reactivation more likely
Two traps this diagram blocks
- Do not call TB a virus that lyses lung cells like a phage.
- Do not say a person with only latent infection is spreading it by coughing.
From the diagram alone
Question
Describe how M. tuberculosis infects lung tissue and how that infection can cause death. [4]
Show a model answer
Inhaled bacteria are taken up by alveolar macrophages and can survive inside them. Tubercles form and may stay latent. If the bacteria later multiply, lung tissue is destroyed and gas exchange is lost. Death can follow from respiratory failure or from spread of the infection.
| State | Where the bacteria are | Spread by cough? |
|---|---|---|
| Uptake | Alveolar macrophages | Not yet a full disease story |
| Latent in tubercles | Contained in lung tissue | No |
| Active lung disease | Multiplying, tissue destroyed | Yes, droplets |
Make your reasoning visible
Name the cell
Macrophage, not “the lung in general”.
Finish at gas exchange
Death answers need lost alveolar surface or spread, not only “they are ill”.
Key explanations
- The organism has a cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting. and DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code.. It is not a lytic virus.
Avoid this misconception
The claim: TB is caused by a virus that lyses lung cells like λ phage.
The correction: M. tuberculosis is a bacterium that survives inside macrophages. Lytic viral cycles are 6.5(ii), not this infection.
Put the course of tuberculosis in order
Mycobacterium tuberculosis survives inside the cells that should destroy it. Order the course of an infection that becomes active disease.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the course of a tuberculosis infection in order.
1 of 7 in the right place
Most infected people never develop active TB, and active TB can be treated with a long course of antibiotics. Death is shown as the untreated end point.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Name the organism that causes TB and the organ it typically infects first.
Show suggested answer
Mycobacterium tuberculosis; the lungs.
Practice question
Retrieval and explanation · suggested answer
How does survival inside macrophages help the infection persist?
Show suggested answer
The bacterium resists digestion, is hidden from antibodies, and can remain latent in tubercles.
Practice question
Retrieval and explanation · suggested answer
Explain how TB infection can result in death.
Show suggested answer
Tubercles destroy lung tissue so gas exchange falls; the person may die of respiratory failure or from spread of the bacteria.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student writes that latent TB “cannot matter because CDC says it does not spread”. Separate the public-health fact from the 6.6 cell story.
Show a hint
Spread versus persistence in macrophages.
Explore the reasoning
Latent infection is not a droplet source, which is why it does not spread that way. 6.6 still cares because bacteria remaining alive inside macrophages can later multiply, destroy alveoli and then become both a disease and a source of droplets.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does survival of M. tuberculosis inside macrophages help the infection persist?
TB and HIV infection
Practice question
Retrieval and explanation · suggested answer
How does survival of M. tuberculosis inside macrophages help the infection persist?
Lesson: 6A 7: Case Studies of Disease: Tuberculosis
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Through the air, not from surfaces
6A · Case Studies of Disease: TuberculosisCDC states that TB germs spread through the air when a person with active lung disease coughs, speaks or sings, and that inactive (latent) TB does not spread…Specification 6.6Read & use the scienceClose article
Why this matters
CDC states that TB germs spread through the air when a person with active lung disease coughs, speaks or sings, and that inactive (latent) TB does not spread that way. Handshake and shared-cup myths are rejected there. Use that as enrichment for the inhalation route. It does not replace macrophages and tubercles, and it is not a diagnosis of anyone you know. Airborne transmission is about inhaling small particles from an infectious source. It does not imply that every brief encounter causes infection; exposure, ventilation and host response all matter.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.6 understand how Mycobacterium tuberculosis and human immunodeficiency virus (HIV) infect human cells, causing symptoms that may result in death
Use the science
Why is “we shared a cup, so it must be TB” a weak 6.7/6.6 answer even after reading CDC?
Explore a possible response
CDC treats TB as airborne from active lung disease, not as a surface or cup organism. A 6.6 answer still needs M. tuberculosis, macrophages and lung damage.
Explore the source
Inside an infected lung cell
6A · Case Studies of Disease: TuberculosisCDC’s clinical overview explains that inhaled Mycobacterium tuberculosis can reach the alveoli and be taken up by macrophages. Some infections remain latent, whereas active disease can damage lung…Specification 6.6Read & use the scienceClose article
Why this matters
CDC’s clinical overview explains that inhaled Mycobacterium tuberculosis can reach the alveoli and be taken up by macrophages. Some infections remain latent, whereas active disease can damage lung tissue and spread through the air. A clinician must distinguish exposure, infection and active disease rather than equating every positive test with infectious coughing. For the Unit 4 mechanism, the key is a bacterium interacting with human cells and the resulting tissue response; it is not a virus and is not spread by casual contact with a surface. The medical decisions are more complex than the school diagram.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.6 understand how Mycobacterium tuberculosis and human immunodeficiency virus (HIV) infect human cells, causing symptoms that may result in death
Use the science
Why does finding TB infection not automatically mean a person is currently releasing infectious bacteria into the air?
Explore a possible response
Latent infection can persist without active lung disease; airborne spread is associated with infectious active disease.