The big idea: An antibiotic is a medicine that kills bacteria (or stops them growing).
It works by attacking something that only bacteria have — for example a cell wall to build, or their own ribosomes and enzymes.
Because viruses are not cells and have none of these structures of their own, antibiotics cannot treat a viral infection such as flu.
Sometimes a disease can pass straight from an animal to a human — this is called a zoonosis.
- Antibiotic
- A medicine that kills bacteria, or stops them growing, by attacking a structure or process that only bacteria have.
- Bacterium (plural: bacteria)
- A single-celled living organism with a cell wall, ribosomes and its own metabolism. Bacteria can be killed by antibiotics.
- Virus
- A tiny non-living particle that is NOT a cell. It has no cell wall, no ribosomes and no metabolism of its own — it copies itself only inside a host cell.
- Antibiotic resistance
- The ability of some bacteria to survive an antibiotic that would normally kill them.
- Resistant strain
- A type of bacteria that can survive a particular antibiotic, so that antibiotic no longer works against it.
- Zoonosis (plural: zoonoses)
- An infectious disease that can be transmitted directly from an animal to a human.
One word, one idea: Antibiotic = anti-bacteria (it has no effect on viruses).
Resistance = the bacteria survive the drug.
Zoonosis = a disease that comes from an animal ('zoo' means animal).
To understand this micro you need two cause-and-effect chains: why antibiotics work on bacteria but not viruses, and why resistance appears and spreads.
Why antibiotics kill bacteria but not viruses: Antibiotics attack features that are unique to bacterial cells — most often the building of the cell wall, but also bacterial ribosomes and enzymes.
Human cells do not have these exact targets, so the antibiotic harms the bacteria and not us.
A virus is not a cell: it has no cell wall, no ribosomes and no metabolism of its own — it simply uses the host cell's machinery. With no bacterial target to attack, an antibiotic does nothing to a virus. That is why flu and colds cannot be treated with antibiotics.
| Feature antibiotics attack | Bacteria | Viruses / human cells |
|---|---|---|
| A cell wall to build | Yes — many antibiotics block cell-wall building | Viruses have no cell wall; human cells have no wall to attack |
| Their own ribosomes / enzymes | Yes — bacterial ribosomes differ from ours, so they can be blocked | Viruses have none of their own; they hijack the host cell's machinery |
| Their own metabolism | Yes — bacteria are living cells that carry out their own reactions | Viruses are not cells and carry out no metabolism of their own |
| Can an antibiotic harm it? | Yes — there is a bacteria-only target to attack | No — there is no bacterial target, so the antibiotic does nothing |
| Bacterial infection | Viral infection | |
|---|---|---|
| Example | Tuberculosis, bacterial pneumonia | Influenza (flu), the common cold, HIV |
| Is it a living cell? | Yes — a complete cell with a wall, ribosomes and metabolism | No — not a cell; uses the host cell's machinery to copy itself |
| Treated by antibiotics? | Yes — antibiotics have a bacteria-only target | No — there is no bacterial structure for the antibiotic to attack |
How resistance evolves — by natural selection: Resistance is not something an individual bacterium learns. It happens by natural selection:
1. Variation — in a large population, a few bacteria already carry a gene (often from a random mutation) that makes them resistant.
2. Selection — when the antibiotic is used, it kills the non-resistant bacteria, but the resistant ones survive.
3. Reproduction — the survivors reproduce, passing the resistance gene to their offspring, so the next generation is mostly resistant.
Over time, the resistant strain becomes common, and the antibiotic no longer works.
Antibiotic resistance evolves by natural selection: a varied population already contains a few resistant bacteria; the antibiotic kills the non-resistant ones; the survivors reproduce, so resistant bacteria become common.
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Antibiotic on bacteria
- Bacteria are living cells with their own structures
- There is a bacteria-only target (cell wall, ribosomes, enzymes)
- The antibiotic kills or stops the bacteria
- Works — unless the strain is resistant
Antibiotic on a virus
- A virus is not a cell — no wall, no ribosomes, no metabolism
- There is no bacterial target to attack
- The antibiotic has nothing to act on
- Does nothing — flu and colds are unaffected
Why the same antibiotic can later fail: If an antibiotic worked once but fails against a second infection, it usually means a resistant strain has been selected.
The drug killed off the bacteria it could, leaving the resistant survivors to multiply — so the same drug no longer clears the infection.
Zoonoses — diseases that jump from animals: A zoonosis is a disease that can pass directly from an animal to a human.
Examples include rabies (from a bite), some forms of tuberculosis (from cattle) and Japanese encephalitis (an animal reservoir in pigs and birds).
What they all share is an animal source — they are not spread only between humans.
| Disease | Animal source it can pass from | Zoonosis? |
|---|---|---|
| Rabies | Dogs, bats and other mammals (a bite) | Yes — passes from an animal to a human |
| Tuberculosis (some strains) | Cattle (e.g. through unpasteurised milk) | Yes — can transfer from an animal to a human |
| Japanese encephalitis | Pigs and birds (via mosquitoes) | Yes — has an animal reservoir and reaches humans |
| Measles | None — spreads only between humans | No — it is not a zoonosis |
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How this is tested: Paper 1 often gives a one-mark prompt asking why antibiotics treat bacterial infections but not viral ones — your answer must name a bacteria-only target (cell wall, ribosomes, metabolism) that a virus lacks.
On Paper 1A you may have to explain why the same antibiotic later failed against a second infection (a resistant strain was selected), or identify a zoonosis / state what diseases such as rabies, TB and Japanese encephalitis share (an animal source).
A Paper 3 data question can show bacteria still growing despite an antibiotic and ask you to suggest why — the answer is that those colonies were resistant.
IB-style question — why antibiotics fail against a virus
A patient with influenza (a viral infection) is given an antibiotic, but it has no effect on the illness. Explain why the antibiotic does not work against the influenza virus. [3]
How to score all three marks
- Say what antibiotics attack. Antibiotics work by attacking a structure or process that is unique to bacteria — for example building a cell wall, or bacterial ribosomes / enzymes.
- Say why a virus is different. A virus is not a cell: it has no cell wall, no ribosomes and no metabolism of its own — it uses the host cell's machinery instead.
- Link the two. Because the virus has no bacterial target for the antibiotic to act on, the antibiotic has nothing to attack and the infection is unaffected. (Mark 1: antibiotics target a bacteria-only structure/process. Mark 2: a virus is not a cell / lacks a wall, ribosomes, metabolism. Mark 3: no target, so no effect.)
Final answer
Antibiotics attack a structure unique to bacteria (e.g. the cell wall or bacterial ribosomes); a virus is not a cell and has none of these, so there is no target for the antibiotic to act on — it has no effect.
✓ Why this scores full marks: It does not just say 'antibiotics only kill bacteria' — it gives the reason: a bacteria-only target exists, a virus lacks it, so there is nothing to attack.
Marks are lost when students simply assert 'viruses are different' without naming the missing target (cell wall / ribosomes / metabolism).