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Topic 15 · 15.1

Drugs and antibiotics

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe a drug, and the use of antibiotics to treat bacterial infections.
  • State that some bacteria are resistant to antibiotics, and that antibiotics kill bacteria but do not affect viruses.
  • Explain how using antibiotics only when essential limits the development of resistant bacteria such as MRSA EXTENDED.

📚The biology

A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Medicines, caffeine and alcohol are all drugs.

Antibiotics

Antibiotics are used to treat bacterial infections: they kill bacteria or stop them growing, for example by damaging their cell walls. They do not affect viruses: viruses are not cells and reproduce inside the host’s own cells, so they have none of the structures antibiotics target. Colds and flu, caused by viruses, cannot be cured by antibiotics.

Left: antibiotic molecules attacking the cell wall of a bacterium. Right: virus particles inside a host cell; an antibiotic molecule outside has no target.
Antibiotics act on bacterial structures that human cells and viruses do not have.

Some bacteria are resistant to antibiotics, which reduces how effective the antibiotics are.

How resistance spreads EXTENDED

Three dishes of bacteria. First, a random mutation makes one bacterium resistant among many that are not. Second, an antibiotic kills the others and the resistant one survives. Third, it reproduces, so most of the population is now resistant.
EXTENDED Natural selection (topic 18): the antibiotic is the selection pressure.

EXTENDED A random mutation may make a bacterium resistant. When an antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive and reproduce, passing on the resistance allele, so a resistant strain such as MRSA develops. Every use of an antibiotic selects for resistance, so using antibiotics only when essential — not for viral infections, and not routinely in farm animals — and always finishing the course reduces the chance of resistant strains developing and spreading.

✏️Worked example

A patient with a sore throat asks a doctor for antibiotics. Tests show the infection is caused by a virus. (a) Explain why the doctor refuses. [2] (b) EXTENDED Explain how prescribing antibiotics unnecessarily can make them less effective in future. [3]

(a) Antibiotics kill bacteria but do not affect viruses, so they would not treat this infection.

(b) Antibiotics kill bacteria that are not resistant; any resistant bacteria survive and reproduce, passing on resistance. The proportion of resistant bacteria increases, so antibiotics fail more often.

Check it. The antibiotic does not cause the resistance mutation; the mutation happens at random, and the antibiotic only selects the bacteria that already have it.
“The patient becomes resistant.” It is the bacteria that become resistant, not the person.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which describes a drug? A: any substance that causes disease. B: any substance taken into the body that modifies or affects chemical reactions in the body. C: a medicine made by bacteria. D: a substance that kills viruses.
B.
2. (Theory.) State which type of infection antibiotics are used to treat. [1]
Bacterial infections.
3. (Theory.) Explain why antibiotics cannot be used to treat influenza. [2]
Influenza is caused by a virus; antibiotics kill bacteria but do not affect viruses.
4. (Theory.) State what is meant by an antibiotic-resistant bacterium, and one problem it causes. [2]
A bacterium that is not killed by an antibiotic. Infections become harder to treat, because the antibiotic is less effective.
5. (Theory.) EXTENDED Give two ways of reducing the development of resistant bacteria. [2]
Any two: use antibiotics only when essential; do not use them for viral infections; finish the full course; limit their use in farming.
6. (Theory.) EXTENDED Name a strain of bacterium that is resistant to many antibiotics. [1]
MRSA.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • World Health Organization — antimicrobial resistance fact sheet
  • e-Bug — teaching resources on antibiotics and microbes