Antibiotic resistance is an increasing problem worldwide. Explain how antibiotic-resistant bacteria develop and spread within a population.

Eduqas GCSE Biology — 3.3 Treating, curing and preventing disease · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Random mutation(s) in bacterial DNA create a variant that is resistant to the antibiotic.
2. When the antibiotic is used, the susceptible bacteria are killed, leaving only the resistant ones – natural selection.
3. The surviving resistant bacteria reproduce, passing the resistance gene to their daughter cells.
4. Resistant bacteria spread between people and across populations, raising the proportion of resistant bacteria.

Examiner tips

Use the exact wording from the mark scheme (e.g., "random mutation", "natural selection", "reproduce", "spread"),
Show the cause–effect sequence: mutation → selection → reproduction → spread.
Keep each point concise – one sentence per mark.
Mention both within a population and between populations to cover all points.

Common mistakes

Confusing mutation with horizontal gene transfer; the question focuses on mutation.
Using vague terms like "evolution" or "selection" without linking to antibiotic use.
Failing to state that the resistance gene is inherited by daughter cells.

Mark scheme (4 marks)

Random mutation(s) occur in bacteria, producing a variant that is resistant to the antibiotic
When an antibiotic is used, non-resistant bacteria are killed / only the resistant bacteria survive (natural selection)
Resistant bacteria reproduce, passing the resistance to offspring / the resistant gene is inherited by daughter cells
Resistant bacteria can spread between people / between populations, increasing the proportion of resistant bacteria

Key terms in this question

antibiotic resistance

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