Explain how antibiotic-resistant bacteria, such as MRSA, develop in a population of bacteria.

Cambridge International IGCSE Biology (0610) — 15.1 Drugs · 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. A mutation occurs in a bacterial cell that confers resistance to an antibiotic.
2. When the antibiotic is applied, susceptible bacteria are killed while the resistant cell survives.
3. The surviving resistant bacterium reproduces, passing the resistance gene to its progeny.
4. Over time the proportion of resistant bacteria rises, making the antibiotic ineffective and leading to infections such as MRSA that are difficult to treat.

Examiner tips

Use numbered points to match the scheme; include the key terms ‘mutation’, ‘survive’, ‘reproduce’, ‘population increases’.
Show the causal chain: mutation → survival → reproduction → population change.
Mention MRSA as an example of a resistant strain.
Keep each point concise and directly linked to the scheme.

Common mistakes

Confusing mutation with horizontal gene transfer; both can occur but the scheme focuses on mutation. Failing to mention that the antibiotic kills non‑resistant bacteria. Using vague language such as ‘bacteria become resistant’ without explaining the survival and reproduction steps.

Mark scheme (4 marks)

A mutation occurs in a bacterial cell that makes it resistant to an antibiotic
When the antibiotic is used, non-resistant bacteria are killed but the resistant bacterium survives
The resistant bacterium reproduces, passing on the resistance to offspring
Over time the population of resistant bacteria increases, making the antibiotic ineffective / leading to infections like MRSA that are very difficult to treat

Key terms in this question

antibiotic · antibiotic-resistant bacteria · MRSA