A patient is prescribed a course of antibiotics to treat a bacterial infection. After a few days, the patient begins to feel better and stops taking the antibiotics early. Explain why stopping a course of antibiotics early can lead to antibiotic-resistant bacteria developing in the population.

Pearson Edexcel A-Level Biology A (9BI0) — 5.4 Antibiotics and resistance · Explain · 5 marks · View as Markdown

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

Antibiotic resistance is an increasing global health concern. Doctors advise patients to always complete a full course of antibiotics, even if they feel better before the course has ended.

Model answer (5 marks)

Some bacteria in the infection may have a random mutation that makes them less susceptible to the antibiotic. If the course is stopped early, not all bacteria are killed, so some resistant bacteria survive. The surviving resistant bacteria reproduce, passing on the resistance gene or mutation to their offspring. Natural selection means the resistant bacteria have a survival advantage and are more likely to survive and reproduce than non‑resistant bacteria. Over time the proportion of resistant bacteria in the population increases, making the antibiotic less effective or ineffective at treating the infection.

Examiner tips

  • Use the exact terms ‘mutation’, ‘resistant’, ‘survive’, ‘reproduce’, ‘natural selection’, ‘proportion increases’
  • Show the causal chain: mutation → survival → reproduction → selection → population change
  • Keep the answer concise – 5 points, no extra explanation

Common mistakes

  • Using vague terms like ‘stronger’ instead of ‘resistant’
  • Failing to mention natural selection as the mechanism
  • Not linking the early stop to incomplete killing of bacteria

Mark scheme (5 marks)

  1. Some bacteria in the infection may have a random mutation that makes them less susceptible / resistant to the antibiotic
  2. If the course is stopped early, not all bacteria are killed / some resistant bacteria survive
  3. The surviving resistant bacteria reproduce / multiply, passing on the resistance gene / mutation to offspring
  4. Natural selection means resistant bacteria have a survival advantage / are more likely to survive and reproduce than non-resistant bacteria
  5. Over time the proportion of resistant bacteria in the population increases, so the antibiotic becomes less effective / ineffective at treating the infection

Key terms in this question

antibiotic · antibiotic-resistant bacteria · population

Related

More Antibiotics and resistance questions

▶ Try answering this question with AI marking (free) →