A patient is diagnosed with a bacterial infection and is prescribed a course of antibiotics. A friend advises the patient to stop taking the antibiotics as soon as they feel better, rather than completing the full course. Explain why it is important that the patient completes the full course of antibiotics.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Antibiotic resistance is an increasing global health concern. Bacteria can become resistant to antibiotics through natural selection.
Model answer (5 marks)
1. Some bacteria in the infection may be naturally resistant to the antibiotic.
2. If the course is stopped early, not all bacteria are killed – some survive.
3. The surviving resistant bacteria reproduce and pass on the resistance gene to their offspring.
4. Natural selection then increases the proportion of resistant bacteria in the population.
5. A new or different antibiotic would be required to treat the resistant strain, which may not be available or effective.
2. If the course is stopped early, not all bacteria are killed – some survive.
3. The surviving resistant bacteria reproduce and pass on the resistance gene to their offspring.
4. Natural selection then increases the proportion of resistant bacteria in the population.
5. A new or different antibiotic would be required to treat the resistant strain, which may not be available or effective.
Examiner tips
- Use the exact sequence of points from the mark scheme; each point earns one mark.
- Show clear cause‑effect links (e.g., ‘if … then …’).
- Keep sentences short and to the point; avoid unnecessary words.
Common mistakes
- Mixing up ‘natural selection’ with ‘mutation’ – only natural selection is required for the answer.
- Failing to mention that the surviving bacteria are the resistant ones, not just any survivors.
Mark scheme (5 marks)
- Some bacteria in the infection may be naturally resistant to the antibiotic
- If the course is stopped early, not all bacteria are killed / some survive
- The surviving resistant bacteria reproduce / pass on the resistance gene to offspring
- Natural selection leads to an increase in the proportion of resistant bacteria in the population
- A new / different antibiotic would be needed to treat the resistant strain, which may not be available / effective
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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