A new strain of tuberculosis (TB) bacteria has emerged that is resistant to several antibiotics. Explain how this resistant strain developed and why completing a full course of antibiotics is important in preventing the spread of resistant strains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis. Treatment requires patients to take a combination of antibiotics for at least six months. In some countries, patients stop taking their antibiotics once they feel better, after only a few weeks.
Model answer (5 marks)
Some bacteria in the original population have a random mutation that gives resistance to an antibiotic.
The antibiotic kills the susceptible bacteria, leaving the resistant ones alive.
The surviving resistant bacteria reproduce and pass the resistance gene to their offspring.
If a patient stops the antibiotic course early, some non‑resistant bacteria may survive and could mutate, or the remaining resistant bacteria are not fully eliminated.
Completing the full course ensures all bacteria are killed, reducing the chance of resistant strains developing and spreading to others.
The antibiotic kills the susceptible bacteria, leaving the resistant ones alive.
The surviving resistant bacteria reproduce and pass the resistance gene to their offspring.
If a patient stops the antibiotic course early, some non‑resistant bacteria may survive and could mutate, or the remaining resistant bacteria are not fully eliminated.
Completing the full course ensures all bacteria are killed, reducing the chance of resistant strains developing and spreading to others.
Examiner tips
- Use the exact terms ‘mutation’, ‘resistant’, ‘susceptible’, ‘reproduce’, ‘allele’, ‘full course’
- Show the cause–effect chain: mutation → selection → reproduction → spread
- Keep the answer to 5 points, one per mark
Common mistakes
- Mixing up ‘mutation’ with ‘acquisition’ of resistance
- Forgetting to mention that the antibiotic kills susceptible bacteria
- Not linking early stopping to the survival of resistant bacteria
Mark scheme (5 marks)
- Some bacteria in the original population have a random mutation that gives resistance to an antibiotic
- Antibiotic kills non-resistant bacteria / susceptible bacteria are killed, leaving resistant bacteria to survive
- Resistant bacteria reproduce and pass on the resistance gene / allele to offspring
- If a patient stops the antibiotic course early, some non-resistant bacteria may survive and could mutate / resistant bacteria are not fully eliminated
- Completing the full course ensures all bacteria are killed, reducing the chance of resistant strains developing and spreading to others
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Antibiotics and resistance questions
- A hospital records an increase in the number of patients infected with a strain …
- Scientists are concerned that some strains of bacteria have become resistant to …
- Explain why the development of antibiotic-resistant bacteria is a serious threat…
- A patient is prescribed a course of antibiotics to treat a bacterial infection. …