A hospital notices that a particular antibiotic is no longer effective against a strain of bacteria that it was once able to treat. Explain how this bacterial strain could have become resistant to the antibiotic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hospitals routinely use antibiotics to treat bacterial infections. In recent decades, strains of bacteria have been identified that are no longer killed by antibiotics that previously worked against them. Scientists are concerned that antibiotic-resistant strains are spreading between patients in hospital wards.
Model answer (5 marks)
Some bacteria in the original population had a random mutation that gave resistance to the antibiotic.
When the antibiotic was used, non‑resistant bacteria were killed (or their growth was prevented).
The resistant bacteria survived and reproduced – natural selection favoured them.
The resistance gene was passed on to offspring and subsequent generations.
Over time the proportion of resistant bacteria in the population increased, so the antibiotic is no longer effective against that strain.
When the antibiotic was used, non‑resistant bacteria were killed (or their growth was prevented).
The resistant bacteria survived and reproduced – natural selection favoured them.
The resistance gene was passed on to offspring and subsequent generations.
Over time the proportion of resistant bacteria in the population increased, so the antibiotic is no longer effective against that strain.
Examiner tips
- Use the exact sequence of points from the mark scheme; each point counts as one mark.
- Show the cause (mutation) and the effect (selection and spread) clearly.
- Mention that the gene is inherited by offspring to justify the increase in resistance.
Common mistakes
- Failing to mention the initial random mutation.
- Confusing the antibiotic’s effect with the bacteria’s reproduction.
- Not stating that the resistance gene is passed to offspring.
Mark scheme (5 marks)
- Some bacteria in the original population had a random mutation that gave resistance to the antibiotic
- When the antibiotic was used, non-resistant bacteria were killed (or their growth was prevented)
- Resistant bacteria survived and reproduced (natural selection)
- The resistance gene was passed on to offspring / subsequent generations
- Over time the proportion of resistant bacteria in the population increased, so the antibiotic is no longer effective against that strain
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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