A scientist is investigating antibiotic resistance in a population of bacteria. Over several generations, the proportion of antibiotic-resistant bacteria in the population increases significantly, even though no new mutations have occurred during the investigation. Explain how the proportion of antibiotic-resistant bacteria in the population increased over several generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A scientist treats a large population of bacteria with an antibiotic. Within the population, a small number of bacteria already carry a mutation that makes them resistant to the antibiotic. After several generations, almost all bacteria in the population are resistant.
Model answer (5 marks)
The antibiotic kills or inhibits the growth of the non‑resistant bacteria, so they die or stop reproducing.
The resistant bacteria survive the treatment and are able to reproduce.
When they reproduce, they pass the resistance gene (allele) to their offspring.
This is an example of natural selection – the resistant bacteria have a selective advantage.
Over successive generations the proportion of resistant bacteria rises because they reproduce more successfully than the non‑resistant ones.
The resistant bacteria survive the treatment and are able to reproduce.
When they reproduce, they pass the resistance gene (allele) to their offspring.
This is an example of natural selection – the resistant bacteria have a selective advantage.
Over successive generations the proportion of resistant bacteria rises because they reproduce more successfully than the non‑resistant ones.
Examiner tips
- Use the exact terms ‘kill’, ‘inhibit’, ‘survive’, ‘reproduce’, ‘pass on the resistance gene’, ‘natural selection’, and ‘selective advantage’.
- Show the causal chain: antibiotic → death of susceptible → survival of resistant → reproduction → gene transmission → increased proportion.
Common mistakes
- Confusing ‘mutation’ with ‘gene transmission’; students say new mutations occur when none do.
- Omitting the step that the resistant bacteria reproduce and pass on the gene, or saying only that they ‘survive’ without mentioning reproduction.
Mark scheme (5 marks)
- The antibiotic kills (or inhibits the growth of) non-resistant bacteria
- The resistant bacteria survive and are able to reproduce
- The resistant bacteria pass on the resistance gene / allele to their offspring
- This is an example of natural selection — the resistant bacteria have a selective advantage
- Over generations, the proportion of resistant bacteria in the population increases as resistant bacteria reproduce more successfully than non-resistant bacteria
Key terms in this question
antibiotic · antibiotic resistance
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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