# 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.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.4 Antibiotics and resistance · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The antibiotic kills (or inhibits the growth of) non-resistant bacteria
2. The resistant bacteria survive and are able to reproduce
3. The resistant bacteria pass on the resistance gene / allele to their offspring
4. This is an example of natural selection — the resistant bacteria have a selective advantage
5. Over generations, the proportion of resistant bacteria in the population increases as resistant bacteria reproduce more successfully than non-resistant bacteria

## Key terms

- [antibiotic](https://www.gradenine.co.uk/glossary/antibiotic)
- [antibiotic resistance](https://www.gradenine.co.uk/glossary/antibiotic-resistance)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-investigating-antibiotic-resistance-e8d5d6e4) · Published by Druglandscape Ltd.