# Scientists are concerned that some strains of bacteria have become resistant to multiple antibiotics. Explain how a population of bacteria can become resistant to an antibiotic through natural selection.

> Pearson Edexcel A-Level Biology A (9BI0) — 5.4 Antibiotics and resistance · Explain · 5 marks

> In 2023, the World Health Organization reported that antibiotic-resistant infections cause over one million deaths per year globally. Researchers have found that the more widely an antibiotic is used, the faster resistance spreads through bacterial populations.

## Mark scheme (5 marks)

1. Random mutation(s) occur in the DNA of some bacteria in the population
2. The mutation gives some bacteria resistance to the antibiotic, meaning the antibiotic cannot kill them / cannot prevent their growth
3. When the antibiotic is present, non-resistant bacteria are killed / cannot grow, so resistant bacteria have a survival advantage (natural selection)
4. Resistant bacteria reproduce and pass the resistance gene / mutation on to offspring (through reproduction / binary fission)
5. Over generations, the proportion of resistant bacteria in the population increases, so the antibiotic becomes less effective at treating the infection

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/scientists-are-concerned-that-some-strains-99eb6afc) · Published by Druglandscape Ltd.