# 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.

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

> 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.

## Mark scheme (5 marks)

1. Some bacteria in the original population had a random mutation that gave resistance to the antibiotic
2. When the antibiotic was used, non-resistant bacteria were killed (or their growth was prevented)
3. Resistant bacteria survived and reproduced (natural selection)
4. The resistance gene was passed on to offspring / subsequent generations
5. Over time the proportion of resistant bacteria in the population increased, so the antibiotic is no longer effective against that strain

## Key terms

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

## 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-hospital-notices-that-a-particular-6dab1d97) · Published by Druglandscape Ltd.