# A hospital records an increase in the number of patients infected with a strain of Staphylococcus aureus that is no longer killed by the antibiotic methicillin. Explain how this antibiotic-resistant strain of Staphylococcus aureus is likely to have developed and spread within the hospital.

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

> Staphylococcus aureus is a bacterium commonly found on human skin. Methicillin-resistant Staphylococcus aureus (MRSA) is a major concern in hospitals worldwide because it cannot be treated with many commonly used antibiotics.

## Mark scheme (5 marks)

1. Within the original population of Staphylococcus aureus, some individuals had a random mutation that gave resistance to methicillin
2. When methicillin was used, non-resistant bacteria were killed / only the resistant bacteria survived (natural selection)
3. The resistant bacteria reproduced, passing the resistance gene / mutation to offspring
4. Over successive generations the resistant strain became more common in the population
5. The resistant strain spread within the hospital through direct contact between patients / via hospital staff / via contaminated equipment

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-hospital-records-an-increase-in-cdb0ed50) · Published by Druglandscape Ltd.