Explain how MRSA becomes resistant to antibiotics and why MRSA poses a particular danger to patients in hospitals.

Cambridge International IGCSE Biology (0610) — 15.1 Drugs · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

MRSA (methicillin-resistant Staphylococcus aureus) is a type of bacterium commonly found in hospitals that is resistant to almost all antibiotics.

Model answer (4 marks)

A mutation in the bacterial DNA changes the target of the antibiotic, so the drug can no longer bind and kill the cell.
When the antibiotic is given, susceptible bacteria are destroyed but the mutant survives.
The surviving MRSA cell divides, passing the resistance gene to its progeny, so the population becomes resistant.
Because MRSA is resistant to almost all antibiotics, infections it causes are very hard to treat and can lead to serious complications in hospital patients.

Examiner tips

Use the word ‘mutation’ and explain the target change; link survival to reproduction; mention ‘almost all antibiotics’ to show breadth of resistance.
Show the sequence: mutation → survival → reproduction → treatment difficulty.
Keep answer to 4 short sentences to match the marks.

Common mistakes

Confusing mutation with plasmid transfer; not specifying the antibiotic target change.
Failing to mention that MRSA is resistant to many antibiotics, not just one.
Writing a long paragraph instead of concise 4‑point answer.

Mark scheme (4 marks)

A mutation occurs in a bacterial cell that makes it resistant to an antibiotic
When the antibiotic is used, non-resistant bacteria are killed but the resistant bacterium survives
The resistant bacterium reproduces, passing on resistance to offspring / producing more resistant bacteria
MRSA is resistant to almost all antibiotics, so infections are very difficult or impossible to treat

Key terms in this question

antibiotic · MRSA