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 · View as Markdown

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

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.

Model answer (5 marks)

Some Staphylococcus aureus cells in the hospital population had a random mutation that conferred resistance to methicillin. When methicillin was administered, the susceptible bacteria were killed while the resistant ones survived – natural selection. The surviving resistant bacteria multiplied, passing the resistance gene/mutation to their offspring. Over successive generations the proportion of resistant bacteria increased, so the resistant strain became dominant. The strain then spread through direct patient contact, via staff hands or instruments, and by contamination of hospital surfaces and equipment.

Examiner tips

  • Use the word ‘mutation’ and ‘natural selection’ – key terms. Show the sequence: mutation → selection → reproduction → spread. Mention both vertical (reproduction) and horizontal (contact) transmission.
  • common_mistakes
  • :
  • Confusing mutation with acquisition of a plasmid. Forgetting to mention the role of natural selection. Failing to explain how the strain spreads (e.g., via staff or equipment).

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 in this question

antibiotic · resistant strain

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