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C3.2.14—Antibiotic resistance evolution

Antibiotic resistance evolution explains how a specific barrier, pathogen interaction, immune mechanism or treatment response contributes to protection against infectious disease.

Syllabus
First assessment 2025
Objective
C3.2.14
Level
SL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–5

Common command terms

  • Explain
  • Compare
  • Discuss
  • Suggest
  • Identify
  • Describe
  • State
  • Outline

Scoring notes

Common mistake
Saying antibiotics cause directed mutations instead of selecting pre-existing or transferred resistance.

Recent exam appearances

May 2025Paper1A ["SL"] · TZ217[ 1 ]C3.2.14—Antibiotic resistance evolution
November 2024Paper2 ["SL"] · TZ17(c)[ 5 ]C3.2.14—Antibiotic resistance evolution
November 2024Paper3 ["SL"] · TZ22(c)[ 1 ]C3.2.14—Antibiotic resistance evolution
May 2024Paper2 ["SL"] · TZ23(b)(ii)[ 3 ]C3.2.14—Antibiotic resistance evolution
November 2022Paper1 ["SL"] · TZ027[ 1 ]C3.2.14—Antibiotic resistance evolution
Practice this objective

Coverage 2010–2025 · Updated 16 Jul 2026

Resistance spreads when antibiotics select variants

Antibiotic exposure selects resistant bacterial variants, allowing strains resistant to several antibiotics to evolve and spread.

Resistance genes arise through mutation or are acquired from other bacteria. An antibiotic kills susceptible cells, while resistant survivors reproduce and pass resistance vertically or transfer genes horizontally.

Existing genetic variation → antibiotic selection → resistant survival → reproduction/gene transfer → resistance frequency rises → different resistance genes can accumulate in one multiresistant strain.

Repeated exposure to different antibiotics can successively select a pathogenic strain carrying resistance to several drug classes, leaving fewer effective treatments.

Antibiotics do not direct bacteria to mutate. Careful use—only when appropriate, with the prescribed agent and regimen—reduces unnecessary selection pressure but cannot make evolution impossible.

Antibiotic resistance evolution

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Compare / Discuss.

Command terms

Explain / Compare / Discuss / Suggest / Identify / Describe / State / Outline

What earns marks

Mark schemes repeatedly reward named details, correct sequence and clear distinctions between similar processes.

Watch for

Saying antibiotics cause directed mutations instead of selecting pre-existing or transferred resistance.

Representative question

Question 1

[Maximum number: 9]

Explain how natural selection can lead to evolution using antibiotic resistance in bacteria as an example.

Defence Against Disease

  • Skin, mucus, cilia, lysozyme and clotting form primary barriers against pathogens.
  • Innate immunity is rapid and broad: phagocytes recognize, engulf and digest pathogens. Adaptive immunity is antigen-specific and forms memory.
  • Helper T-cells coordinate responses; activated B-cells undergo clonal selection, producing antibody-secreting plasma cells and memory cells. A second exposure therefore triggers a faster, stronger response.
  • HIV infects CD4 helper T-cells; their loss weakens immune coordination and can lead to AIDS.
  • Antibiotics target bacterial processes, not viruses. Antibiotic exposure selects resistant variants, which can spread by reproduction or plasmid transfer.
  • Vaccination creates active artificial immunity; high population immunity can indirectly protect susceptible people.
  • Evaluate disease and vaccine claims using reliable sources, trends, controlled comparisons, incidence and efficacy—not raw totals alone.

Concept essentials

  • Antibiotic resistance evolution has to be linked to the exact pathogen, cell, molecule or population process involved.
  • The biological effect of antibiotic resistance evolution depends on the sequence from trigger to protective outcome.
  • Clear distinctions within antibiotic resistance evolution prevent confusion with neighbouring immune responses.
  • Evidence for antibiotic resistance evolution is strongest when mechanism and consequence are explained together.