Scientists are concerned that some strains of bacteria have become resistant to multiple antibiotics. Explain how a population of bacteria can become resistant to an antibiotic through natural selection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In 2023, the World Health Organization reported that antibiotic-resistant infections cause over one million deaths per year globally. Researchers have found that the more widely an antibiotic is used, the faster resistance spreads through bacterial populations.
Model answer (5 marks)
1. Random mutations in bacterial DNA create variants.
2. Some mutants acquire a change that makes the antibiotic ineffective, allowing them to survive.
3. When the antibiotic is applied, non‑mutants are killed or stop growing, giving the mutants a survival advantage (natural selection).
4. The surviving mutants reproduce by binary fission, passing the resistance mutation to their progeny.
5. Repeated cycles raise the proportion of resistant bacteria, reducing the antibiotic’s effectiveness over time.
2. Some mutants acquire a change that makes the antibiotic ineffective, allowing them to survive.
3. When the antibiotic is applied, non‑mutants are killed or stop growing, giving the mutants a survival advantage (natural selection).
4. The surviving mutants reproduce by binary fission, passing the resistance mutation to their progeny.
5. Repeated cycles raise the proportion of resistant bacteria, reducing the antibiotic’s effectiveness over time.
Examiner tips
- Use the exact sequence of mutation → survival advantage → reproduction → increase in proportion to hit all points.
- Show the link between antibiotic presence and selective pressure.
- Mention binary fission as the mode of bacterial reproduction.
Common mistakes
- Confusing mutation with selection; forgetting to state that the antibiotic is present.
- Failing to mention that resistance is passed to offspring.
- Using vague terms like "changes" instead of "mutations".
Mark scheme (5 marks)
- Random mutation(s) occur in the DNA of some bacteria in the population
- The mutation gives some bacteria resistance to the antibiotic, meaning the antibiotic cannot kill them / cannot prevent their growth
- When the antibiotic is present, non-resistant bacteria are killed / cannot grow, so resistant bacteria have a survival advantage (natural selection)
- Resistant bacteria reproduce and pass the resistance gene / mutation on to offspring (through reproduction / binary fission)
- Over generations, the proportion of resistant bacteria in the population increases, so the antibiotic becomes less effective at treating the infection
Key terms in this question
antibiotic · natural selection
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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