Explain selection of resistant bacteria
Trace differential survival and reproduction instead of purposeful adaptation.
Section 15.1 · Drugs
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish bacteria from viruses; recall enzymes, chemical reactions, pathogens and reproduction of bacteria.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the starting condition in this selection model?
What you will learn
- Begin with bacterial variation before exposure
- Connect survival and reproduction with a changed population
Explain selection of resistant bacteria
| Feature | Meaning |
|---|---|
| Before | Some bacteria already carry resistance traits |
| Exposure | Susceptible bacteria killed; resistant bacteria survive |
| Afterwards | Survivors reproduce and pass traits to descendants |
| Outcome | Resistant proportion can increase |
Further reading: CDC explanation of antimicrobial resistance.
Begin with bacterial variation before exposure
A bacterial population can contain variation in susceptibility before an antibiotic is applied. Some bacteria may already carry a resistance trait arising through genetic change or acquired genetic material. In the taught selection model, the antibiotic does not give every bacterium a purposeful instruction to become resistant. It changes which existing variants survive the exposure.
The specified antibiotic kills susceptible bacteria while resistant bacteria survive in the supplied model. This is differential survival under a selection pressure. The resistant survivors can reproduce and pass resistance traits to descendants, increasing the resistant proportion in a later population. Survival during exposure and reproduction afterwards are separate stages; a higher proportion need not require an immediate increase in the absolute resistant count.
Connect survival and reproduction with a changed population
Unnecessary exposure provides additional opportunities for this selection process without an appropriate treatment benefit. Even bacteria other than the pathogen being targeted can be exposed, so the biological consequences of use are broader than a symptom disappearing. Essential clinical use can still create selection pressure, which is why the aim is appropriate use rather than claiming that all antibiotic treatment should stop.
The complete explanation links variation, selective killing, survival, reproduction and a change in population composition. It does not describe the human body becoming resistant, individual bacteria deciding to adapt or every antibiotic directly converting all cells. Resistant bacteria can also spread between hosts, so limiting selection and reducing transmission are complementary ways of limiting the problem.
Pause and explain
Which sequence explains development of a resistant population?
Worked example
Explain how a resistant minority can become a larger proportion after exposure.
Show the worked solution
- Susceptible bacteria are killed while resistant bacteria survive.
- Resistant survivors reproduce and pass the trait to descendants.
Answer Selective killing leaves resistant survivors; their reproduction can increase the resistant proportion in the population.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain selection and reproduction of resistant bacteria in a complete chain.
Give me a hint
Begin before exposure and end with descendants.
Compare my reasoning
- Some bacteria are already resistant; the drug kills susceptible bacteria.
- Resistant survivors reproduce and pass traits to descendants, increasing the resistant proportion.
A population has susceptibility variation. The antibiotic kills susceptible bacteria while resistant bacteria survive; survivors reproduce and pass resistance traits to descendants, increasing the resistant proportion.
Look for these in your work
- Include initial variation and selective survival
- Include reproduction and population change
Why can the resistant proportion rise during exposure even before resistant bacteria reproduce?
Give me a hint
The susceptible denominator can decrease.
Compare my reasoning
- Susceptible bacteria are removed.
- The surviving population is therefore more strongly composed of resistant bacteria.
Removing susceptible bacteria changes population composition, so the resistant proportion can rise without an immediate increase in resistant count. Reproduction is a later stage.
Look for these in your work
- Distinguish proportion from count
- Separate survival and reproduction
Correct: antibiotics make each bacterium choose resistance, and avoiding all antibiotics is the only solution.
Give me a hint
Replace purposeful adaptation and retain appropriate essential use.
Compare my reasoning
- Antibiotics select among variants through differential survival.
- Essential treatment remains appropriate; unnecessary exposure should be avoided.
Antibiotics select among bacterial variants rather than making every cell choose resistance. Appropriate essential treatment remains valuable; avoiding unnecessary exposure reduces avoidable selection opportunities.
Look for these in your work
- Explain selection without purpose
- Retain essential clinical use
Common mistakes
- Saying bacteria decide to become resistant because they need to.
- Confusing an immediate proportion change with reproduction.
What can you explain now?
Which sequence explains development of a resistant population?
Compare with the explanation
Variation → selective killing → resistant survival → reproduction
It includes both selection and descendants.
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Make it stick
Explain the complete selection chain and distinguish a change in proportion from an immediate increase in resistant count.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.