Explain antibiotic resistance and its control
Explain how resistant bacterial populations arise, why resistance matters and how its impact can be reduced.
Section 10.2 · Antibiotics
Cambridge International AS & A Level Biology 9700, 2028–2030 syllabus, Version 1. Educational biology content only: it does not diagnose illness or replace advice from a qualified health professional.
Before you begin
- A pathogen is a disease-causing microorganism or infectious agent; transmission describes how it passes between hosts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which population becomes resistant: the patient or the bacteria?
What you will learn
- Explain selection and inheritance of antibiotic resistance
- Discuss consequences of resistance and actions that reduce its spread and impact
Build the explanation
A bacterial population may contain heritable variation, including a mutation or resistance gene that lets some bacteria survive an antibiotic. When the antibiotic is used, susceptible bacteria are more likely to die while resistant bacteria survive, reproduce and pass resistance to descendants. Resistance can also spread between bacteria through gene transfer. Antibiotic exposure selects among bacteria; it does not cause a patient's body to become resistant. As resistant bacteria spread, infections can become harder to treat and effective options may become fewer.
Apply it carefully
Reduce selection and transmission together. Antibiotics should be used only when indicated by qualified health professionals and according to their directions; do not share or use leftover medicines. Infection prevention, vaccination where appropriate, hygiene, diagnostic testing, surveillance and responsible prescribing reduce unnecessary exposure and spread. No single action removes all resistance, and a resistant infection needs clinical assessment rather than self-treatment.
Pause and explain
What is natural selection's role in resistance?
Worked example
How can antibiotic use increase the proportion of resistant bacteria in a population?
Show the worked solution
- Start with variation in the population.
- Treatment selects survivors, which reproduce and pass on resistance.
Answer Susceptible bacteria are reduced, while bacteria with heritable resistance survive and reproduce. Their descendants make resistant bacteria a larger proportion of 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 how resistance can become more common during antibiotic exposure.
Give me a hint
Use variation, differential survival and inheritance.
Compare my reasoning
- Some bacteria already carry heritable resistance.
- Antibiotic exposure removes more susceptible bacteria; resistant survivors reproduce and pass resistance on.
Some bacteria have heritable resistance before treatment. Antibiotic exposure makes susceptible bacteria less likely to survive, while resistant bacteria survive and reproduce, passing resistance to descendants. The resistant type becomes a larger proportion of the population.
Look for these in your work
- Variation before selection stated
- Differential survival explained
- Inheritance and population change included
A headline says 'the patient became resistant to antibiotics'. Rewrite it scientifically.
Give me a hint
Identify the organism on which resistance acts.
Compare my reasoning
- Bacteria, not the patient's body, acquire or carry resistance.
- The infection may be caused by resistant bacteria.
The patient is infected with bacteria that are resistant to one or more antibiotics; it is the bacterial population, not the patient's body, that has resistance.
Look for these in your work
- Resistance assigned to bacteria
- Patient/infection wording clarified
A clinic wants to reduce resistance while still treating infections. Give two complementary strategies and explain why both matter.
Give me a hint
Combine reducing unnecessary selection with preventing spread.
Compare my reasoning
- Use antibiotics responsibly under qualified clinical direction and support appropriate diagnosis.
- Prevent infection and transmission through hygiene, vaccination where appropriate, surveillance and infection control.
Use antibiotics responsibly when clinically indicated, supported by diagnosis and appropriate prescribing, to reduce unnecessary selection. Also prevent infections and resistant-organism spread through hygiene, vaccination where appropriate, infection control and surveillance. Reducing selection and transmission address different parts of the problem.
Look for these in your work
- Responsible use strategy stated
- Infection prevention strategy stated
- Selection and transmission distinguished
Common mistakes
- Claiming antibiotics make an individual patient resistant
- Saying bacteria deliberately adapt because they need to
- Suggesting self-stopping or sharing prescribed medicine as a control measure
What can you explain now?
Which population becomes resistant: the patient or the bacteria?
Compare with the explanation
The bacterial population
Resistance is a heritable trait in microbes, not a change in the patient's body.
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Make it stick
How does selection increase resistant bacteria, and what can reduce selection and spread?
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