Identify inherited resistance variation in bacteria
Natural selection can act on bacterial variation; antibiotics do not instruct bacteria to mutate.
Section B17.2 · Selection
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Use the Cambridge syllabus and CDC’s public explanation of antimicrobial resistance. This is educational content, not medical advice or a laboratory protocol.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is antibiotic resistance in the syllabus example?
What you will learn
- Explain resistance as a bacterial trait in a population
- Distinguish variation from selection by an antibiotic
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Before exposure | Resistant and susceptible variants are stated |
| Selection factor | A particular antibiotic |
| Biological subject | Bacterial strains |
| Not implied | Drug-directed mutation or universal resistance |
Explain resistance as a bacterial trait in a population
Bacterial populations can contain heritable differences in resistance to a particular antibiotic. Some resistance traits are encoded in DNA; variation may arise through genetic change and resistance information can also move between bacteria. For the syllabus example, a population with resistant and susceptible bacteria provides variation on which selection can act.
An antibiotic does not give each bacterium a purposeful instruction to become resistant. A resistant variant may be present before exposure, or resistance information may be acquired by other biological routes. Keep the origin of a variant separate from the environmental selection that changes its representation.
Distinguish variation from selection by an antibiotic
Resistance belongs to the bacteria, not to a person’s body becoming resistant to an antibiotic. The CDC describes antimicrobial resistance as germs defeating the drugs intended to affect them, and notes that resistant germs can survive, multiply and spread resistance traits. The classroom model here focuses on bacterial strains and a stated drug context.
The effect is specific: a strain may resist one antibiotic and remain susceptible to another. Do not say that all bacteria are resistant or that every treatment produces the same outcome. This lesson explains a population example; it does not offer treatment advice or instructions to grow microorganisms.
Sources for this lesson
Pause and explain
What does an antibiotic do in the natural-selection model?
Worked example
A question states that a bacterial population already contains inherited resistant and susceptible variants before exposure to a named antibiotic. Identify the variation and explain what exposure can select.
Show the worked solution
- Name the two resistance phenotypes that the case supplies.
- The antibiotic context can change relative survival; it does not create a needed allele by intention.
Answer The population contains inherited resistance variation. Exposure to the stated antibiotic can select for bacteria with greater resistance in that context; it does not instruct individual bacteria to mutate.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
A bacterial strain has 5 resistant and 95 susceptible cells before a stated drug exposure. What variation is supplied, and what must not be inferred?
Give me a hint
Use the two stated phenotypes and stay specific to that drug.
Compare my reasoning
- The population includes resistant and susceptible bacteria.
- It does not show the origin of resistance or resistance to other drugs.
The supplied variation is resistance to the stated drug; the count does not show that the antibiotic caused the variant or that the strain resists all drugs.
Look for these in your work
- Both variants identified
- Drug and origin limits
A student says, “My body gets used to antibiotics, so it is resistant.” Correct the biological subject.
Give me a hint
Identify which organism the resistance term describes.
Compare my reasoning
- The bacteria are the germs whose susceptibility changes.
- Resistance is a property of bacterial strains, not a person becoming resistant.
Antibiotic-resistant bacteria can survive a drug that affects susceptible bacteria; a person’s body is not what becomes antibiotic-resistant.
Look for these in your work
- Correct subject
- Bacterial trait
The CDC notes that resistance traits are in germ DNA and can spread between germs. How does that differ from an antibiotic deliberately producing a useful mutation?
Give me a hint
Separate inheritance or gene transfer from purposeful instruction.
Compare my reasoning
- A trait may be transmitted biologically.
- The drug does not select a mutation because bacteria need it.
DNA-based resistance can be inherited or spread between germs, while the antibiotic supplies a selection context rather than a purposeful instruction to mutate.
Look for these in your work
- Transmission mechanism
- Selection versus directed change
Common mistakes
- Saying a person becomes antibiotic-resistant
- Saying antibiotics deliberately create needed mutations
- Calling a strain resistant to every antibiotic
What can you explain now?
What does an antibiotic do in the natural-selection model?
Compare with the explanation
Creates a selection context for existing differences
The drug can affect survival of variants; need does not direct mutation.
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Make it stick
Without your notes, explain resistance as a bacterial trait in a population and distinguish variation from selection by an antibiotic. Explain the mechanism, use the given population or breeding records and state what the evidence does not establish.
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.