By the end of this lesson
- Explain antibiotic resistance by natural selection.
- Distinguish mutation from selection.
- State measures that slow resistance spread.
BIG QUESTION
Why can an antibiotic become less effective even though the medicine has not changed?
Resistance variants arise by mutation or gene transfer; antibiotic exposure selects them to become common.
Resistance exists before treatment
A mutation can give a bacterium an allele that reduces antibiotic entry, alters the target, destroys the drug or pumps it out. Bacteria can also acquire resistance genes by horizontal gene transferPassing genes, such as antibiotic-resistance genes on plasmids, from one bacterium to another, rather than from parent to offspring..
The antibiotic does not purposefully cause a bacterium to become resistant because it needs to survive.
Antibiotic exposure selects resistant cells
Susceptible bacteria are killed or inhibited. Resistant bacteria are more likely to survive, reproduce rapidly and pass resistance genes to descendants or other bacteria.
The resistant strain therefore becomes a larger proportion of the bacterial population.
Stewardship reduces selection pressure
Use antibiotics only when indicated, use the right drug, dose and duration as prescribed, and prevent infections through hygiene and vaccination. Antibiotics do not treat viral infections.
Reducing unnecessary exposure slows selection but does not eliminate resistance already present.
Resistance evolution sequence
Separate origin of resistance from selection of it.
| Stage | Bacterial population | Antibiotic role |
|---|---|---|
| Before treatment | Rare resistant variants may exist. | No selection or low selection. |
| Treatment | Susceptible cells die/inhibited. | Selection pressure. |
| Survival | Resistant cells remain. | Drug has reduced effect on them. |
| Reproduction/transfer | Resistance genes spread. | No new need-directed mutation required. |
| Later infection | Resistant strain more common. | Treatment may be less effective. |
Language that earns precision
- antibiotic resistance
- Ability of bacteria to survive or grow despite an antibiotic concentration that previously inhibited them.
- horizontal gene transfer
- Transfer of genetic material between organisms other than parent-to-offspring inheritance.
- selection pressure
- Environmental factor changing relative survival or reproduction.
- antimicrobial stewardship
- Careful use of antimicrobial medicines to preserve effectiveness.
Common misconceptions
Claim: Antibiotics make bacteria resistant on purpose.
Correction: They select bacteria that already have resistance variants or acquire genes.
Claim: People become resistant to antibiotics.
Correction: Bacteria become resistant; people may carry resistant bacteria.
Claim: Stopping antibiotics early is always harmless once symptoms improve.
Correction: Follow the prescriber’s instructions; inappropriate use can leave infection inadequately treated and select survivors.
Match the answer to the command word
Random variation → antibiotic kills susceptible → resistant survive/reproduce → frequency rises.
Write “bacteria evolve resistance”, not “patients become resistant”.
Recall, apply, then justify
Explain how antibiotic resistance evolves in a bacterial population.
Hint
Variation, selection, survivors, reproduction, frequency.
Suggested marking points
- Some bacteria have resistance alleles through mutation or acquired genes.
- The antibiotic kills or inhibits susceptible bacteria.
- Resistant bacteria survive.
- They reproduce and pass/transfer resistance genes.
- The resistant bacteria become a larger proportion of the population.
State two measures that can reduce the spread of antibiotic resistance.
Hint
Appropriate use and infection prevention.
Suggested marking points
- Use antibiotics only when indicated and as prescribed.
- Prevent infection/spread through hygiene, vaccination or infection control.
Evaluate why combining two antibiotics with different targets can sometimes slow resistance evolution.
Planning hint
Probability of resistance to both, selection and limitations.
What a strong design includes
- A bacterium resistant to one drug may still be killed by the other.
- Simultaneous resistance to two different targets can be rarer than resistance to one.
- The surviving population may therefore be smaller.
- Fewer survivors reduce opportunities for reproduction and transfer.
- Combination treatment is not suitable for every infection and can have side effects.
- Bacteria can still evolve multi-drug resistance.
- Use should follow evidence and clinical guidance.