3.7.9Additional lessons
Antibiotic Resistance — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 101
Part 1 of 5How Resistance Arises
Resistance is an example of evolution by natural selection acting in a short time span.
1
Within a bacterial population there is variation, produced by random mutations in genes during DNA replication.
2
By chance, a mutation in one bacterium gives resistance to an antibiotic such as penicillin, for example by making an enzyme that destroys it.
3
When the population is treated with the antibiotic, sensitive bacteria die while resistant ones survive.
4
The survivors face less competition and reproduce, passing the resistance allele to their offspring.
5
After several generations the population is mostly resistant, because bacteria divide very quickly.
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Resistance genes can also pass to other bacteria by horizontal transfer, such as plasmids, including to different species.
Part 2 of 5Why Resistance Is a Problem
Treatment fails
- Infections that were once easily cured need stronger, costlier drugs or can no longer be treated at all.
MRSA
- Methicillin-resistant Staphylococcus aureus is resistant to many antibiotics and spreads in hospitals.
Few new drugs
- Few new classes of antibiotic reach the market, so the supply of effective drugs is shrinking.
Global spread
- Travel and trade carry resistant strains quickly between countries.
Part 3 of 5Reducing Resistance
Reducing Resistance
Prescribe with care
- Antibiotics should be used only for bacterial infections and never for viral illnesses such as colds.
Finish the course
- Completing the dose kills the more tolerant bacteria that would otherwise survive and multiply.
Farm use
- Restricting the routine use of antibiotics in healthy livestock lowers selection pressure for resistance.
Hygiene and surveillance
- Hand washing, isolation of infected patients and tracking resistant strains slow the spread.
| Factor | Effect on resistance |
|---|---|
| Overuse of antibiotics | Increases selection pressure |
| Incomplete course | Leaves survivors that can breed |
| Hygiene measures | Reduces spread |
| Horizontal gene transfer | Spreads resistance genes between species |
Part 4 of 5Evidence and Interpreting Data
Evidence and Interpreting Data
Rapid evolution
- Bacteria can divide every twenty minutes or so under good conditions,
- so natural selection produces visible change in days rather than millennia.
Reading a graph
- If the percentage of resistant isolates rises as antibiotic use rises, this suggests a link,
- but correlation alone does not prove cause.
Hospital settings
- Intensive antibiotic use and close contact between vulnerable patients make hospitals ideal places for resistant strains to spread.
Nature of science
- Evidence from many countries and long-term data lets scientists and governments agree on policy, such as restricting antibiotic use.
Part 5 of 5Exam Checklist & Tips
3.7.9 – Key Points for the Exam
- Resistance The ability of bacteria to survive an antibiotic that would normally kill them.
- Mutation A random change in DNA base sequence.
- Selection pressure An environmental factor, here the antibiotic, that favours survival of some variants.
- Plasmid A small circular DNA molecule that can carry resistance genes between bacteria.
- MRSA A bacterial strain resistant to methicillin and many other antibiotics.
Common Mistakes
- Saying that bacteria 'learn' or choose to become resistant;
- resistance comes from random mutation followed by selection.
- Stating that the patient becomes resistant rather than the bacteria.
- Forgetting that the antibiotic does not cause the mutation but only selects for those already present.
- Ignoring horizontal transfer, which allows resistance to spread without reproduction.
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