17.2.417-18 · Selection, Evolution, Classification and Conservation
Natural Selection- Antibiotic Resistance — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 10 of 36 Unit quiz
Lesson summary
17.2.4 Natural Selection: Antibiotic Resistance — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how antibiotic resistance evolves and spreads in bacteria through natural selection.Antibiotics — chemical substances that inhibit or kill bacterial cells with li…
- Rapid reproduction
- Vertical transmission
- Horizontal transmission
- Bactericidal
- Bacteriostatic
- Horizontal gene transfer
Part 1 of 8Antibiotics
Antibiotics
- Antibiotics chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue.
- They come from natural substances that harm prokaryotic cells but not eukaryotic cells.
- Penicillin was the first antibiotic, discovered by Alexander Fleming in 1928.
Bactericidal antibiotics kill bacteria; bacteriostatic antibiotics inhibit growth processes.
They target prokaryotic features, so they can affect both pathogenic and helpful bacteria.
Penicillin prevents cross-links forming in the peptidoglycan cell wall, so growing bacteria burst by osmosis.
Antibiotics do not act on viruses, so they do not treat colds or flu.
Part 2 of 8How resistance evolves
1
Genetic diversity exists in a bacterial population; random mutation gives some cells an allele for resistance.
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The alleles are not caused by the antibiotic; the antibiotic only raises their frequency.
3
Bacteria have one loop of DNA with one copy of each gene, so a new allele is immediately expressed in the phenotype.
4
The antibiotic acts as a selection pressure and kills non-resistant bacteria.
5
Resistant bacteria survive, divide by binary fission and pass on the allele.
6
Over several generations the whole population may become resistant, an example of directional selection.
Part 3 of 8Speed of evolution in bacteria
Rapid reproduction
- Rapid reproduction some species divide every 20-30 minutes in optimum conditions.
- One resistant bacterium can produce 10 000 million resistant descendants in a day.
Large populations mean one infected person has billions of bacteria, so resistant mutants are likely to be present.
There is often a cost of resistance, so resistant bacteria may grow more slowly when no antibiotic is present.
Staphylococcus has strains resistant to penicillin, called resistant strains.
Part 4 of 8Spread of resistance
Vertical transmission
- Vertical transmission resistance alleles pass to daughter cells during binary fission.
Horizontal transmission
- Horizontal transmission resistance alleles pass between bacteria, even of different species.
- Resistance genes are often on plasmids, small circular pieces of DNA separate from the main chromosome.
- In conjugation a pilus forms and a plasmid copy passes to another bacterium.
- Transformation (uptake of DNA from the environment) and transduction (carried by bacteriophages) also transfer DNA.
Bacteria can gather several resistance genes, giving multi-drug resistance such as MRSA and multi-drug resistant tuberculosis.
Part 5 of 8The problem of resistance
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New resistant strains keep emerging because of overuse of antibiotics, which exerts selective pressure.
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Finding new antibiotics is expensive and time-consuming.
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Multi-resistant strains cause infections that are very difficult to treat.
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Many infections may become untreatable with current medicines less than a century after antibiotics were discovered.
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Routine surgery depends on effective antibiotics.
Part 6 of 8Reducing the spread of resistance
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Prescribe antibiotics only for bacterial infections.
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Patients should complete the full course so all bacteria are killed.
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Use narrow-spectrum antibiotics where possible and avoid routine use in livestock feed.
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Rotate or combine antibiotics so a bacterium would need several mutations at once.
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Use good hygiene, hand washing and isolation of infected patients.
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Develop new antibiotics, faster diagnostic tests, vaccination and international surveillance.
Part 7 of 8Investigating antibiotics on agar plates
1
Spread a lawn of bacteria on agar using aseptic technique.
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Place discs soaked in antibiotic on the lawn.
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Incubate at about 25 °C (not above 30 °C in schools).
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Measure the clear inhibition zone around each disc; a larger zone means a more effective antibiotic.
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Calculate the zone area as πr2 to compare antibiotics; colonies inside the zone are likely resistant mutants.
Part 8 of 8More from the specification
More from the specification
Bacterial colony survival yield
- Bacterial colony survival yield indiscriminate antibiotic exposure exerts intense selective pressures,
- increasing the phenotypic yield of drug-resistant microbial strains.
Natural Selection and Environmental Selection Pressures
- Environmental selection factors favor individuals possessing adaptive phenotypic traits, granting them higher reproductive fitness and an increased chance of survival to pass beneficial alleles to their offspring.
Evolution of Antibiotic Resistance in Bacteria
- Random chromosomal mutations or horizontal gene transfer generate antibiotic resistance alleles in bacterial individuals;
- under antibiotic treatment, non-resistant bacteria perish while resistant strains survive and proliferate.
Artificial Selection vs Natural Fitness
- In artificial selection, human breeders consciously select individual animals or crop plants exhibiting desirable agricultural traits, often reducing genetic variation compared to populations shaped by natural selection.
- (selective breeding)
Genetic Drift in Small Populations
- In small natural populations,
- such as isolated colonies of wild rabbits, random chance events rather than differential selection can cause significant fluctuations in allele frequencies between generations.
Bottleneck effect
- Loss of genetic variation after a sudden fall in population size.
Hardy-Weinberg principle
- Equations predicting allele and genotype frequencies in a non-evolving population.
Stabilising selection
- Selection favouring the average phenotype.
Disruptive selection
- Selection favouring both extreme phenotypes.
Recessive
- Expressed only when no dominant allele is present.
Organism
- A living thing, such as a plant or an animal.
Variation
- Differences between individuals of the same species.
Dominant
- Expressed when present in the genotype.
Selective breeding
- Humans choosing which individuals breed.
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
Genetic drift
- Change in allele frequency due to chance, most important in small populations.
Founder effect
- Loss of genetic variation when a few individuals found a new population.
Key terms
| Term | Meaning |
|---|---|
| Antibiotic | Chemical that kills or inhibits growth of bacteria with little harm to human tissue. |
| Antibiotic resistance | Ability of a bacterium to survive exposure to an antibiotic, due to a resistance allele. |
| Bactericidal | Describes an antibiotic that kills bacteria. |
| Bacteriostatic | Describes an antibiotic that inhibits bacterial growth. |
| Plasmid | Small circular DNA molecule, separate from the main chromosome, that often carries resistance genes. |
| Horizontal gene transfer | Transfer of alleles between bacteria, e.g. by conjugation, rather than by reproduction. |
| Multi-drug resistance | Resistance of a strain to several antibiotics. |
Avoid these mistakes
1
Saying the antibiotic causes the mutation — the resistance allele arises by random mutation before exposure.
2
Saying individual bacteria become resistant — the population changes as resistant cells survive and reproduce.
3
Forgetting horizontal gene transfer on plasmids — resistance can pass to other species.
4
Saying antibiotics treat viral infections — they act on bacterial features only.
5
Stopping a course early — surviving partly resistant bacteria are selected.
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