The Antibiotic That Stopped Working
A patient with a throat infection is prescribed a 7-day course of an antibiotic. After 3 days the fever subsides and the patient stops taking the tablets, keeping the rest 'for next time'. Two months later the same patient has a similar infection; the same antibiotic is prescribed but the symptoms do not improve. A swab shows a bacterial population in which a small fraction of cells survive the drug. The patient says: 'My body has become immune to the antibiotic.'
1. Explain, using the principles of variation and selection, how a drug-resistant bacterial population can arise in this patient.
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Reasoning chain:
- In any large bacterial population, random mutations (and gene transfer between bacteria) produce variants. A few cells happen to carry traits that reduce the drug's effect - e.g. an altered target protein or an enzyme that destroys the drug.
- The antibiotic is a selective pressure: it kills susceptible cells.
- The rare resistant cells survive, and with competitors removed they reproduce rapidly (bacteria divide by binary fission, often in under an hour).
- The population that regrows is descended from survivors, so it is now largely resistant - natural selection in action, not a change in the patient's own immunity.
Crucial correction: the patient's immune system is not involved in 'becoming immune' to an antibiotic - the antibiotic does not act on human cells at all.
2. Explain why stopping the course after 3 days made resistance more likely, and why the full course matters.
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Reasoning chain:
- Symptoms improving at day 3 means the most susceptible bacteria are dead - but the population is not eliminated. The hardiest and least susceptible cells remain.
- Stopping early removes the selective pressure exactly when the survivors are the least susceptible ones, freeing them to multiply without competition.
- A full course keeps the drug concentration above the level needed to kill the remaining cells for long enough to clear them.
Important nuance: the modern message is 'take the drug exactly as prescribed - right drug, right dose, right duration', because unnecessary or incomplete use both enrich resistant strains. Leftover antibiotics should never be reused for a later illness.
3. The swab shows the bacteria are still a normal throat species, not a new species. Explain why the population is now resistant although the species is unchanged.
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Reasoning chain:
- Species identity is defined by the whole genome and its relationships; resistance is a property of particular alleles carried by individuals.
- Selection changes allele frequencies within a population, not necessarily the species label. The resistant strain is the same species with a different gene variant.
- Therefore no new species has 'evolved' in two months - but microevolution has occurred: the frequency of resistance alleles has risen sharply.
Conclusion: 'the species is unchanged' and 'the population is resistant' are fully compatible. This is a change in a population's genetic composition, which is exactly what evolution is at its smallest scale.