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3.6.9Additional lessons

Antibiotic Resistance — IB Biology SL Revision Notes & Topical Questions

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Part 1 of 5How Resistance Evolves

3.6.9 Antibiotic Resistance

The antibiotic does not cause the mutation; it selects for it.

1
Mutations occur at random in a bacterial population, and a few produce resistance to an antibiotic.
2
When the antibiotic is used, non-resistant bacteria are killed, while resistant bacteria survive.
3
Survivors reproduce with less competition, so resistance genes are passed on in much greater numbers.
4
Because bacteria have a single copy of each gene, a resistance mutation has an effect at once.
5
After several generations most of the population is resistant, which is evolution by natural selection.

Part 2 of 5Mechanisms of Resistance

Enzymes
  • Some bacteria make β-lactamase, an enzyme that breaks down penicillin.
Gene transfer
  • Resistance genes can pass between bacteria on plasmids, even between different species.
Rapid reproduction
  • Bacteria divide quickly, so mutations spread through a population in a short time.
MRSA
  • Methicillin-resistant Staphylococcus aureus resists several antibiotics and is hard to treat.

Part 3 of 5Causes and Consequences

Overuse
  • Frequent use of antibiotics exerts strong selection pressure for resistant strains.
Misuse
  • Using antibiotics for viral infections or not finishing a prescribed course helps resistance to spread.
Agriculture
  • Routine use of antibiotics in farm animals contributes to resistant bacteria.
Consequences
  • Infections become harder to treat, hospital stays lengthen and death rates rise.

Part 4 of 5Reducing the Problem

Reducing the Problem
Prescribing
  • Avoid prescribing antibiotics for non-serious or non-bacterial infections.
Hygiene
  • Strict hygiene in hospitals reduces the spread of resistant bacteria.
Farming
  • Minimise routine antibiotic use in animals.
New drugs
  • Research into new antibiotics is costly and slow, but it is needed to stay ahead of resistance.

Part 5 of 5Exam Checklist & Tips

3.6.9 – Key Points for the Exam

  • Antibiotic resistance The ability of bacteria to survive exposure to an antibiotic.
  • Selection pressure An environmental factor that favours certain individuals.
  • Plasmid A small circular DNA molecule that can carry resistance genes between bacteria.
  • MRSA A strain of Staphylococcus aureus resistant to multiple antibiotics.

Common Mistakes

  • Writing that bacteria become resistant because they choose to adapt;
    • mutation is random and selection acts afterwards.
  • Saying an individual bacterium develops resistance during treatment, when it is the population that evolves.
  • Thinking humans become resistant to antibiotics; it is the bacteria that become resistant.
  • Forgetting that selection needs an antibiotic present for resistance to be favoured.

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