AQA GCSE Combined Science: Biology

Revision Notes

6.3.7 Resistant Bacteria

Mutations & Bacterial Evolution

  • The theory of evolution by natural selection is now widely accepted and many sources of data are now available to support the theory of evolution
  • One very clear piece of evidence for evolution is antibiotic resistance in bacteria
  • An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria
  • Antibiotics are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease
  • Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span
  • Like all other organisms, within a population, there will be variation caused by mutations
  • A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)
  • When the population is treated with this antibiotic, the resistant bacteria do not die
  • This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead
  • Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
  • Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment

Antibiotic_resistance, IGCSE & GCSE Biology revision notes

Development of antibiotic resistance in bacteria


  • This is an example of natural selection that humans have helped to develop due to overuse of antibiotics in situations where they were not really necessary, for example:
    • For the treatment of non-serious infections
    • Routine treatment to animals in agriculture
    • Failure to finish the prescribed course of antibiotics


  • Increases in the population of antibiotic-resistant bacteria cause infections and diseases which are harder to control as it is difficult to find antibiotics that certain strains of bacteria are not resistant to
  • An example of this is MRSA, a very dangerous bacterial strain that is resistant to most antibiotics
  • If someone gets infected with MRSA they cannot be treated easily

Antibiotic Development

  • The increase we are currently seeing in antibiotic resistance is encouraging drug companies to develop new antibiotics that are effective against these new resistant strains, such as MRSA
  • However, the number of new antibiotics discovered has slowed significantly
  • Developing new antibiotics is also a very costly process
  • Some scientists are worried we may not be able to keep up with the demand for new antibiotics, as more and more antibiotic-resistant strains evolve

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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