CIE A Level Biology (9700) 2019-2021

Revision Notes

10.2.2 Antibiotic Resistance

Resistance to Antibiotics: An Outline

  • Within a bacterial population, there is variation caused by mutations (as occurs in populations of all species)
  • 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
    • For example, a mutation may change an existing gene within the bacterial genome, causing it to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic being used
  • This means the resistant bacteria can continue to reproduce with less competition from the 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
    • As bacteria only have one copy of each gene, a mutant gene will have an immediate effect on any bacterium possessing it
  • Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment
  • This is an example of evolution by natural selection
  • Some pathogenic bacteria have become resistant to penicillin as they have acquired genes that code for the production of the enzyme β-lactamase (also known as penicillinase), which breaks down penicillin

Vertical transmission

  • Bacteria reproduce asexually by binary fission (the DNA of the bacterial chromosome is replicated and the bacterial cell divides in two, with each daughter cell receiving a copy of the chromosome)
  • Bacteria reproduce like this very rapidly (on average, every 20 minutes)
  • If one bacterium contains a mutant gene that gives it antibiotic resistance, all of its descendants (millions of which can be produced in a matter of hours) will also have the antibiotic resistance
  • This method of spreading antibiotic resistance within a bacterial population is known as vertical transmission

Horizontal transmission

  • Plasmids (the small rings of DNA present in bacterial cells) often contain antibiotic-resistant genes
  • These plasmids are frequently transferred between bacteria (even from one species to another)
  • This occurs during conjugation (when a thin tube forms between two bacteria to allow the exchange of DNA) – DNA from the bacterial chromosome can also be transferred in this way
  • In this way, a bacterium containing a mutant gene that gives it antibiotic resistance could pass this gene on to other bacteria (even those from a different species). This is how ‘superbugs’ with multiple resistance have developed (e.g. methicillin-resistant Staphylococcus aureus – MRSA)
  • This method of spreading antibiotic resistance within or between bacterial populations is known as horizontal transmission
  • Antibiotic resistance in bacteria is an example of natural selection that humans have helped to develop. This is due to the overuse of antibiotics in situations where they were not really necessary or the incorrect use of antibiotics, for example:
    • For treatment of non-serious infections
    • Routine treatment to animals in agriculture
    • Failure to finish the prescribed course of antibiotics

Exam Tip

Remember, vertical transmission is only responsible for passing on antibiotic resistance within a single bacterial population, whereas horizontal transmission can spread antibiotic resistance within a single bacterial population, between two populations of the same species of bacteria, or occasionally between populations of different species of bacteria.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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