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First teaching 2023

First exams 2025

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Antibiotic Resistance (CIE A Level Biology)

Revision Note

Naomi H

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Naomi H

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Resistance to Antibiotics: An Outline

  • Within a bacterial population, genetic variation exists between individuals; this is the result of random mutation
  • A chance mutation might give rise to a new allele that causes some bacteria to become resistant to an antibiotic
    • Some pathogenic bacteria have become resistant to penicillin as they have acquired an allele that codes for the production of the enzyme β-lactamase (also known as penicillinase), which breaks down penicillin
  • When the bacterial population is treated with this antibiotic, the bacteria with the resistance allele do not die
    • The antibiotic acts as a selection pressure, effectively selecting the resistant bacteria
  • This means the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead
  • Therefore the allele for antibiotic resistance is passed on with much greater frequency to the next generation
    • As bacteria only have one allele of each gene, a new allele will have an immediate effect; it cannot be masked by a second allele, as it might be in eukaryotes
  • Over time, the antibiotic resistance allele increases in frequency, and a higher proportion of the bacterial population is resistant to the antibiotic
  • This is an example of evolution by natural selection

Antibiotic resistance diagram

Antibiotic resistanceBacteria evolve rapidly due to their high mutation rate and short generation times; this means that antibiotic resistance can arise quickly when a population is exposed to a new antibiotic

Vertical and horizontal gene transmission in bacteria

  • Bacteria can evolve by natural selection very quickly, this is because:
    • They reproduce very fast, carrying out what is known as vertical gene transmission where genes are passed from one generation to the next 
      • Frequent DNA replication means that mutation rate is high, so the probability of an allele for resistance arising is increased
      • In asexual binary fission the offspring are all clones of the parent, so all the offspring of an individual with a resistance allele will also have the resistance allele
      • Bacteria can divide as often as every 20 minutes, so the number of individuals with a new resistance allele can increase very fast
    • Bacteria can also carry out horizontal gene transmission
      • Plasmids may contain antibiotic-resistant genes
      • These plasmids are frequently transferred between bacterial individuals in the same generation
        • This can occur between bacteria of the same species, or bacteria of different species
      • This process is known as conjugation
      • In this way, a bacterium containing an allele for antibiotic resistance could pass the allele on to other bacteria

Vertical and horizontal gene transmission diagramvertical-and-horizontal-gene-transfer

Antibiotic resistance in bacteria can spread by vertical or horizontal transmission

  • Humans have caused an increase in the occurrence of antibiotic resistance; this is because whenever bacteria are treated with antibiotics, humans are providing a selection pressure which drives natural selection
  • This problem is made worse when antibiotics are used improperly, e.g.
    • For treatment of non-serious infections
    • For treatment of viral infections
    • Routine treatment to animals in agriculture

Examiner Tip

Be careful when describing the development of antibiotic resistance; there are two major pitfalls to look out for:

  • Antibiotic resistance mutations occur by chance, and not as the result of exposure to antibiotics; you should NEVER say that antibiotics cause mutation
  • Mutation gives rise to a new resistance allele, and not a new gene; it is always safer to refer to the resistance allele rather than to the resistance 'gene'

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.