Antibiotic Resistance (CIE A Level Biology)

• 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

Bacteria 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 diagram

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