CIE A Level Biology (9700) exams from 2022

Revision Notes

17.2.4 Natural Selection: Antibiotic Resistance

Natural Selection: Antibiotic Resistance

  • When humans experience a pathogenic bacterial infection they are often prescribed antibiotics by a healthcare professional
  • Antibiotics are chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue
    • Antibiotics are derived from naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells
    • The aim of antibiotic use is to aid the body’s immune system in fighting a bacterial infection
  • Penicillin is a well-known example; it was the first antibiotic to be discovered in 1928 by Sir Alexander Fleming
  • Antibiotics are either described as being bactericidal (they kill) or bacteriostatic (they inhibit growth processes), they target prokaryotic features but can affect both pathogenic and mutualistic bacteria living on or in the body
  • However, like in all species, there exists genetic diversity within populations, and the same applies to disease-causing bacteria
  • Individual bacterial cells may possess alleles that confer resistance to the effects of the antibiotic
    • These alleles are generated through random mutation and are not caused by antibiotic use, but antibiotic use exerts selection pressures that can result in the increase in their frequency
  • Bacteria have a single loop of DNA with only one copy of each gene so when a new allele arises it is immediately displayed in the phenotype
  • When an antibiotic is present:
    • Individuals with the allele for antibiotic resistance have a massive selective advantage so they are more likely to survive, reproduce and pass genome (including resistance alleles)
    • Those without alleles are less likely to die and reproduce
    • Over several generations, the entire population of bacteria may be antibiotic-resistant
  • Antibiotic resistance is an important example of natural selection


  • There are known populations of the bacterium Staphylococcus that possess alleles which make them resistant to the effects of penicillin
    • These are known as resistant strains
  • Due to the rapid reproduction rate of bacteria (generations of 20-30 minutes for some species in optimal conditions) a single resistant bacterium can produce 10 000 million resistant descendants within a day

The future of antibiotic resistance

  • Antibiotic-resistant strains are a major problem in human medicine
  • New resistant strains are constantly emerging due to the overuse of antibiotics
    • By using antibiotics frequently, humans exert a selective pressure on the bacteria, which supports the evolution of antibiotic resistance
  • Scientists are trying hard to find new antibiotics that bacteria have not yet been exposed to, but this process is expensive and time-consuming
  • Some strains of bacteria can be resistant to multiple antibiotics and they create infections and diseases which are very difficult to treat
  • When antibiotics were discovered, scientists thought they would be able to eradicate bacterial infections, but less than a century later a future is being imagined where many bacterial infections cannot be treated with current medicines

Exam Tip

Bacteria pass on alleles for antibiotic resistance through reproduction (vertical gene transfer) but they can also do it in another way.

Bacterium possess plasmids which are a small circular piece of DNA that is not the main chromosome. Alleles for antibiotic resistance are often found on these plasmids. Plasmids can be easily transferred from one bacterium to another, even between different species. This is an example of horizontal gene transfer.

This means that alleles for antibiotic resistance can be passed one from species of bacteria to another species.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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