Evolution in Action (WJEC GCSE Biology)

Antibiotic resistance

• Antibiotics are chemical substances made by certain fungi or bacteria that affect the working of bacterial cells, either by disrupting their structure or function or by preventing them from reproducing
• Antibiotics are effective against bacteria but not against viruses
• Antibiotics target processes and structures that are specific to bacterial (prokaryotic) cells; as such they do not generally harm animal cells

Antibiotic action diagram

How antibiotics work

• The use of antibiotics has increased exponentially since they were first introduced in the 1930s
• In that time they have saved millions of lives

Reduction in deaths in the US due to antibiotics graph

The introduction of antibiotics has had one of the largest impacts on global health, shown by this example in the USA

• However, since their discovery and widespread use, antibiotics have been overused and antibiotic resistance has developed in many different types of bacterial species
  • Bacteria, like all organisms, have random mutations in their DNA
  • One of these mutations may give them resistance to an antibiotic
  • If an organism is infected with bacteria and some of them have resistance, they are likely to survive treatment with antibiotics
  • The population of the resistant bacteria will increase, especially if competition with other bacteria in the area is reduced
  • If the resistant strain is causing a serious infection then another antibiotic will be needed
  • A strain of Staphylococcus aureus has developed resistance to a powerful antibiotic methicillin, this is known as MRSA (Methicillin resistant Staphylococcus aureus)
  • MRSA can infect wounds and is difficult to treat without antibiotics

Antibiotic resistance diagram

Bacteria evolve rapidly as they reproduce quickly and acquire random mutations – some of which confer resistance

Warfarin resistance

• Warfarin is the active ingredient used in rat poison
• Random mutations in the DNA of rats have produced new alleles and resulted in some individuals being naturally resistant to warfarin
• This means that those resistant individuals are more likely to survive and reproduce
  • They have a selective advantage over those who are not resistant to warfarin
• Rats that reproduce will pass on their advantageous alleles for resistance, to the next generation
• Over time, the allele for resistance becomes more common in the population
• The population of rats becomes resistant to rat poison

Pesticide resistance

• Pesticides are applied to crops to prevent damage by insects
• Random mutations in the DNA of insects have produced new alleles and resulted in some individuals being naturally resistant to pesticides
• This means that those resistant individuals are more likely to survive and reproduce
  • They have a selective advantage over those who are not resistant to the pesticide
• Insects that reproduce will pass on their advantageous alleles for resistance, to the next generation
• Over time, the allele for resistance becomes more common in the population
• The population of  insects becomes resistant to pesticide