Natural Selection: Antibiotic Resistance (CIE A Level Biology)

• When humans experience a pathogenic bacterial infection they are often prescribed antibiotics by a doctor
• 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 an 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 their genome (including resistance alleles) on
  • Those without alleles are less likely to survive and reproduce
  • Over several generations, the entire population of bacteria may become antibiotic-resistant

• Antibiotic resistance is an important example of natural selection

Staphylococcus

• 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 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 envisaged where many bacterial infections cannot be treated with current medicines