Antibiotics (Edexcel International A Level Biology)

• When humans experience a bacterial infection they are often prescribed antibiotics 
• Antibiotics are chemical substances that damage bacterial cells with little or no harm to human tissue
  • Penicillin is a well-known example; it was discovered in 1928 by Sir Alexander Fleming
• Antibiotics are either  
  • Bactericidal; they kill bacterial cells
  • Bacteriostatic; they inhibit bacterial growth processes
    • Note that bacteriostatic antibiotics given at a high enough dose will result in the death of bacterial cells
• Antibiotics work by interfering with the growth or metabolism of the target bacterium e.g.
  • Inhibiting bacterial enzymes needed to form bonds in the cell walls; this prevents bacterial growth and can cause death
    • Cell walls are weakened and burst under the pressure of water entering the cell by osmosis
  • Binding to ribosomes and preventing protein synthesis; this inhibits enzyme production, stopping metabolic processes in the bacterial cell
  • Damaging cell membranes, leading to loss of useful metabolites or uncontrolled entry of water
  • Preventing bacterial DNA from coiling into rings, meaning that it no longer fits into the bacterial cell
• Since mammalian cells are eukaryotic, they will not be damaged by antibiotics
  • They do not have cell walls
  • They have different enzymes
  • They have different ribosomes
• Viruses do not have cellular structures such as enzymes, ribosomes, and cell walls so they are not affected by antibiotics

Penicillin prevents the formation of a strong cell wall in prokaryotes, ultimately leading to the death of the cell by lysis