Antibiotics (Edexcel International A Level Biology): Revision Note

Antibiotics

• 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