CIE A Level Biology (9700) 2019-2021

Revision Notes

10.2.1 Penicillin

Penicillin: Why it Affects Bacteria & Not Viruses

  • Antibiotics are drugs that kill or stop the growth of bacteria (prokaryotes) but do not harm the cells of the infected organism
  • Some antibiotics are derived from living organisms (eg. penicillin is produced by certain fungi in the genus Penicillium), whilst other are made synthetically (in a laboratory)
  • Antibiotics work by interfering with the growth or metabolism of the target bacterium. Antibiotics target a variety of processes including:
    • synthesis of bacterial cell walls
    • activity of proteins in bacterial cell surface membranes
    • bacterial enzyme action
    • bacterial DNA synthesis
    • bacterial protein synthesis

How penicillin affects bacteria

  • Bacterial cell walls are composed of peptidoglycans (long molecules of peptides and sugars)
  • These peptidoglycan molecules are held together by cross-links that form between them
  • When a new bacterial cell is growing, it secretes enzymes known as autolysins that create small holes in the bacterial cell wall
  • These holes allow the bacterial cell wall to stretch, with new peptidoglycan molecules then joining up via the cross-links described above
  • Penicillin stops these cross-links forming by inhibiting the enzymes that catalyse their formation
  • However, the autolysins keep creating holes in the bacterial cell wall, making the walls weaker and weaker
  • As bacteria live in watery environments and take up water by osmosis, their weakened cell walls eventually burst as they can no longer withstand the pressure exerted on them from within the cell
  • This means penicillin is only effective against bacteria that are still growing (autolysins no longer create holes and no more cross-links between peptidoglycan molecules are formed once the growth of a bacterium is complete, as the bacterial cell wall no longer needs to expand)
  • Penicillin (and other antibiotics) do not affect viruses as they do not have cells (or cell walls) and therefore cannot be targeted in any of the ways that an antibiotic targets a bacterial cell
  • When a virus replicates, it uses the host cell’s mechanisms for transcription and translation, so not even these processes can be targeted as antibiotics do not bind to the proteins that host cells use in these processes
  • Penicillin is not effective against all bacteria (eg. tuberculosis) because the bacteria may have:
    • Thick cell walls which reduce permeability
    • Enzymes which breakdown penicillin

Exam Tip

Antibiotics are only effective against bacteria, not viruses. For example, cholera and TB can be treated with antibiotics, whereas measles and smallpox cannot, as they are viral infections.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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