Penicillin (Cambridge (CIE) AS Biology)

Penicillin: Why it Affects Bacteria & Not Viruses

• Antibiotics are drugs that kill or stop the growth of bacteria but that do not harm the cells of the infected organism

• Some antibiotics are derived from living organisms, e.g. penicillin is produced by some 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, e.g.

  • 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 peptidoglycan

• Peptidoglycan molecules are held together by cross-links

• 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 as the cell grows, and new peptidoglycan molecules then join 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, as autolysins no longer create holes and no more cross-links form once the growth of a bacterium is complete

The effect of penicillin diagram

Penicillin prevents the formation of cross-linkages in bacterial cell walls during cell growth, resulting in a weakened cell wall

• Antibiotics, such as penicillin, do not affect viruses as they do not have cells 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

• Penicillin is not effective against all bacteria due to:

  • Thick cell walls which reduce permeability to penicillin

  • Enzymes which break down penicillin

• When one antibiotic is not effective against a particular bacterial species, an antibiotic with a different mechanism of action can be used