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Immunology and Disease: Antibiotics

Penicillin and the bacterial cell wall

  • 3 things to know
  • Component 3

Overview

Recall questions on peptidoglycan and how the wall is cross-linked, how penicillin prevents that, why the effect is bactericidal only in growing cells, why human cells are unaffected, and why Gram negative bacteria are less susceptible.

What you need to know

Answer each one before you reveal it.

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Key term

Which type of organism produces penicillin naturally?

Key term

Which polymer forms the wall of a bacterial cell?

Key term

Explain how the inhibition of cross-linking leads to the death of the bacterium.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Which type of organism produces penicillin naturally?

    Model answer

    Penicillin is produced by moulds of the genus Penicillium, which secrete it to inhibit the bacteria competing with them.

  2. 2

    Which polymer forms the wall of a bacterial cell?

    Model answer

    The wall is made of peptidoglycan, also called murein, in which parallel chains of sugar units are held together by short peptide cross-links.

  3. 3

    What does penicillin inhibit in a growing bacterium?

    Model answer

    It inhibits the enzyme that forms the peptide cross-links between the sugar chains of the peptidoglycan wall.

  4. 4

    Explain how the inhibition of cross-linking leads to the death of the bacterium.

    Model answer

    The new wall laid down as the cell grows has no cross-links and is too weak to resist the pressure inside, so water entering by osmosis from the surrounding dilute solution swells the cell until it bursts.

  5. 5

    Why is penicillin classed as bactericidal rather than bacteriostatic?

    Model answer

    It does not simply hold the population steady, because the affected cells actually lyse and the number of living bacteria falls.

  6. 6

    Why does penicillin only affect bacteria that are growing and dividing?

    Model answer

    The drug blocks the making of new wall material and does nothing to wall that is already cross-linked, so a cell that is not enlarging keeps a wall strong enough to survive.

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