10.2.1—Penicillin action on bacteria

Penicillin stops new peptidoglycan molecules from forming the cross-links that strengthen a growing bacterial cell wall.

• A growing bacterium makes small holes in its wall so the wall can stretch; enzymes called autolysins continue to create these holes.
• Normally, new peptidoglycan is added and cross-linked to reinforce the wall. Penicillin prevents the cross-linking step.
• The holes therefore accumulate while reinforcement is reduced, so the wall becomes progressively weaker.
• Bacteria take up water by osmosis. A weakened wall cannot withstand the pressure from inside indefinitely, so a growing susceptible bacterium can burst.

The effect depends on growth: once bacterial growth is complete, autolysins no longer create new holes and no further cross-links form, so this penicillin mechanism is most effective while the bacterium is growing.

Penicillin weakens a bacterial wall by interrupting its reinforcement; it does not dissolve every bacterium immediately. This card stops at the cell-wall mechanism: the distinction between bacterial and viral targets and the development of antibiotic resistance belong to the neighbouring cards.