Infectious Disease: Question 6

Worked solution

Step 1: Recall the target of penicillin

Penicillin belongs to a group of antibiotics that act on the bacterial cell wall. A bacterial cell wall gets its strength from peptidoglycan (murein), long chains of sugars and amino acids that are cross-linked together by specific enzymes as the cell grows and divides, forming a rigid mesh around the cell.

Penicillin works by inhibiting these cross-linking enzymes. This mainly affects bacteria that are actively growing and dividing, since it is exactly when new peptidoglycan is being laid down and cross-linked that the antibiotic has an effect; it does little to a wall that is already fully formed and not being remodelled.

Step 2: Link this to why the bacterium dies

Bacterial cytoplasm is more concentrated than the surrounding fluid, so water constantly tends to move into the cell by osmosis. Normally the rigid, cross-linked wall resists this and stops the cell bursting. Without proper cross-linking, the new wall is too weak to withstand this osmotic pressure, so water continues to enter, the cell swells, and it eventually bursts (lyses), killing the bacterium.

Step 3: Evaluate each option

• A: correct. This describes inhibition of peptidoglycan cross-linking during cell wall synthesis, leading to osmotic lysis, which is exactly how penicillin kills susceptible bacteria.
• B: incorrect. Penicillin does not break down existing peptidoglycan (that is closer to how an enzyme such as lysozyme works); it prevents new cross-links from forming.
• C: incorrect, this describes a different class of antibiotic that targets bacterial ribosomes/protein synthesis, not penicillin’s mode of action on the cell wall.
• D: incorrect. Penicillin does not act as a physical coating on the outside of the cell; it acts inside the cell wall assembly process itself.

Final answer

• Penicillin prevents cross-linking of peptidoglycan during cell wall synthesis, so a weakened wall allows water to enter by osmosis and the cell bursts, option A.