Question

How does an antibiotic penicillin work against bacteria?

• A. Inhibit overall DNA synthesis
• B. Selectively inhibit bacterial DNA synthesis
• C. Inhibit peptidoglycan synthesis
• D. Does not affect bacteria

Hint:

Beta-lactam antibiotics (penicillins, cephalosporins) are bactericidal because they target cell wall (peptidoglycan) synthesis.
Since animal cells do not possess peptidoglycan cell walls, these drugs have a high margin of safety.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Beta-lactam antibiotics, including penicillin, target the integrity of the bacterial cell wall.
Unlike animal cells, which lack a cell wall, bacteria depend on a rigid outer peptidoglycan cell wall to maintain osmotic stability and cellular shape.
Inhibiting the assembly of this structure leads to cell lysis and death.

Step 2: Detailed Explanation:
The bacterial cell wall consists of a complex mesh-like polymer known as peptidoglycan.
Peptidoglycan is made of alternating chains of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
These glycan chains are cross-linked to one another by short peptide bridges, which provide structural strength.
The final step in peptidoglycan synthesis, the cross-linking of the peptide side chains, is catalyzed by enzymes called transpeptidases, also known as penicillin-binding proteins (PBPs).
Penicillin possesses a highly reactive four-membered beta-lactam ring.
This ring structurally mimics the D-alanyl-D-alanine terminus of the nascent peptidoglycan peptide chains.
Because of this structural similarity, penicillin binds covalently and irreversibly to the active site of PBPs.
This binding blocks transpeptidase activity, halting the cross-linking of peptidoglycan polymers.
As the bacterium continues to grow and attempt cell division, autolysins degrade the existing cell wall, but new peptidoglycan cannot be synthesized to replace it.
The loss of cell wall integrity causes the bacterium to swell and burst due to internal osmotic pressure, leading to bactericidal action.
Thus, penicillin works by inhibiting peptidoglycan synthesis.

Step 3: Final Answer:
The correct option is (C).