What is the mechanism of action of vancomycin?

• A. Inhibits DNA replication
• B. Inhibits protein synthesis
• C. Inhibits cell wall synthesis
• D. Inhibits RNA transcription

Answer: (C)

Vancomycin works by binding to the terminal D-Ala-D-Ala dipeptide of the peptidoglycan precursors used to build the bacterial cell wall. This blocks the enzymes that assemble and cross-link the wall (the transglycosylase and related steps), so the cell wall cannot be formed properly. Without a sturdy wall, the bacteria, especially Gram-positive ones, cannot maintain integrity and die as they attempt to grow or divide. This action is different from agents that inhibit DNA replication, protein synthesis, or RNA transcription, which target nucleic acids or ribosomes rather than the cell wall. Vancomycin is most effective against Gram-positive organisms and is less active against Gram-negatives due to permeability barriers; resistance can occur when the D-Ala-D-Ala terminus is altered to D-Ala-D-Lac, reducing vancomycin binding.