What is the mechanism of action of vancomycin?

• A. Disrupts cell membrane integrity
• B. Inhibits peptidoglycan synthesis in the bacterial cell wall; primarily active against Gram-positive organisms
• C. Inhibits DNA replication
• D. Inhibits protein synthesis at the 50S ribosomal subunit

Answer: (B)

Vancomycin blocks the synthesis of the bacterial cell wall by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors. This prevents these units from being incorporated into the growing wall, stopping the final steps of wall assembly and cross-linking. As a result, the cell wall becomes weak and the bacterium is susceptible to lysis, with Gram-positive organisms being most affected because their cell envelopes expose the wall more directly and they lack an outer membrane barrier. The outer membrane of many Gram-negative bacteria prevents vancomycin from reaching its target, which is why its activity is mainly against Gram-positive organisms. Other mechanisms, like disrupting membranes, inhibiting DNA replication, or inhibiting protein synthesis at the 50S subunit, belong to different antibiotic classes and are not how vancomycin works.