Which antibiotic used for Gram-positive ocular infections inhibits cell wall synthesis by binding D-Ala-D-Ala termini?

• A. Cefazolin
• B. Aztreonam
• C. Vancomycin
• D. Tobramycin

Answer: (C)

Vancomycin works by directly binding to the D-Ala-D-Ala termini of the peptidoglycan precursors in the bacterial cell wall. This blocks the enzymes responsible for cross-linking and elongation of the cell wall, effectively halting wall synthesis and causing death of the bacterium. This mechanism is particularly important for Gram-positive organisms like Staphylococcus and Streptococcus, which is why vancomycin is used for Gram-positive ocular infections. Gram-negative bacteria are less affected because their outer membrane prevents vancomycin from reaching its target. Other drugs listed either inhibit cell wall synthesis in a different way (beta-lactams) or inhibit protein synthesis and are not specifically acting on the D-Ala-D-Ala termini, so they don’t fit this mechanism.