Which class of antibiotics inhibits bacterial cell wall synthesis by interfering with peptidoglycan cross-linking?

• A. Beta-lactams
• B. Macrolides
• C. Fluoroquinolones
• D. Aminoglycosides

Answer: (A)

The question tests how certain antibiotics block cell wall synthesis by stopping peptidoglycan cross-linking. Cross-linking is done by penicillin-binding proteins that act as transpeptidases to join peptide chains and strengthen the wall. Beta-lactam antibiotics resemble the normal D-Ala-D-Ala end of the peptidoglycan precursor, bind irreversibly to these PBPs, and inhibit the transpeptidase step. Without cross-links, the cell wall is weak, especially as the bacterium grows, leading to lysis.

Other drug classes inactivate their targets instead of cross-linking—macrolides inhibit protein synthesis at the 50S ribosomal subunit, fluoroquinolones disrupt DNA replication by inhibiting DNA gyrase/topoisomerase IV, and aminoglycosides cause misreading of mRNA at the 30S subunit.