Mechanism of action of vancomycin is:
**Core Concept:** Vancomycin is a glycopeptide antibiotic that exerts its antimicrobial action by inhibiting bacterial cell wall synthesis. It competes with D-alanyl-D-alanine, the terminal dipeptide of the bacterial cell wall precursors, for binding to the D-alanyl-D-alanine ligase enzyme. As a result, cell wall synthesis is impaired, leading to cell lysis and death.
**Why the Correct Answer is Right:** Vancomycin primarily targets Gram-positive bacteria, as they possess a thicker cell wall composed of peptidoglycan. The presence of cross-linking of peptidoglycan strands by the enzyme, transpeptidase, and the absence of the outer membrane in Gram-positive bacteria make them more susceptible to vancomycin.
**Why Each Wrong Option is Incorrect:**
A. **Incorrect**: This option is wrong because vancomycin does not directly target Gram-negative bacteria, which have an outer membrane and different cell wall composition that renders them less susceptible to vancomycin.
B. **Incorrect**: Although vancomycin interferes with cell wall synthesis, it does not act on the ribosome, a site of action for many other antibiotics like beta-lactams and macrolides.
C. **Incorrect**: Vancomycin inhibits transpeptidation, but it does not target the peptidoglycan synthase enzyme, which is responsible for the synthesis of peptidoglycan precursors.
D. **Incorrect**: Unlike many other antibiotics, vancomycin does not inhibit protein synthesis, as its mechanism targets the cell wall rather than the cytoplasmic membrane or nucleic acids.
**Clinical Pearl:** Vancomycin should be considered when treating infections caused by resistant strains of Gram-positive bacteria like methicillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile, and Enterococcus species. It is crucial to differentiate between the correct mechanism and the incorrect mechanisms provided by options A, B, C, and D, as understanding this distinction is crucial for appropriate antibiotic selection and treatment of specific infections.