Which one of the following antibiotics inhibits the final peptide bond between d-alanine and glycine?
**Core Concept**
The final peptide bond between d-alanine and glycine is a critical step in bacterial cell wall synthesis. Inhibiting this bond prevents the formation of the bacterial cell wall, ultimately leading to cell lysis and death. This mechanism is exploited by certain antibiotics that target the bacterial cell wall synthesis pathway.
**Why the Correct Answer is Right**
The antibiotic that inhibits the final peptide bond between d-alanine and glycine is Vancomycin. Vancomycin binds to the d-alanyl-d-alanine terminus of the cell wall precursor, preventing the enzyme transpeptidase from forming the final peptide bond. This action disrupts cell wall synthesis, ultimately leading to bacterial cell death. Vancomycin's mechanism of action is particularly effective against Gram-positive bacteria, which lack a outer membrane and are more susceptible to cell wall inhibitors.
**Why Each Wrong Option is Incorrect**
**Option A:** Penicillin inhibits the synthesis of the cell wall by binding to the penicillin-binding proteins (PBPs) and preventing the cross-linking of peptidoglycan chains. This is a different mechanism of action compared to Vancomycin.
**Option B:** Cephalosporins, like Penicillin, also inhibit cell wall synthesis by binding to PBPs and preventing the cross-linking of peptidoglycan chains.
**Option C:** Aminoglycosides inhibit protein synthesis by binding to the bacterial 30S ribosomal subunit, causing misreading of mRNA and premature termination of protein synthesis.
**Clinical Pearl / High-Yield Fact**
Vancomycin resistance in bacteria, particularly MRSA (methicillin-resistant Staphylococcus aureus), is a growing concern. Vancomycin-resistant enterococci (VRE) are also increasingly reported. The increasing prevalence of antibiotic-resistant bacteria underscores the need for judicious antibiotic use and infection control measures.
**Correct Answer:** C. Vancomycin. Vancomycin inhibits the final peptide bond between d-alanine and glycine.