A 45-year-old male admitted with severe pneumonia is found to have methicillin-resistant Staphylococcus aureus (MRSA) on blood culture. The resistance mechanism in this organism involves acquisition of the mecA gene, which encodes an altered penicillin-binding protein (PBP-2a) with significantly reduced affinity for β-lactam antibiotics. The structure marked **B** in the diagram represents the normal penicillin-binding proteins that are the target of β-lactam antibiotics. Which of the following best explains why MRSA with altered PBP-2a remains resistant to conventional β-lactams including cephalosporins, yet may respond to newer agents like ceftaroline?

Explanation

## Why PBP-2a has low affinity for β-lactams, and ceftaroline has enhanced affinity for PBP-2a is right MRSA acquires the mecA gene encoding PBP-2a, an altered penicillin-binding protein with structurally modified active site that confers LOW AFFINITY for conventional β-lactams (methicillin, oxacillin, nafcillin, and most cephalosporins). This explains pan-β-lactam resistance in MRSA. Ceftaroline (5th-generation cephalosporin) and ceftobiprole have enhanced structural features (bulky C-3 side chain) that allow them to bind and inhibit PBP-2a with sufficient affinity, restoring bactericidal activity against MRSA. The mechanism remains the same — covalent inhibition of transpeptidation — but the drug-target interaction is optimized. This directly expresses the clinical anchor: altered PBPs confer resistance, and newer agents overcome this by enhanced PBP-2a binding (KD Tripathi 9e Ch 53; Murray 9e). ## Why each distractor is wrong - **β-lactamase production is upregulated in MRSA, and ceftaroline is resistant to β-lactamase degradation**: β-lactamase is a separate resistance mechanism (not the primary mechanism of MRSA resistance). MRSA resistance is PBP-mediated, not β-lactamase-mediated. While ceftaroline is stable to β-lactamase, this is not the reason for its activity against MRSA. - **PBP-2a undergoes enzymatic inactivation of β-lactams more rapidly than wild-type PBPs, and ceftaroline inhibits this inactivation**: PBPs do not inactivate β-lactams; rather, β-lactams inactivate PBPs by covalent binding. This reverses the mechanism and is biochemically incorrect. - **MRSA produces altered transpeptidases that cannot cross-link peptidoglycan, and ceftaroline restores normal cell wall synthesis**: This is backwards. PBP-2a is still capable of cross-linking (it is a functional transpeptidase), but it has low affinity for β-lactams. β-lactams inhibit transpeptidation; they do not restore it. **High-Yield:** MRSA resistance = altered PBPs with low β-lactam affinity; ceftaroline/ceftobiprole = enhanced PBP-2a binding → overcome MRSA resistance. Separate from β-lactamase resistance. [cite:KD Tripathi 9e Ch 53; Murray 9e]