## mecA Gene and MRSA Resistance Mechanism **Key Point:** The mecA gene encodes an altered penicillin-binding protein (PBP2a) that has dramatically reduced affinity for β-lactam antibiotics, allowing continued cell wall synthesis even in the presence of β-lactams. ### Molecular Basis of mecA-Mediated Resistance 1. **Normal PBPs (PBP1, PBP2, PBP3):** Bind β-lactams with high affinity → inhibition of transpeptidation → cell wall disruption 2. **PBP2a (encoded by mecA):** - Has altered active site geometry - Binds β-lactams with very low affinity (Km ~1000× higher) - Maintains transpeptidase activity despite β-lactam presence - Allows continued peptidoglycan cross-linking - Result: **Resistance to all β-lactams including methicillin, oxacillin, nafcillin** ### Comparison of S. aureus Resistance Mechanisms | Mechanism | Gene(s) | Antibiotic Class | Resistance Pattern | |---|---|---|---| | **PBP2a alteration** | **mecA** | **β-lactams** | **Broad (MRSA phenotype)** | | β-lactamase production | blaZ | β-lactams (not β-lactamase inhibitors) | Penicillin, ampicillin only | | Ribosomal methylation | ermC, ermA | Macrolides, lincosamides | MLSB resistance | | Efflux pumps | norA, norB | Fluoroquinolones | Reduced fluoroquinolone susceptibility | | Vancomycin resistance | vanA, vanB | Glycopeptides | Rare in S. aureus | **High-Yield:** MRSA is defined by mecA presence (genotypic) or oxacillin/cefoxitin resistance (phenotypic). PBP2a is the sole mechanism — not β-lactamase. **Mnemonic:** **mecA = Modified Enzyme Creates Antibiotic resistance** — the altered PBP2a is the "modified enzyme." **Clinical Pearl:** Vancomycin or linezolid are used for MRSA infections because they bypass the PBP2a mechanism entirely — vancomycin inhibits earlier steps in peptidoglycan synthesis (D-Ala-D-Ala), and linezolid inhibits protein synthesis.
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