MRSA
One-Sentence Definition
MRSA (methicillin-resistant Staphylococcus aureus) denotes S. aureus isolates resistant to nearly all β-lactams via acquired PBP2a (encoded by mecA or mecC), typically carried on the SCCmec genomic island.
Simple Explanation
The staph rewires its cell-wall-building enzyme so methicillin/oxacillin (and most related β-lactams) no longer bind — a genetic cassette makes ordinary penicillins useless.
Detailed Scientific Explanation
| Element | Role |
|---|---|
| mecA / mecC | Encodes PBP2a / PBP2c with low β-lactam affinity |
| SCCmec | Resistance island integrating at orfX |
| mecR1 / mecI | Regulation of mecA induction |
| HA-MRSA vs CA-MRSA | Hospital clones (often MDR) vs community (often PVL+, narrower resistance) |
Phenotypic detection: cefoxitin screen, oxacillin MIC, PBP2a immunoassays; confirm with mec PCR when discrepant. Borderline oxacillin-resistant S. aureus (BORSA) lacks mecA — different mechanism.
Mechanism
SCCmec acquisition → PBP2a takes over transpeptidation in presence of β-lactams → cell wall synthesis continues → methicillin/oxacillin/nafcillin and most cephalosporins fail (ceftaroline/ceftobiprole are anti-MRSA cephalosporin exceptions).
Clinical Importance
- Empiric therapy for severe staphylococcal disease must consider local MRSA rates.
- Infection control: contact precautions, decolonization protocols in outbreaks.
- Distinguishes MSSA (anti-staph β-lactams preferred) from MRSA.
Research Importance
- SCCmec typing, CC5/CC8/CC30 genomics, vaccine and anti-virulence research.
- Model for Genomic Islands and Horizontal Gene Transfer in Gram-positives.
Diagnostic Relevance
- Rapid mecA/PBP2a assays guide therapy hours before AST finalization.
- WGS for outbreak typing of MRSA clones (Population Structure and Clustering).
AMR Relevance
Flagship Gram-positive AMR. Often co-resists macrolides, fluoroquinolones, sometimes vancomycin-intermediate (VISA) phenotypes via cell-wall thickening — distinct from mec.
Related Organisms
Related Methods
Related MOCs
Learning Aids
Clinical Example
Example
Case: Purulent cellulitis; wound grows S. aureus; cefoxitin screen resistant; mecA PCR positive.
Question: Is oxacillin an option?
Answer: No — MRSA; use anti-MRSA agents (vancomycin, daptomycin, linezolid, TMP-SMX, doxycycline, ceftaroline — site-dependent).
Active Recall Questions
- What protein does mecA encode?
- HA-MRSA vs CA-MRSA — typical clinical/genetic contrast?
- Why can cefoxitin be a better screen than oxacillin disks?