Morphology: Gram-positive cocci in clusters, methicillin-resistant.
MRSAGramPosOther
Typical drugs
- #1Vancomycin— First-line for hospitalized MRSA.
- #2Daptomycin— Bacteremia / endocarditis. NOT for pneumonia.
- #3Linezolid— Pneumonia, IV→PO switch.
- #4Doxycycline— Mild SSTI outpatient.
Empiric therapy when resistant
Vancomycin (target AUC 400–600), daptomycin (8–10 mg/kg, NOT for PNA), linezolid, ceftaroline. For mild outpatient SSTI: TMP-SMX or doxycycline.
Resistance mechanisms
Target alteration
PBP2A — alternative penicillin-binding protein with low β-lactam affinity (encoded by mecA)
Example: All β-lactams ineffective except ceftaroline.
Resistance notes
Hospital-acquired MRSA often more resistant than community-acquired. Watch for heteroresistance (hVISA) and increasing vancomycin MIC.
Common syndromes
Pearls
Nasal MRSA PCR has high NPV — useful to rule OUT MRSA in pneumonia and de-escalate empirics.
References
- IDSA MRSA guidelines (2011)