Morphology: Gram-positive cocci in clusters, catalase-positive, coagulase-positive.
MSSAGramPosOther
Typical drugs
- #1Cefazolin— Drug of choice for MSSA bacteremia.
- #2Ampicillin-sulbactam— When mixed flora suspected.
Empiric therapy when resistant
MSSA: cefazolin 2g IV q8h or nafcillin 2g IV q4h. Avoid vancomycin — inferior to β-lactams for MSSA.
Resistance mechanisms
Enzymatic degradation
Beta-lactamase production (blaZ)
Example: PCN resistance — not active against cefazolin or anti-staph penicillins.
Resistance notes
Almost all S. aureus produce β-lactamase (resistant to penicillin G).
Common syndromes
Pearls
Cefazolin > nafcillin in observational data — better tolerated, lower mortality. AVOID vancomycin for MSSA bacteremia if you confirm methicillin-susceptibility.