Staphylococcus aureus

MICROBIOLOGY

• Gram-positive cocci, usually seen in clusters.
• Easily grown on blood agar or other conventional media.
  • Coagulase positive and thermonuclease positive.
• Antimicrobial resistance: rare isolates remain penicillin susceptible. In many areas, MRSA > MSSA.
  
  • Penicillin resistance (MSSA) conferred by penicillinase production which can be overcome by the addition of a beta-lactamase inhibitor (e.g., amoxicillin/clavulanate, ampicillin/sulbactam) or use of a penicillinase-resistant penicillin (e.g., oxacillin, nafcillin).
  • Methicillin resistance (MRSA) conferred by presence of mecA gene that encodes penicillin binding protein 2a, an enzyme that has low affinity for beta-lactams and thus leads to resistance to methicillin, oxacillin, nafcillin, and cephalosporins.
  • Community-acquired MRSA (CA-MRSA) isolates (strains include USA 300 as most common in US, also USA 500, USA1000): often maintain susceptibility to tetracyclines (tetracycline, doxycycline, minocycline, tigecycline) and TMP/SMX.
    • Clindamycin susceptibilities vary geographically. If isolate is erythromycin resistant, must confirm clindamycin susceptibility with D-test.