ID Board Review/Bacterial Diseases/Gram-positive cocci (GPC)/Staphylococcus aureus

Staphylococcus aureus
S. aureus
Staphylococcus aureus is Gram Stain positive (above), and β-hemolytic on blood agar (below)
Microlab Descriptions
Preferred Media/Setting	Readily grows well on animal blood agar (aerobic or anaerobic conditions). Isolates on 7.5-10% NaCl with mannitol.
Histology	GPC, "cluster of grapes"
Species Identification	β-hemolytic Catalase (+) Coagulase (+) Oxidase (-) PYR (-)
Normal Environment	Skin flora
Associated topics
MRSA

Clinical Presentations

• Skin Infections (most common presentations of S. aureus)
  • Cellulitis
  • Impetigo
  • Folliculitis
  • Mastitis
  • Infection Surgical Wounds
• Infective Endocarditis, especially patients with left-sided valvular heart disease and patients with IV drug use
• S. aureus is often a cause of Osteomyelitis, and is the most common cause of Septic Arthritis
• S. aureus is the most common cause of Spinal Epidural Abscesses and Suppurative Intracranial Phlebitis
  • Rarely causes meningitis, but when it does, it's usually after trauma, or Neurosurgery
• S. aureus rarely causes community pneumonia, but is often the cause of Nosocomial (Hospital Acquired) Pneumonia
• Septic Shock
• Prosthetic Joint Infection

Risk Factors

• Skin/mucosal breaks or tears
• Foreign body/implant presence
• Prior viral illness (especially superimposed bacterial pneumonia)
• Defects in cellular or humoral immunity

Diagnosis

• Bacteremia: Blood culture; low threshold for getting infective endocarditis blood cultures
  • Sensitivities to differentiate between MSSA and MRSA
• MRSA Nares Swab PCR (to r/o MRSA pneumonias)

Differential Diagnoses

• For skin infections without pus, consider Streptococcus

Resistance Mechanism(s)

• Mec A gene → MRSA