Staphylococcus aureus

Microbiology

• Facultative anaerobic, Gram-positive coccus
• Catalase and coagulase positive
• Microscopy: groups or clusters ("cluster of grapes")
• Colony morphology: large, round, golden yellow colonies, often hemolytic on blood agar

Pathophysiology

Virulence factors

• Surface proteins for fibrinogen and other substances (used for binding to host endothelial cells)
• Membrane-damaging (hemolytic) toxins
• Exfoliative toxins (A and B) which disrupt the junction between epidermal cells
• Superantigens that bind to MHC molecules and T-cell receptors, leading to release of huge amounts of cytokines

Clinical Presentation

Skin and soft tissue infections

• Boils and carbuncles

Scalded-skin syndrome

• Aka Ritter disease
• Sunburn-like, bullous rash that can lead to skin loss (most common in kids with Staph infections of the nasopharynx or skin)

Bacteremia

• See [Staphylococcus aureus bacteremia](Staphylococcus aureus bacteremia.md)

Infective endocarditis

• Usually more acute presentation
• High mortality

Myriad other infections

• Osteomyelitis
• Native and prosthetic joint infections
• Pyomyositis
• Deep organ abscesses
• Surgical site infections
• Pneumonia, especially ventilator-associated or influenza-related

Superantigen-related syndromes

• Food poisoning
• Staphylococcal toxic-shock syndrome

Management

Methicillin-susceptible Staphylococcus aureus (MSSA)

• First-line: cloxacillin, then cefazolin or cephalexin
• Others: clindamycin, fluoroquinolones, Septra, doxyxycline, erythromycin

Methicillin-resistant Staphylococcus aureus (MRSA)

• First-line: vancomycin
• Others: linezolid (if lungs) and daptomycin (if blood), but also consider Septra, ciprofloxacin, doxycycline, and clindamycin