Staphylococcus aureus

Background

Microbiology

• Facultative anaerobic, Gram-positive coccus
• Catalase-positive 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 toxins (hemolysins), including Panton-Valentine leukocidin (PVL)
• Exfoliative toxins (A and B) which disrupt the junction between epidermal cells, causing scalded skin syndrome
• Superantigens that bind to MHC molecules and T-cell receptors, leading to release of huge amounts of cytokines
  • Toxic shock syndrome toxin 1 (TSST-1) is implicated in Staphylococcal toxic shock syndrome
  • Exotoxins B and C cause Staphylococcal toxic shock syndrome and also food poisoning

Antibiotic Resistance

• Mechanisms of resistance are myriad
• Common ones:
  • bla gene encodes penicillinase, conferring resistance to penicillin
  • mecA encodes PBP2a, conferring broad β-lactam resistance (MRSA)
  • vanA on a plasmid confers vancomycin resistance (VRSA)

Clinical Manifestations

Colonization

• About 20-30% of people carry it in their nares
• See Staphylococcus aureus decolonization

Skin and Soft Tissue Infections

• Boils and carbuncles
• Cellulitis, usually purulent

Scalded-Skin Syndrome

• Aka Ritter disease
• Caused by exfoliative toxin A or B
• Essentially bullous impetigo
• 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

Infective Endocarditis

• Usually more acute presentation
• High mortality
• See Staphylococcus aureus endocarditis

Other Syndromes

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

Superantigen-Related Syndromes

• Food poisoning
• Staphylococcal toxic shock syndrome
  • Classically tampon-associated
  • Post-surgical (ENT): more of historic with changes in packing
  • Surgical site infections, even without obvious signs of localized infection

Management

• Duration depends on clinical syndrome

Methicillin-Susceptible Staphylococcus aureus (MSSA)

• First-line: cloxacillin, then cefazolin or cephalexin (for oral)
• Others: clindamycin, fluoroquinolones, TMP-SMX, doxyxycline, erythromycin

Methicillin-Resistant Staphylococcus aureus (MRSA)

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