General Characteristics
- Gram stain: Gram-positive cocci in clusters
- Enzymes: Catalase positive, Coagulase negative, Urease positive
- Biochemical tests:
- Novobiocin: Sensitive
- Mannitol fermentation: Negative (does not ferment mannitol, unlike S. aureus)
Laboratory Differentiation
| Feature | S. epidermidis | S. aureus |
|---|---|---|
| Coagulase | Negative | Positive |
| Novobiocin | Sensitive | Resistant (S. saprophyticus is resistant) |
| Mannitol fermentation | No | Yes |
Clinical Relevance
- Normal flora: Part of skin microbiota
- Common contaminant: Often a cause of false-positive blood cultures
- Pathogenic potential:
- Infects prosthetic devices (hip implants, heart valves)
- Infects IV catheters
- Pathogenesis via biofilm formation (adherence to surfaces, resistance to antibiotics, and host defenses)
Learning Objective (USMLE Step 1)
Understand the distinguishing features of Staphylococcus epidermidis, including its laboratory identification (coagulase-negative, novobiocin-sensitive, mannitol non-fermenter), clinical significance (prosthetic and catheter infections via biofilm formation), and its role as a skin commensal and blood culture contaminant.
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