Why Skin Abscesses Yield Only Sparse Growth of Staphylococcus epidermidis
When a skin abscess culture grows only a few colonies of Staphylococcus epidermidis, this most likely represents contamination from normal skin flora rather than the true pathogen causing the abscess.
The Clinical Reality
Skin abscesses are predominantly caused by Staphylococcus aureus (not S. epidermidis), with S. aureus and Streptococcus pyogenes being the primary pyogenic bacteria responsible for these infections 1. The IDSA guidelines explicitly state that furuncles, carbuncles, and abscesses are "usually caused by S. aureus" 1.
Why S. epidermidis Appears in Low Numbers
Contamination vs. True Pathogen
S. epidermidis is one of the most abundant colonizers of healthy human skin 2, 3. When only sparse growth occurs from an abscess culture, this pattern indicates:
- Surface contamination during specimen collection - The organism was picked up from normal skin flora during the incision and drainage procedure
- Not the causative pathogen - True pathogens causing purulent infections typically grow in high colony counts
- Commensal carryover - The specimen collection inadvertently sampled the skin surface rather than pure abscess contents
S. epidermidis as an "Accidental" Pathogen
S. epidermidis has evolved NOT to cause aggressive tissue infections 4. This organism:
- Lacks aggressive virulence determinants needed for abscess formation
- Functions primarily as a commensal maintaining skin homeostasis 3
- Causes disease mainly in the context of foreign body/implant-associated infections where biofilm formation is key 5, 6, 7
- Requires implanted devices or severe immunocompromise to establish significant infection 6
Clinical Interpretation Algorithm
When you see sparse S. epidermidis growth from an abscess:
Disregard it as a contaminant - The IDSA guidelines specifically state that cultures of inflamed epidermoid cysts (which can mimic abscesses) are not recommended precisely because S. epidermidis contamination confuses interpretation 1
Treat empirically for S. aureus - The true pathogen is almost certainly S. aureus, which may have been:
- Overgrown by faster-growing contaminants
- Present in the specimen but not adequately sampled
- Killed by the host immune response while S. epidermidis persisted as a bystander
Consider MRSA coverage based on clinical factors - If the patient has SIRS, immunocompromise, or other risk factors 1
Important Caveats
The only scenario where S. epidermidis should be considered a true abscess pathogen:
- Presence of an implanted foreign body (catheter, prosthetic device) 6, 7
- Severe immunocompromise (neutropenia, chemotherapy) 8
- Recurrent infections at the same site with consistent isolation 1
Even in these cases, S. epidermidis typically requires biofilm formation on foreign material to cause persistent infection 5, 7. In native tissue without implants, sparse growth represents contamination until proven otherwise.
Bottom line: Sparse S. epidermidis growth from a skin abscess = contamination. Treat for S. aureus as the actual pathogen 1.