Cellulitis

• Localized pain
• Erythema (redness)
• Swelling
• Heat

Etiology

1. Indigenous (endogenous) flora colonizing the skin and appendages — most commonly Staphylococcus aureus and Streptococcus pyogenes
2. Exogenous bacteria — a wide variety, depending on environmental exposure and epidemiological context

Diagnostic Challenges

• When drainage, an open wound, or a clear portal of entry is present, Gram stain and culture can provide a definitive diagnosis.
• In the absence of these findings, identifying the specific bacterial etiology is difficult.
• Even with needle aspiration of the leading edge or a punch biopsy of the cellulitis tissue, cultures are positive in only ~20% of cases — suggesting that relatively low numbers of bacteria are present in the tissue.