Erysipelas is most commonly caused by which organism?

• A. Staphylococcus aureus
• B. Streptobacillus moniliformis
• C. Streptococcus agalactiae
• D. Streptococcus pyogenes

Answer: (D)

Erysipelas is a superficial skin infection of the upper dermis and lymphatics, and the most common bacterial cause is Streptococcus pyogenes, a Group A Streptococcus. This organism tends to invade the superficial skin layers and lymphatic channels, producing a sharply defined, raised, erythematous plaque often with warmth and fever. Staphylococcus aureus can cause skin infections too, but erysipelas classically points to GAS because of its propensity for upper dermal involvement and rapid, well-demarcated borders. The other organisms listed are associated with different clinical syndromes—rat-bite fever with Streptobacillus moniliformis, and neonatal or invasive infections with Streptococcus agalactiae—making Streptococcus pyogenes the best fit for erysipelas.