Cellulitis

Cellulitis is an acute, spreading bacterial infection of the dermis and subcutaneous tissue, most commonly caused by beta-hemolytic streptococci (Group A Strep, S. pyogenes) and Staphylococcus aureus (including MRSA). Bacteria enter through breaks in the skin barrier (trauma, surgical wounds, tinea pedis, insect bites, venous stasis ulcers) and spread laterally through tissue planes via hyaluronidase and other enzymes that degrade extracellular matrix. The infection triggers an intense inflammatory response with neutrophil infiltration, vasodilation, and increased capillary permeability, producing the cardinal signs of erythema, warmth, swelling, and tenderness. Unlike erysipelas (which involves the upper dermis and has sharply demarcated, raised borders), cellulitis extends deeper into subcutaneous tissue and has indistinct, non-elevated borders. Purulent cellulitis (with abscess or drainage) is more likely MRSA; non-purulent cellulitis is more likely streptococcal.