Cellulitis

Cellulitis is an acute bacterial infection involving the epidermis, dermis, and superficial subcutaneous tissue. Bacteria usually enter through a break in the skin barrier: a fissure between the toes, tinea pedis, eczema, an ulcer, insect bite, surgical wound, injection site, or minor trauma that the patient may not remember. The most common organisms are *Streptococcus pyogenes*, other beta-haemolytic streptococci, and *Staphylococcus aureus*, including methicillin-resistant *S. aureus* (MRSA). Once organisms enter tissue, the local immune response produces vasodilation and increased capillary permeability. That is why the area becomes red, warm, swollen, and painful: more blood reaches the site, and fluid and inflammatory cells move out of the vessels into the tissue. The typical pattern is diffuse erythema with indistinct, non-raised borders. Erysipelas is more superficial and usually has a bright-red, sharply demarcated, raised edge. An abscess is different again: it is a localized collection of pus, often with fluctuance, a central pustule, drainage, or focal tenderness. Antibiotics may not clear a walled-off collection without drainage. Non-purulent cellulitis is more often streptococcal. Purulence, an abscess, or purulent drainage raises concern for *S....