Cellulitis

Cellulitis is a diffuse bacterial infection of the epidermis, dermis, and superficial subcutaneous tissue. Bacteria usually enter through a break in the skin—such as a fissure, ulcer, wound, insect bite, or injection site—and spread through the surrounding tissue. The inflammatory response explains the bedside picture. Increased blood flow produces erythema and warmth; capillary leakage produces swelling; inflammatory mediators stimulate pain and tenderness. Because cellulitis spreads through tissue rather than forming a contained pocket, the redness is usually diffuse and may enlarge beyond its original boundary. The most common pathogens are *Streptococcus pyogenes*, other beta-haemolytic streptococci, and *Staphylococcus aureus*, including MRSA. Diabetes, chronic oedema, poor perfusion, immunosuppression, and a previous MRSA infection can make infection more likely or more difficult to control. An abscess is different: it is a localized collection of pus that may feel fluctuant or produce purulent drainage. Antibiotics alone may not address a walled-off collection, which is why recognizing a possible abscess changes the diagnostic and treatment plan.