Cellulitis is a bacterial infection involving the inner layers of the skin. It specifically affects the dermis and subcutaneous fat. Signs and symptoms include an area of redness which increases in size over a couple of days. The borders of the area of redness are generally not sharp and the skin may be swollen.
Cellulitis is caused by a type of bacteria entering the skin, usually by way of a cut, abrasion, or break in the skin. This break does not need to be visible. Group A Streptococcus and Staphylococcus are the most common of these bacteria, which are part of the normal flora of the skin, but normally cause no actual infection while on the skin's outer surface. Dental infections account for about 80% of cases of Ludwig's angina, or cellulitis of the submandibular space. Mixed infections, due to both aerobes and anaerobes, are commonly associated with the cellulitis of Ludwig's angina. Typically, this includes alpha-hemolytic streptococci, staphylococci, and bacteroides groups.
Cellulitis is most often a clinical diagnosis, readily identified in many people by history and physical examination alone, with rapidly spreading areas of cutaneous edema, redness and heat, occasionally associated with inflammation of regional lymph nodes. While classically distinguished as a separate entity from erysipelas by spreading more deeply to involve the subcutaneous tissues, many clinicians may classify erysipelas as cellulitis. Both are often treated similarly, but cellulitis associated with furuncles, carbuncles, or abscesses is usually caused S. aureus.