What causes cellulitis?
Cellulitis is caused by different types of bacteria. For example; if cellulitis develops due to a common household cut, the bacteria responsible is usually either Streptococcus pyogenes or Staphylococcus aureus. If cellulitis develops due to a cat bite, the culprit is usually Pasteurella multocida. The most common infecting organisms are beta-haemolytic Streptococcus pyogenes and Staphylococcus aureus. Less common bacteria include Pseudomonas aeruginosa, particularly following a
puncture wound involving the foot or hand, and Haemophilus influenzae in children with facial cellulitis. There are many other ways in which bacteria can get through the skin, such as dog bites or through surgical wounds.
The lower extremities are the most common site of infection. A skin abnormality (e.g., skin trauma, ulceration, tinea pedis, or dermatitis) often precedes the infection. Scars from saphenous vein removal for cardiac or vascular surgery are common sites for recurrent cellulitis, especially if tinea pedis is present. Frequently, however, no predisposing condition or site of entry is evident. Streptococcus pyogenes is the most common cause of superficial cellulitis with diffuse spread of infection. Staphylococcus aureus occasionally produces a superficial cellulitis typically less extensive that of streptococcal origin and usually only in association with an open wound or cutaneous abscess. Cellulitis occurring after animal bites may be caused by other bacteria, especially Pasteurella multocida from dogs and cats.
Sometimes cellulitis develops even when there is no apparent skin trauma or injury. This is because the cellulitis-causing bacteria can enter through microscopic openings in the skin. People who have poor immune systems, such as people with AIDS, are more prone to cellulitis.