The majority of cellulitis infections are caused by either strep (Streptococcus) or staph (Staphylococcus) bacteria.

The most common bacteria that cause cellulitis are beta-hemolytic streptococci (groups A, B, C, G, and F). A form of rather superficial cellulitis caused by strep is called erysipelas and is characterized by spreading hot, bright red circumscribed area on the skin with a sharp, raised border. Erysipelas is common in young children. The so-called "flesh-eating bacteria" are, in fact, also a strain of strep that can sometimes rapidly destroy tissues underneath the skin.

Staph (Staphylococcus aureus), including methicillin-resistant strains (MRSA), is another common type of bacteria that causes cellulitis. There is a growing incidence of community-acquired infections due to methicillin-resistant S. aureus (MRSA), a particularly dangerous form of this bacteria that is resistant to many antibiotics and is more difficult to treat.

Cellulitis can be caused by many other types of bacteria. In children under 6 years of age, H. flu (Hemophilus influenzae) bacteria can cause cellulitis, especially on the face, arms, and upper torso. Cellulitis from a dog or cat bite or scratch may be caused by the Pasteurella multocida bacteria, which has a very short incubation period of only four to 24 hours. Aeromonas hydrophilia, Vibrio vulnificus, and other bacteria are causes of cellulitis that develops after exposure to freshwater or seawater. Pseudomonas aeruginosa is another type of bacteria that can cause cellulitis, typically after a puncture wound.

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