What is etiology of cellulitis?

• 80% of cases are caused by gram positive organisms • Group A streptococcus is most common; other strep less so • Staphylococcus aureus is less common but occurs with open wound or penetrating trauma as with needle injection with drug abuse • Think of other organisms if there have been unusual exposures or conditions: Pasteurella multocida (animal bites) • Eikenella corrodens (human bites) • MRSA (with concurrent MRSA elsewhere/illicit drug use/purulent drainage)