Cellulitis is a bacterial infection of the skin. It’s more superficial than deeper infections which are abscesses, although there’s a range of depths and terms may overlap. The main bacteria involved are Streptococcus pyogenes (“Strep,” same species that causes Strep Throat), and Staphylococcus aureus (“Staph“). Special cases might involve rarer germs, like infections from bites, from plants, those acquired in lakes or oceans, from bedsores in the very debilitated, etc.
Symptoms are primarily pain at the site of infection. Diagnosis is made by simply observing a painful red patch of skin that feels warm to touch, and hurts when touched. An older term Erysipelas refers to a rapidly-spreading cellulitis, which is almost always due to Strep, as is “lymphangitis,” a red streak extending from the infection up an arm or leg. If pus is visible, or an abscess appears present, it’s most likely Staph. Distinguishing between the two is important, because the antibiotics which can treat the one bacteria usually don’t work on the other.
The danger of these infections is if they spread from skin to blood. That can cause sepsis, which can be fatal. The complication is very uncommon, but we’d worry more about the possibility if fever is developing. If pain seems disproportionate to what we see on exam, we’d worry about rare deeper complications like gas gangrene, bone or joint infections, and infected veins among IV drug injectors.
Misdiagnoses are common. The one I often note is that patients think they just have an insect bite; the main difference is that bites itch, infections hurt. Clinicians often mistake cellulitis for an allergy (allergies itch, not hurt). One patient complained of a painful “bruise” on his foot; there was a dark spot tender to touch. But bruises don’t usually hurt after the fact. Also, if you press on a bruise, the color doesn’t disappear for a split second & then return (called “blanching”); the color simply remains. This man’s dark spot turned white for a second when I pressed (& pressing hurt!). It was cellulitis; he began to get better after 1 day of antibiotics.