Ecthyma gangrenosum is an infection of the skin typically caused by Pseudomonas aeruginosa. It is often seen in immunocompromised patients such as those with neutropenia. Ecthyma gangrenosum presents as a round or oval lesion, 1 cm to 15 cm in diameter, with a halo of erythema. A necrotic center is usually present with a surrounding erythematous edge, representing where the organism invaded blood vessels and caused infarctions. These ulceritic lesions are single or multiple and heal with scar formation, although sepsis resulting from other gram negative bacteria can also cause this condition.
The lesion is caused by perivascular invasion of bacteria in the dermis and subcutaneous tissue. The mechanism of tissue destruction is Pseudomonas exotoxin A, a toxin similar to Corynebacterium diphtheriae toxin that causes inactivation of elongation factor 2 thus inhibition of protein synthesis, elastase, an enzyme which degrades elastin in blood vessels wall, phospholipase C, which degrades phospholipids in cell membranes and pyocyanin which generates reactive oxygen species.