Ecthyma gangrenosum is caused by:

Ecthyma gangrenosum is a localized cellulitis with necrotizing and ulcerative evolution occurring in immunocompromised or neutropenic patients, mainly caused by Pseudomonas aeruginosa. Two pathogenic mechanisms are recognized; one occurring as the result of the presence of P. aeruginosa during bacteriemia, and the second as a primary lesion. Treatment is systemic antibiotics. Ref: Dermatology by jean bolognia 2nd e p. 1093 ; Life-Threatening Dermatoses and Emergencies in Dermatology edited by Jean Revuz, Jean-Claude Roujeau, Francisco Kerdel, Laurence Valeyrie-Allanore, 2009, Page 126