Polymyxin-Resistant Clinical Isolates of Escherichia coli

A surveillance study to identify patients from the community with Escherichia coli resistant to broad-spectrum cephalosporins discovered two isolates that were also resistant to polymyxin B and colistin. One isolate from a patient in the community and a second from a patient who received multiple courses of polymyxin B also possessed a CTX-M-15 enzyme. Resistance to cationic peptides in E. coli is unusual, and testing for susceptibility to these agents should be performed.