Introduction A 62-year-old woman presents to the emergency room with fevers, abdominal pain, and frequent urination. She has a past medical history of diabetes mellitus, chronic obstructive pulmonary disease, and frequent urinary tract infections. On physical exam, there is suprapubic tenderness and right-sided costovertebral angle tenderness. Urine culture reveals multidrug-resistant Pseudomonas aeruginosa, sensitive only to polymyxins. Introduction Drugs polymyxin B polymyxin E (colistin) Mechanism of action these drugs are cation polypeptides that bind to cell membrane phospholipids that disrupt the cell membrane causing leakage of cell contents Clinical use used as a last-resort therapy for multidrug-resistant gram-negative organisms multidrug-resistant Pseudomonas aeruginosa multidrug-resistant Actinobacter baumannii also used for superficial skin infections as a topical ointment Adverse effects nephrotoxicity neurotoxicity respiratory tract paralysis and failure gastrointestinal upset (C. difficile diarrhea)