 Colistin is a polymixin antibiotic used in the treatment of multidrug-resistant gram-negative infections. Given the limited use of colistin and previous challenges with laboratory assays to determine plasma concentrations, there is a lack of knowledge of the pharmacokinetic profile of colistin. The purpose of the investigators observational prospective pharmacokinetic cohort study is to examine the steady-state pharmacokinetic and pharmacodynamic properties of intravenous colistimethate sodium in cystic fibrosis and critically ill patients. 