 The investigators study aims to determine steady-state colistin A and colistin B concentrations in cystic fibrosis and critically ill patients, evaluate pharmacodynamic parameters and relationship to microbiologic success, and monitor for the incidence of nephrotoxicity. The findings of this pilot study will be used to develop dosing recommendations for future pharmacokinetic studies of colistimethate sodium in cystic fibrosis and critically ill patients. 