Polymyxin B Monotherapy vs Combination Therapy in Critically Ill Patients With Multi-drug Resistant Pathogens

Inclusion Criteria: * 21 years or older admitted to the Intensive Care Unit of the Puerto Rico Trauma Hospital ° Consent form signed, * Clinical and microbiological evidence of a MDR infection related to HAP, VAP, cUTI or BSI. * The pathogen should be resistant to almost all antibiotics, AND/OR intermediate resistant to some of the antibiotics, AND/OR susceptible only to a class of antibiotic (i.e. aminoglycosides which are NOT recommended as monotherapy), AND/OR the clinician decision is to start the patient on polymyxin B due to severity of the infection. * Patient with a diagnosis of MDR infection, who have not received antibiotics at all; OR if received would be \< 72 hours with polymyxin B or imipenem at/or after the diagnosis of MDR AND/OR at the time of randomization * Have a life expectancy of \> 24 hours according to the attending physician's criteria. Exclusion Criteria: * Pregnant woman * Prisoners * Severe hepatic failure (defined by serum conjugated bilirubin \> 3 mg/dL) * End-stage renal disease requiring hemodialysis * Hypersensitivity to any study drug * Septic shock at the moment of randomization * Died within 48 hours of starting the study