Multicenter Open-label Randomized Controlled Trial (RCT) to Compare Colistin Alone Versus Colistin Plus Meropenem

Inclusion Criteria: * Adult inpatients * Clinically significant, microbiological-documented infection caused by carbapenem-resistant and colistin-susceptible Gram-negative bacteria and identified according to CDC criteria- blood stream infections, hospital acquired pneumonia, ventilator associated pneumonia, and urinary tract infections * Patient recruitment will occur only after microbiological documentation and susceptibility testing. Patients will be included within 96 hours of the time the index culture was taken (typically within 48 hours of isolate identification), regardless of the antibiotic treatment administered during this time period. Exclusion Criteria: * Previous inclusion in the trial. Patients will be included in the RCT only once for the first identified episode of infection * Pregnant women * Epilepsy or prior seizures * Known allergy to colistin or a carbapenem