Patient description: The patient is a 55 year old man visiting his primary care physician for lower urinary tract symptoms including frequency, urgency, weak stream, incomplete emptying and intermittent flow for the past 9 months. Further evaluation revealed: IPSS score : 15 Post-void residual: 70 mL Prostate volume (TRUS): 60 mL Prostate-specific antigen (PSA) level: 3.2 ng/mL 10 mL/sec of maximum flow rate when urine volume was 130 mL He is otherwise healthy only using Vit D 1000 units daily. His recent blood chemistry (3 days ago) was normal: Hgb: 13.5 g/dl WBC: 135000 /mm3 Plt: 350000 /ml PT: 11 second PTT: 35 second INR: 0.9 Creatinine: 0.5 mg/dl BUN: 10 mg/dl U/A: Color: yellow Appearance: cloudy PH: 5.3 Specific gravity: 1.010 Glc: 100 Nitrite: negative Ketone: none Leukocyte esterase: negative RBC: negative WBC: 2 WBCs/hpf U/C: negative

Clinical trial eligibility criteria: eligible ages (years): 18.0-95.0,  Pneumonia Blood Stream Infection Hospitalized Adults (> 18 years to 95 years of age), at one of the study sites Diagnosis of BSI and/or pneumonia due to a preliminary result of gram-negative non-lactose fermenter that is oxidase negative; or a final results of XDR-A. baumannii; carbapenem-resistant Enterobacteriaciae; or XDR P. aeruginosa and/or patients with suspected BSI and/or HAP and who have had a prior history (within last 6 months) of XDR-GNB that was susceptible to colistin. o If final results do not indicate that the pathogen is an XDR-GNB, and identifies alternative treatment options, the patient would be eligible for the study if the subject is allergic to all the alternative treatment options Patients with polymicrobial respiratory or blood infections, including XDR-GNB and one or more pathogens, will be included in the study, as long as the XDR-GNB is determined to be a true pathogen (AB, CRE or PA). Other pathogens will be treated with antimicrobial agents as determined by the treating physician If more than one XDR-GNB study pathogens is identified as a study pathogen causing BSI and/or pneumonia, then the first study pathogen recovered will be considered as the primary study pathogen. If more than one study pathogen is recovered from the same culture, then the infection will be categorized as being caused by multiple study pathogens Patients with a life expectancy of > 24 hours Signed written informed consent and HIPAA Authorization form (US sites)  Female patients who are pregnant Female patients who are nursing Patients who are prisoners Patients who are less than 18 years of age or greater than or equal to 96 years of age Patients with neutropenia (WBC < 500 cells/mm3) The presence of any of the following known clinical syndromes involving XDR-GNB as a pathogen which necessitate durations of antimicrobial therapies greater than 14 days: endocarditis, osteomyelitis, prosthetic joint infections, meningitis and/or other central nervous system infections Patients receiving valproic acid (with or without a known seizure disorder) Patients who received 72 hours or more of polymyxin treatment (intravenous or inhaled [pneumonia]) within 96 hours of enrollment Patients who have end-stage renal disease requiring hemodialysis, will be excluded from evaluation pertaining to nephrotoxicity in the per protocol population Patients with known Type 1 or other severe drug allergy to either of the study drugs or to β-lactams