This is a Phase 3 study containing a randomized open-label superiority cohort (Cohort 1) comparing the efficacy and safety of plazomicin with colistin when combined with a second antibiotic (either meropenem or tigecycline) in the treatment of patients with bloodstream infection (BSI), hospital acquired bacterial pneumonia (HABP), or ventilator-associated bacterial pneumonia (VABP) due to CRE. An additional cohort of patients with BSI, HABP, VABP, cUTI, or AP due to CRE, not eligible for inclusion in the other cohort, will be enrolled into a single arm (Cohort 2) and treated with plazomicin-based therapy. Therapeutic drug management (TDM) will be used to help ensure that plazomicin exposures lie within an acceptable range of the target mean steady-state area under the curve (AUC).