Choosing the Best Antibiotic to Protect Friendly Gut Bacteria During the Course of Stem Cell Transplant

Inclusion Criteria: * Age ≥ 18 years * Patients with any hematologic malignancy undergoing Exclusion Criteria: * Patients with severe allergies to piperacillin-tazobactam, cefepime, aztreonam or vancomycin. Severe reactions include anaphylaxis and Stevens-Johnson syndrome (SJS)/toxic epidermal necrolysis (TEN). * Patients with unconfirmed allergies to piperacillin-tazobactam, cefepime, aztreonam or vancomycin can be evaluated by an Allergy/Immunology specialist, after which they may become eligible by a consensus of the treating physician, trial investigator and and the Allery * Prolonged antibiotic treatment ( ≥10 days, within 3 weeks of enrollment) as prevention or suppression of an ongoing infection, where treatment involves gut-perturbing anti anaerobic antibiotics * Patients with history of infection with extended-spectrum beta-lactamase producing organism. Patients known to be colonized with multi-drug resistant gram-negative organisms or with history of infection with multi-drug resistant organisms will be evaluated case by case and discussed with infectious disease specialist before enrollment * Febrile patients * Patients with estimated glomerular filtration rate (eGFR) \< 30 mL/min/1.73m\^2