Gut Decontamination In Pediatric Allogeneic Hematopoietic

Inclusion Criteria: * Eligibility Criteria for Patients Undergoing Allogeneic HSCT * Recipient of 9/10 or 10/10 (HLA-A, -B, -C, -DRB1, -DQB1) matched bone marrow allogeneic hematopoietic stem cell transplantation (HSCT) OR 4/6, 5/6 and 6/6 (HLA-A, -B, -DR) matched cord blood allogeneic HSCT. * Participants may have underlying malignant or non-malignant hematologic disease, except for primary immunodeficiency, as the indication for their allogeneic HSCT. Patients with immune dysregulation such as familial or secondary hemophagocytic lymphohistiocytosis (HLH) are eligible. * Participants must may receive either a myeloablative or non-myeloablative(reduced-intensity) conditioning regimen. Anti-thymocyte globulin (ATG) in the conditioning regimen is permitted. * Graft-versus-host disease (GVHD) prophylaxis with any of the following agents: calcineurin inhibitor, and short-course methotrexate, with or without steroids, mycophenolate mofetil, and sirolimus. * Age ≥ 4 years old and toilet-trained. Participants must be able to deposit stool samples directly into stool collection containers. Stool specimens from diapers are difficult to obtain and are prone to more sampling error, particularly for loose or liquid stools which are common in the peri-transplant period. * Lansky/Karnofsky performance status ≥60% (see Appendix A) * Ability to understand and/or the willingness of their parent or legally authorized representative to sign a written informed consent document * Eligibility Criteria for Healthy Bone Marrow Donors * Healthy individuals, ages ≥ 4 years and toilet-trained, who have been identified by BCH or DFCI providers as 9/10 or 10/10 (HLA-A, -B, -C, -DRB1, -DQB1 matched bone marrow donors for transplantation will also be eligible to participate in this study. Exclusion Criteria: * Patients undergoing allogeneic HSCT for correction of a primary immunodeficiency disorder (e.g. SCID). * Patients with age ≤ 10 years undergoing HSCT with a matched sibling donor. These patients are at very low risk of acute GVHD and do not receive gut decontamination per our institutional standard practice. * Participants receiving GVHD prophylaxis with drugs other than calcineurin inhibitors, methotrexate or steroids.agents listed above (e.g. abatacept). * History of allergic reactions attributed to oral vancomycin or oral polymyxin B. * Participants undergoing active therapy for immune-mediated or infectious colitis upon admission for allogeneic HSCT. * Participants receiving antibiotic therapy for treatment of a bacterial infection or bacterial prophylaxis upon admission for allogeneic HSCT. Use of any agent (e.g. sulfamethoxazole/trimethoprim) for prophylaxis of Pneumocystis jirovecii pneumonia is permitted. Concurrent use of anti-fungal and anti-viral therapies is also permitted. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection or psychiatric illness/social situations that would limit compliance with study requirements.