Effect of Rifaximin on Gut Bacterial Flora Post Stem Cell Transplant in Patients With Acute Leukemia

* Goal: This study is a randomized phase II interventional study. The purpose of this study is to see if addition of oral rifaximin tablets during allogeneic stem cell transplant can improve the quality of gut microbiome and reduce chances of death, infections and graft versus host disease (GVHD) post-transplant. * The study objectives are as follows: * Primary Objective: To determine the impact of rifaximin on gut microbial diversity and compare it with controls. * Secondary Objectives: a. To determine non-relapse mortality at 1-year post transplant in patients who receive peri-transplant transplant rifaximin and compare it with controls. * b. To compare the incidence of severe GVHD in patients who receive peri-transplant rifaximin with the controls. * c. To determine impact of gut decontamination with rifaximin on incidence of MDR sepsis and usage of higher antibiotics (e.g. Carbapenems, colistin, tigecycline, ceftazidime avibactum and ceftriaxone-sulbactam EDTA) in first 6 months post BMT. * d. To determine the impact of rifaximin induced gut manipulation on immune reconstitution, T cell repertoire post-transplant and cytokine profile. * Exploratory objective: To use single cell transcriptomics (SCT) to identify immune cell profile in gut biopsies post allogeneic stem cell transplant whenever biopsy is done, to correlate the impact of microbiome on gut immunity. * Intervention: Tab Rifaximin 200 mg will be given orally twice daily from day -8 to day +60 of allogeneic stem cell transplant in acute leukemia patients. This will be in addition to standard of care post-transplant treatment. * Comparator Agent: Standard of care treatment including standard anti GVHD measures, antibiotic support and transfusions as needed.