Oral Pooled Fecal Microbiotherapy to Prevent Allogeneic Hematopoietic Cell Transplantation Complications (PHOEBUS Trial)

Inclusion Criteria: * Age ≥ 50 years old * Presence of a hematologic malignancy for which an alloHCT is indicated with a reduced toxicity or reduced intensity conditioning regimen * Patients with polynuclear neutrophils \> 0.5 G/L * Patients having received wide spectrum antibiotics within the last 90 days prior to inclusion * Karnofsky index ≥ 70% * Availability of a sibling donor, an unrelated stem-cell donor or a familial haploidentical donor * Written informed consent Exclusion Criteria: * Patients planned to receive a non-myeloablative conditioning regimen (2 Gray total body irradiation (TBI) +/- purine analog, fludarabine + cyclophosphamide or equivalent) * Patients planned to receive a conventional myeloablative conditioning regimen (e.g. high dose cyclophosphamide and high dose TBI (≥10Gy); high dose busulfan (12.8 mg/kg IV) + high dose cyclophosphamide) * Patients receiving a manipulated graft (in-vitro T-cell depletion) * Patients planned to receive a conditioning regimen with alemtuzumab * Patients planned to receive alloHCT with cord blood cells * Patients planned to receive alloHCT from unrelated donor with \>= 3/10 HLA-mismatches * Patients receiving a large spectrum antibiotic at time of randomization * Patients planned to receive vedolizumab or abatacept for GvHD prophylaxis * Creatinine clearance \<30 mL/min * Bilirubin or amino-transferases abnormalities contra-indicating alloHCT * Cardiac ejection fraction less than 40% * Pulmonary impairment with \<50% lung carbon monoxide diffusing capacity (DLCO) * Pregnancy * Confirmed or suspected intestinal ischemia * Confirmed or suspected toxic megacolon or gastrointestinal perforation * Any history of gastro-intestinal surgery in the past 3 months * Any history of chronic digestive disease (Crohn's disease, ulcerative colitis, inflammatory bowel disease or other relevant digestive condition according to physician's judgement) * Known allergy or intolerance to trehalose or maltodextrin * Patients with EBV-IgG negative serology * Any condition that would, in the investigator's judgment, interfere with full participation in the study, including administration of study drug and attending required study visits; pose a significant risk to the subject; or interfere with interpretation of study data. * Vulnerable patients such as: persons deprived of liberty, persons in Intensive Care Unit unable to provide informed consent prior to the intervention.