Altogether, these data suggested that introduction of B. ovatus into meropenem-treated allo-HSCT mice resulted in a carbohydrate-enriched intestinal environment in the colonic lumen by degrading dietary-derived polysaccharides such as xylose-comprising polysaccharides, leading to inhibition of B. theta mucin utilization, ultimately resulting in amelioration of disrupted microbiota-induced severe GVHD. The gene discussed is MUC5AC; the disease is graft versus host disease.