Preonset GVHD clostridia abundance, butyrate producers, and S/F anaerobe ratio predicts survival in allo-HCT patients. OS (A) and GVHD-related mortality (B). Patients were stratified according to the median relative abundances of class Clostridia, butyrate producers, and S/F anaerobe ratio prior to GVHD onset. Figure shows univariable (UVA) and multivariable (MVA) tests of associations for microbial determinants as binary or continuous variables. The proportional hazards multivariable regression models (OS) and the Fine and Gray multivariable regression models (GVHD-related mortality) were adjusted for sex, age, and conditioning regimen. Data corroborates the association of higher relative abundance of butyrate-producing bacteria and S/F ratio to increased OS and reduced GVHD-related mortality.