Forest plot of PFS subgroup analysis. (A) Univariable analysis showed the dose of melphalan conditioning chemotherapy predicted PFS; age, sex, transplant as part of frontline therapy, melphalan dose, nonprophylactic antibiotic exposure (n = 83); race (n = 81); Durie-Salmon stage (n = 76); and high-risk cytogenetics (n =74). (B) On multivariable analyses, the dose of melphalan again predicted PFS and preintervention Faecalibacterium abundance remained prognostic; all variables (n = 79). HR and 95% CI were calculated via Cox regression analysis.