Oral Microbiota Dominance in Allogeneic Hematopoietic Stem Cell Transplantation: Dynamics, Predictors, and Clinical Implications in a Multicenter Study

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important therapeutic strategy for hematological malignancies and other diseases but remains associated with substantial morbidity and mortality. Although microbial dominance has been extensively investigated in the gut microbiota, its dynamics and clinical implications in the oral microbiota remain poorly understood. In this prospective multicenter study, longitudinal oral mucosal samples were collected from pre-conditioning to six months post-transplantation in Brazilian patients undergoing allo-HSCT, and microbiota composition was assessed by 16S rRNA gene sequencing. Associations between oral dominance and death within D+180, disease relapse, and acute graft-versus-host disease (aGvHD) were evaluated using competing-risk methods, while predictors of oral dominance were assessed using mixed-effects logistic regression. Among 90 patients, 244 oral samples were analyzed. Alpha diversity significantly decreased during the first 30 days post-allo-HSCT compared with healthy controls. Oral dominance occurred in 87.8% of patients, with Streptococcus, Veillonella, and Rothia being the most frequent dominant genera. Streptococcus dominance was associated with a higher cumulative incidence of moderate-to-severe aGvHD, whereas Rothia dominance was associated with a higher cumulative incidence of disease relapse. These findings demonstrate substantial oral microbiota disruption during allo-HSCT and suggest that specific taxonomic patterns may be associated with clinically relevant transplant outcomes.