Abstract
In patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT), emerging evidence suggests that the oral microbiota may serve as a predictive biomarker. We conducted a systematic review to provide a critical overview of oral microbiota research in the allo-HSCT setting. We searched PubMed, Embase, and Web of Science from inception to December 2025 to identify studies assessing the oral microbiota in allo-HSCT settings. We included all articles reporting detailed data on the oral microbiota in this context and conducted a qualitative synthesis. Risk of bias was assessed using the JBI critical appraisal tools. From 8160 initially identified records, 35 studies evaluating the oral microbiota in 1964 allo-HSCT patients were included. Of these, 27 studies (77%) assessed temporal oral microbiota dynamics and reported dysbiosis in the early post-transplantation period, followed by gradual recovery. Additionally, 27 studies (77%) evaluated the prognostic value of the oral microbiota, identifying associations with key clinical outcomes such as oral mucositis, overall survival, and graft-versus-host disease. Finally, substantial methodological heterogeneity was observed across studies, including differences in sampling techniques, sampling timepoints, and analytical strategies. This systematic review highlights the prognostic and therapeutic potential of the oral microbiota in allo-HSCT and underscores the need for standardized methodologies.