Abstract
The oral and gastrointestinal mucosae represent the main targets of the toxic effect of chemo and/or radiotherapy administered during the conditioning regimen before hematopoietic stem cell transplant (HSCT). These harmful consequences and the immunological complications that may occur after the transplant (such as Graft versus Host Disease, GvHD) are responsible for the clinical symptoms associated with mucositis during the aplasia phase, like pain, nausea, vomiting, and diarrhea. These toxicities could play a critical role in the oral and gastrointestinal microbiomes during the post-transplant phase, and the degree of microbial dysbiosis and dysregulation among different bacterial species could also be crucial in intestinal mucosa homeostasis, altering the host's innate and adaptive immune responses and favoring abnormal immune responses responsible for the occurrence of GvHD. This prospective pediatric study aims to analyze longitudinally oral and gut microbiomes in 17 pediatric patients who received allogeneic HSCT for malignant and non-malignant diseases. The oral mucositis was mainly associated with an increased relative abundance of Fusobacteria, and Prevotella species, while Streptococcus descendants showed a negative correlation. The fecal microbiome of subjects affected by cutaneous acute GvHD (aGvHD) correlated with Proteobacteria. Oral mucosal microbiota undergoes changes after HSCT, Fusobacteria, and Prevotella represent bacterial species associated with mucositis and they could be the target for future therapeutic approaches, while fecal microbiome in patients with acute GvHD (aGvHD) revealed an increase of different class of Proteobacteria (Alphaproteobacteria and Deltaproteobacteria) and a negative correlation with the class of Gammaproteobacteria.