Impact of Oral and Gut Microbiota Dysbiosis in Patients with Multiple Myeloma and Hematological Malignancies: A Narrative Review

The interplay between the oral and gut microbiota and systemic health has garnered significant attention in recent years, particularly concerning hematological malignancies. Multiple myeloma and other hematological cancers are characterized by immune dysfunction, creating a bidirectional relationship with microbial communities. Dysbiosis, defined as an imbalance in microbial composition, may influence disease progression, treatment response, and overall prognosis. This narrative review is based on a non-systematic search of PubMed and Scopus (2010–2024) using terms related to oral microbiota, gut microbiota, dysbiosis, hematological malignancies, multiple myeloma, immune modulation, and treatment-related complications. Studies were selected for relevance to pathogenesis, immune regulation, clinical implications, and therapeutic interactions. As this is a narrative review, no quantitative synthesis or formal grading of evidence strength was performed; findings are therefore interpreted qualitatively based on the available literature. The role of microbial-derived metabolites, their effects on immune modulation, and their potential as biomarkers for disease and treatment outcomes have been explored. Specific attention is given to the implications of dysbiosis in chemotherapy-induced complications, such as mucositis and infections, and emerging therapeutic strategies, including probiotics, prebiotics, and fecal microbiota transplantation. Additionally, the influence of anticancer therapies on microbial ecosystems has been highlighted and the bidirectional impact of host–microbe interactions in shaping disease trajectory has been discussed. Understanding these complex interactions could lead to novel diagnostic and therapeutic approaches, ultimately improving patient outcomes. This review aims to provide clinicians and researchers with a comprehensive overview of current knowledge and future perspectives on the role of oral and gut microbiota in the context of hematological malignancies.