Gut microbiota in immunomodulation and infection prevention among multiple myeloma patients after chemotherapy: current evidence and clinical prospects.

Multiple Myeloma (MM) is the second most common hematological malignancy, with its pathogenesis involving complex cytogenetic variations, tumor clonal evolution, and dynamic interactions between tumor cells and bone marrow stromal microenvironment. Recent studies highlight the role of the intestinal microbiota, a key component of the tumor-associated microenvironment, in regulates MM occurrence, progression, and treatment response via the "gut-bone marrow axis". Under physiological conditions, it protects the local microenvironment by regulating host metabolism and maintaining immune homeostasis. However, intestinal dysbiosis causes metabolic disorders and immune surveillance defects, promoting tumor growth, drug resistance, and poor prognosis. Though traditional treatments such as chemotherapy and hematopoietic stem cell transplantation have been optimized, chemotherapy disrupts intestinal mucosal integrity and impairs immunity, significantly increasing post-chemotherapy infections. These infections can interrupt treatment, worsen conditions, and reduce quality of life, leaving MM still intractable. Notably, microbiota-targeted interventions (e.g., probiotics, fecal microbiota transplantation [FMT]) have shown potential to reduce infection risk by restoring microbiota balance and repairing intestinal barriers. These interventions may also exert potential anti-tumor effects through immune microenvironment regulation and alleviate chemo/radiotherapy-related adverse reactions (e.g., nausea, diarrhea), offering a new direction for relapsed/refractory MM. This article summarizes the molecular regulatory network of the intestinal microbiota in the pathogenesis of MM and the research progress of microbiota-based interventions, aiming to provide a foundation for developing novel microbiome-oriented precision treatment regimens and improving chemotherapy tolerance and patient prognosis.