Intratumoral microbiome signatures of multiple myeloma: a cross-sectional study

The tumor microbiome has emerged as a potential contributor to cancer biology, yet the bone marrow microbiome in multiple myeloma (MM) remains poorly characterized. We aimed to profile the bone marrow microbiome in MM patients and subjects with normal bone marrow and to explore its biological plausibility through culture-based validation and in vitro functional assays. Bone marrow mononuclear cells from 50 MM patients and 25 controls were subjected to 16 S rRNA gene sequencing using a stringent contamination-control workflow. Alpha and beta diversities were assessed, differentially abundant taxa were identified using the LEfSe framework, and associations with clinical outcomes were examined. Bacteria were isolated by culture, and a bacterial internalization assay was performed using a murine myeloma cell line. MM was associated with increased species richness driven by gains in rare taxa. Pseudomonadota and Actinomycetota predominated in both groups, with non-significant trends toward enrichment of Pseudomonadota, Bacillota, and Bacteroidota and depletion of Actinomycetota and Fusobacteriota in MM. At the genus level, Phyllobacterium , Mycoplasma , and Streptococcus were enriched in MM, while soil-associated taxa predominated in controls. Mycoplasma and Streptococcus are recognized constituents of the respiratory tract and ileal microbiome, respectively, suggesting plausible routes of entry into the systemic circulation. Furthermore, Sphingomonas melonis , Sphingomonas paucimobilis , and Methylorubrum zatmanii were successfully cultured from a Phyllobacterium -abundant MM sample, confirming the presence of viable bacteria of plant origin in MM bone marrow. Notably, Streptococcus abundance was associated with favorable clinical outcomes, and all tested bacteria from MM- and control-associated taxa were internalized by myeloma cells. This study identified MM-associated alterations in the bone marrow microbial community, with culture and internalization data supporting the biological plausibility of a bone marrow-associated microbiota, warranting prospective investigation into its functional and clinical significance.