<i>Eubacterium limosum</i> modulates tumor microenvironments and produces antitumor metabolites active against colorectal cancer

Gut microbiota play a key role in ameliorating colorectal cancer (CRC). Eubacterium limosum is a potential probiotic with anti-CRC functions. However, the mechanistic basis of its anti-CRC effect remains largely unknown. In vitro, we detected the effects of the E. limosum strain El1405 on cell proliferation, colony formation, cell cycle, and apoptosis of CRC cells, and found that El1405CS specifically suppressed cell proliferation by altering cell cycle distribution and inducing apoptosis. In the CT26 syngeneic mouse model, daily gavage with live El1405, inactivated El1405, culture supernatant of El1405, and El1405-derived indole derivatives, including indole-3-lactic acid (ILA), indole-3-acetic acid (IAA), L-arginine, and butyrate, inhibited tumor growth. Analysis of the 16S rRNA gene sequences revealed that El1405 altered the microbiota compositions within tumors, primarily reducing the abundance of Enterobacter, Pseudomonas, and Staphylococcus. Staphylococcus succinus isolated from the tumors of CT26 syngeneic mice promoted abdominal metastasis of tumors. Moreover, El1405 intervention significantly increased the levels of TNF-α, INF-γ, and CD8 in the tumor microenvironment, while decreasing the levels of CD4, IL-6, IL-10, and TGF-β. Metabolomic analysis indicated that El1405 induced antitumor effects through changing the serum metabolome of mice by producing indole derivatives such as ILA and IAA. Furthermore, 16S rRNA gene sequencing demonstrated that El1405 intervention changed the composition of intestinal flora, significantly increasing the abundance of Roseburia and Eubacterium while decreasing the abundance of Staphylococcus and Enterococcus. These findings suggest that E. limosum El1405 is a potential probiotic candidate for the prevention of CRC.