Association of Enterotoxigenic Bacteroides fragilis with Immune Modulation in Colorectal Cancer Liver Metastasis

<b>Background</b>: Enterotoxigenic <i>Bacteroides fragilis</i> (ETBF) carries the <i>bft</i> toxin gene, which influences the host immune response and inflammatory pathways and promotes colorectal cancer (CRC). This study investigated the potential role of ETBF in CRC liver metastasis. <b>Methods</b>: We reviewed the records of 226 consecutive patients who underwent curative-intent (R0) resection of CRC liver metastases. ETBF DNA in fresh-frozen metastasis specimens was quantified using droplet digital PCR (ddPCR). Patients were grouped into very-low (≤80%; <i>N</i> = 178), low (80-90%; <i>N</i> = 24), and high (>90%; <i>N</i> = 24) ETBF-DNA groups. Three tissue cores per specimen were stained for CD8, CD4, CD20, FOXP3, CD68, and CD163, and immune-cell densities were measured digitally (cells/mm²). <b>Results</b>: ETBF DNA was detected in 219 of 226 lesions (96.9%). The densities of cytotoxic CD8⁺ T-cells, effector CD4⁺ T-cells, CD20⁺ B-cells, and CD163⁺ macrophages did not differ significantly by ETBF-DNA group (<i>P</i>_trend all > 0.12). FOXP3⁺ regulatory T-cells (Tregs) decreased (<i>P</i>_trend = 0.010), and CD68⁺ macrophages increased (<i>P</i>_trend = 0.020) as ETBF-DNA levels increased. ETBF-DNA levels in CRC liver metastases were not associated with disease-free survival or overall survival or serum C-reactive protein levels. <b>Conclusions</b>: ETBF was present in almost all CRC liver metastases. Higher ETBF levels were associated with a tumor-immune microenvironment enriched in CD68⁺ macrophages and deficient in FOXP3⁺ Tregs, suggesting that ETBF facilitates immune evasion without loss of effector lymphocytes. Although ETBF-DNA levels did not predict survival in this single-center cohort, the potential role of ETBF in immune remodeling and as a candidate biomarker and therapeutic target in metastatic CRC warrants further study.