Strain-specific genetic signatures of enterotoxigenic Bacteroides fragilis (ETBF) reveal bft-2 as a candidate biomarker for colorectal cancer.

[BACKGROUND] The role of specific gut microbial strains in colorectal cancer (CRC) remains incompletely understood. Enterotoxigenic Bacteroides fragilis (ETBF), particularly its toxin gene (bft) isotypes, has been linked to CRC, but strain-specific contributions need further elucidation.

[MATERIALS AND METHODS] In this case-control study, stool samples from 60 CRC patients and 60 matched healthy controls were collected at Shahid Beheshti Hospital, Hamadan, Iran (2023-2024). B. fragilis isolates were cultured and confirmed via 16S rRNA gene amplicon sequencing. The presence of bft, fpn, nanH, and leuB genes was assessed using multiplex and conventional PCR. Gene expression was quantified by qRT-PCR, and genetic diversity analyzed with REP-PCR.

[RESULTS] ETBF carriage (any bft) was significantly higher in CRC patients compared to controls (85.0 % vs. 66.7 %; p = 0.036, OR = 2.78). The bft-2 isotype was notably enriched in CRC cases (55.8 % vs. 16.7 %; p < 0.001, OR = 6.27), whereas bft-1 predominated in controls. Multiple bft alleles including rare bft-3 (always co-occurring with bft-2 in metastatic cases) were mainly found in advanced tumors. Accessory virulence genes fpn (31.4 % vs. 17.5 %; p = 0.048) and nanH (56.7 % vs. 26.7 %; p = 0.002) were also more prevalent in CRC isolates. Transcription levels of bft, fpn, and nanH increased 3.2- to 5.1-fold in CRC strains, correlating with tumor stage. No significant differences were seen for leuB. Tumor anatomical location did not affect gene prevalence or expression.

[CONCLUSIONS] bft-2-positive B. fragilis strains are significantly associated with CRC, highlighting bft-2 as a promising biomarker candidate. Longitudinal and mechanistic studies are warranted to confirm its diagnostic and prognostic utility.