Fusobacterium in the Gut–Breast Axis: Interpreting Systemic Dysbiosis in a Romanian Breast Cancer Cohort

Background and Objectives: Fusobacterium nucleatum, an oral anaerobe well-established as an onco-pathogen in colorectal cancer, is increasingly implicated in extra-colonic malignancies, including breast cancer. Despite growing mechanistic evidence, the taxonomic composition of fecal Fusobacterium in breast cancer remains poorly resolved, particularly regarding genus-level signals that may precede Fusobacterium nucleatum enrichment and their relationships with clinicopathological and demographic variables. This study aimed to quantify Fusobacterium (Fs) fecal abundance in a Romanian breast cancer cohort, assess its independence from environmental and demographic factors, and evaluate its association with disease-specific parameters, including TNM staging and BRCA mutational carrier status. Materials and Methods: This retrospective case–control study enrolled 99 women at the University of Medicine and Pharmacy of Craiova between October 2020 and June 2025, comprising 57 breast cancer patients and 42 healthy controls. Fecal samples underwent 16S rRNA gene sequencing targeting the V3–V4 hypervariable regions. Bacterial abundance was encoded ordinally and analyzed using the Mann–Whitney U test, the chi-squared test, Spearman’s Rank Correlation, and Kendall’s Tau test, with stratification by residential environment, age, commensal co-occurrence, and cancer-specific variables. Results: Fs abundance was highly significantly elevated in breast cancer patients relative to controls (Mann–Whitney U: p = 2.433 × 10−12; chi-squared: p = 1.548 × 10−13), with no significant associations identified with residential environment or patient age across all stratified groups. No significant correlations were identified with tumor size, lymph node involvement, or metastatic status. A statistically significant differential abundance was observed between BRCA mutation carriers and non-carriers (Mann–Whitney U: p = 0.00029; chi-squared: p = 0.00076). Conclusions: Fecal Fs abundance demonstrates cancer-specific enrichment independent of demographic and environmental determinants and is significantly associated with BRCA mutational status, positioning it as a candidate non-invasive biomarker and mechanistic contributor within the gut–breast dysbiosis axis, warranting prospective multi-center validation.