Distinct Breast Tissue Microbiota Profiles in Early-Stage Breast Cancer: A Prospective Study in Turkish Women

<b>Background:</b> This pilot study aimed to investigate the relationship between the breast tissue microbiota and breast cancer in Turkish women. We compared cancerous and adjacent normal breast tissues, as well as stool samples, obtained during breast-conserving surgery. <b>Methods:</b> In this prospective study, paired tumor and normal breast tissue samples, together with preoperative stool samples, were analyzed using 16S rRNA sequencing. Diversity indices and relative abundance differences were calculated, with effect sizes, 95% confidence intervals, and false discovery rate (FDR) corrections reported where appropriate. <b>Results:</b> A total of 22 patients with early-stage breast cancer were included (mean age 58.3 ± 12.7 years, mean BMI 28.9 ± 3.1 kg/m²). Distinct compositional shifts were observed between tumor and normal tissues, with <i>Ruminococcus</i>, <i>Eubacterium</i>, <i>Actinobacteria</i> (<i>phylum</i>), and <i>Stenotrophomonas</i> enriched in tumor samples, while <i>Lactobacillus</i>, <i>Staphylococcus</i>, <i>Bifidobacterium</i>, and <i>Faecalibacterium</i> were more abundant in normal tissues. No consistent associations were identified between fecal and breast tissue microbiota. <b>Limitations:</b> The small sample size, absence of healthy tissue or stool controls, and reliance on 16S rRNA sequencing limit the generalizability and functional interpretation of these findings. <b>Conclusions:</b> Despite these limitations, this study demonstrates localized microbial differences between tumor and adjacent normal breast tissues. Larger, multi-center studies with healthy controls and functional omics approaches are warranted to clarify the biological relevance and potential clinical implications.