Tumor-linked microbiota in the breast: a systematic review and meta-analysis

Breast cancer is the most common cancer among women worldwide. Recent research suggests that the microbiota may contribute to tumorigenesis via immune, inflammatory, or metabolic mechanisms. This meta-analysis aimed to compare the relative abundance of bacteria among cancerous(C), adjacent normal (NAT) and non-cancerous(N) breast tissues to identify microbial profiles associated with cancer.

A systematic search was conducted in PubMed and Scopus according to PRISMA guidelines. We have included twenty-nine studies published between 2014 and 2024, using 16S rRNA sequencing. Data were extracted from graphs using ImageJ and subsequently analyzed in Python using log-transformed ratios and statistical models adapted to the degree of heterogeneity (I²).

At the phylum level, Proteobacteria and Firmicutes predominated in tumor tissues without significant differences (p > 0.05). However, significant increases were observed at lower taxonomic levels. Pseudomonadaceae, Corynebacteriaceae, and Staphylococcaceae were enriched in cancerous tissues. The genera Pseudomonas and Lactobacillus were also significantly enriched in the pooled analysis (p < 0.05).

Although methodological heterogeneity was observed among studies, the findings suggest recurrent microbial patterns potentially associated with breast cancer. These observations should be considered exploratory and hypothesis-generating. Future research should prioritize standardized protocols, multi-omic integration, and geographically diverse cohorts to better elucidate causal relationships and clinical implications.