Bacteria Living Inside Tumors May Be Driving Breast Cancer to Spread

Bacteria have been found living inside breast tumors, and a growing body of research suggests they are doing far more than just surviving there. A review by Ruiying Xia and colleagues, published in Frontiers in Immunology, maps the mechanisms by which the intratumoral microbiota (ITM), the community of microorganisms residing within tumor tissue, influences cancer spread, rewires immune defenses, and opens new therapeutic possibilities. The implications reach well beyond basic science.

Intratumoral Microbiota Accelerates Breast Cancer Spread

For years, breast tissue was assumed to be sterile. That assumption has collapsed. Microbial communities are now known to colonize breast tumors, and as research on interactions between tumor microbiota and breast cancer confirms, these microbes can affect disease outcomes by destabilizing the genome, activating carcinogenic pathways, promoting inflammation, and remodeling the immune microenvironment. The Xia review builds on this foundation, focusing on how ITM specifically fuels metastatic progression.

The review identifies four major routes through which ITM promotes spread. First, certain microbes drive epithelial-mesenchymal transition (EMT), the process by which tumor cells lose their adhesive properties and gain the ability to migrate. Second, ITM improves the survival of circulating tumor cells (CTCs), the rare cells that escape the primary tumor and travel through the bloodstream. Third, intratumoral microbes help construct pre-metastatic niches, priming distant organs to receive and harbor tumor cells before they arrive. Fourth, once cells reach a distant site, ITM facilitates their colonization and proliferation there.

One of the review's sharpest examples involves Fusobacterium nucleatum, a bacterium originally associated with periodontal disease. F. nucleatum secretes an adhesin called FadA, which activates the E-cadherin and beta-catenin signaling pathway, a molecular lever that can switch cells toward a more invasive, proliferative state. The bacterium also promotes tumor growth and metastatic progression through additional immune-modulating mechanisms. It is not the only culprit: microorganism-produced toxins can induce DNA double-strand breaks, directly damaging the genome and nudging cells toward malignancy.

Why This Matters for the 30% Who Relapse

Breast cancer metastasis is the central driver of cancer-related death. As data on breast cancer metastasis mortality underlines, over 90% of breast cancer deaths are attributable to metastatic disease, and 30 to 40% of patients experience distant relapse despite advances in treatment. Any biological mechanism that influences whether and where cancer spreads is therefore of urgent clinical interest.

The ITM's leverage over immune regulation makes it especially relevant. The review describes how ITM modulates immune cell function within the tumor microenvironment, including stimulating Th1 cell-mediated inflammatory responses and influencing pattern recognition receptor (PRR)-mediated signaling cascades. These are the same pathways that determine whether a tumor is immunologically "hot" or "cold," a distinction that strongly predicts whether a patient will respond to immunotherapy. Specific microbial taxa within ITM have already been linked to tumor immune status, metastatic potential, and sensitivity to treatment. That means the composition of a patient's intratumoral bacteria could one day inform both prognosis and therapy selection.

Advances in multi-omics tools for tumor microenvironment research have been critical to revealing these microbial communities in the first place, enabling researchers to detect low-biomass, predominantly intracellular organisms that were invisible to older techniques.

How the Microbiota Gets Inside Tumors

Understanding where ITM comes from matters for controlling it. The review points to several probable origins: oral and intestinal microbiota are leading candidates, consistent with the F. nucleatum finding given its oral origins. Microbial migration from adjacent normal breast tissue to the tumor site is another plausible route. Once inside, the hypoxic and immunosuppressive environment of the tumor microenvironment provides conditions that favor microbial growth and persistence, creating a feedback loop in which the tumor shelters bacteria that then further remodel the tumor on the bacteria's behalf.

ITM heterogeneity, meaning variation in microbial composition from tumor to tumor or within different regions of the same tumor, adds complexity. This heterogeneity can affect how precancerous or cancerous cells metabolize nutrients and respond to stress, with downstream effects on progression and treatment response. The review also notes that technologies enabling single-cell and imaging analyses are beginning to give researchers the resolution needed to map these spatial relationships within tumors.

Therapeutic Strategies Targeting Intratumoral Bacteria

The review surveys a range of approaches for manipulating ITM therapeutically. Probiotics could be used to introduce beneficial microbes that counteract pro-tumorigenic strains. Antibiotics might selectively deplete bacteria like F. nucleatum that actively promote progression. Fecal microbiota transplantation (FMT), already showing promise for enhancing responses to immune checkpoint inhibitors by reshaping gut and tumor immune microenvironments, represents a more systemic avenue. More futuristic approaches include engineered bacteria designed to deliver therapeutic payloads directly to the tumor and phage therapy that uses viruses to selectively eliminate specific bacterial strains.

ITM also holds diagnostic promise. The review proposes that microbial composition within tumors could serve as a novel biomarker for molecular subtyping and prognosis, potentially supplementing or refining existing classification systems.

Open Questions About Causality and Human Evidence

The review is candid about what remains unresolved. Most of the supporting evidence comes from preclinical models or correlative studies in human cohorts; no proven causal role for ITM in human breast cancer progression has been established. The low biomass of intratumoral microbiota makes contamination a serious concern and complicates the interpretation of sequencing data. The diversity and biological function of ITM across different patients, tumor subtypes, and treatment contexts remain poorly characterized.

The authors call for rigorous methodologies, particularly better contamination controls, and longitudinal human studies that can track microbial changes alongside clinical outcomes. Until that evidence base matures, the therapeutic strategies outlined are promising but not yet practice-ready. What is already clear, though, is that the old assumption of sterile tumors has been overturned, and that the microbes living inside breast cancers are active participants in how those cancers behave.