Microbiota in the Tumor Metastatic Cascade: Emerging Evidence Across Invasion, Circulation, and Colonization

Emerging evidence suggests that the microbiota may influence cancer metastasis and has been associated with multiple stages of the metastatic cascade. This review synthesizes recent advances using a unified framework centered on the three main stages of metastasis: invasion, circulation, and colonization. We highlight evidence linking several microbial taxa, including Fusobacterium nucleatum, enterotoxigenic Bacteroides fragilis, and Escherichia coli, to metastatic progression, and discuss how they may promote metastasis through distinct mechanisms in specific contexts. We further summarize stage-specific evidence indicating that the microbiota may contribute to local invasion, hematogenous dissemination, and distant colonization, offering a perspective on microbiota-associated metastatic progression. Finally, we discuss major limitations of the current literature, including the preponderance of preclinical and correlative evidence, constraints on causal inference, cancer-type and model dependence, low-biomass contamination, and open questions in interpreting tumor- and blood-derived microbial signals.