The pro-metastatic role of microbiota in tumors: a comprehensive review

Cancer metastasis remains a major obstacle in oncology and significantly worsens patient outcomes. A key challenge lies in understanding when and how tumor cells overcome biological barriers to spread, and to what extent external or internal factors modulate this process. Among these, the human microbiota-comprising trillions of microbes vastly outnumbering human cells-has emerged as a crucial regulator of both health and disease. Recent studies have identified the presence of microbiotas within tumor tissues; these microbes, together with other stromal elements, shape the tumor microenvironment (TME) and critically influence cancer progression and metastasis. Notably, intratumor microbiotas have been implicated in altering local immune responses, inducing DNA damage, and reprogramming cancer-associated pathways. These groundbreaking findings have expanded our understanding of tumor biology and offer promising therapeutic avenues. In this review, we aim to comprehensively summarize current evidence on how intratumor microbes contribute to tumor progression and metastasis. We further highlight how modulation of these microbes may serve as a novel anticancer strategy. We also discuss gaps in the literature and propose future research directions that can bridge the bench-to-bedside translation.