Spatial niches of the colorectal cancer microbiome: differences, interrelationships, and clinical implications of fecal, mucosal, and intratumoral microbiota.

Colorectal cancer (CRC) is one of the most common malignancies worldwide and a leading cause of cancer-related mortality. The gut microbiome has attracted growing attention because of its involvement in CRC initiation and progression, early detection, prognostic evaluation, and therapeutic response. However, CRC microbiome studies continue to face inconsistent findings and limited reproducibility, partly because of differences in specimen type. Directly comparing microbial signals from fecal, mucosal, and intratumoral samples may therefore bias mechanistic interpretation and mislead clinical translation. From a spatial-niche perspective, this review summarizes the distinct characteristics, formation mechanisms, interrelationships, and clinical implications of fecal, mucosal, and intratumoral microbiota during CRC progression. We further propose a "seed bank-colonizers-specialized populations" model to conceptualize the continuous but non-equivalent hierarchy among these niches. By providing a clearer spatially stratified framework, this review aims to help move CRC microbiome research beyond descriptive associations toward precision-oriented applications supported by mechanistic interpretability, reproducible evidence, and clinical translatability.