Tumor-associated Microbiome Patterns in Colorectal Carcinomas

Colorectal cancer (CRC) is a biologically heterogeneous disease in which the intestinal microbiome increasingly emerges as a key modifier of tumor behavior. Specific bacterial taxa have been implicated in colorectal carcinogenesis through distinct mechanisms, including biofilm formation, immune modulation, genotoxic stress, metabolic reprogramming, and immune evasion. We present a case series of five colorectal adenocarcinomas, each associated with a dominant tumor-associated bacterial species representing a unique microbiome-driven phenotype: biofilm-mediated chemoresistance ( Fusobacterium nucleatum ), cytokine-driven tumorigenesis (enterotoxigenic Bacteroides fragilis ), colibactin-associated genotoxicity (adherent-invasive Escherichia coli ), metabolically driven proliferation ( Peptostreptococcus anaerobius ), and immune-evasive aggressive carcinoma ( Porphyromonas gingivalis ). Integrated clinicopathologic, histologic, immunophenotypic, microbiologic, and outcome correlation demonstrated that microbial dominance corresponded to reproducible morphologic patterns, immune microenvironments, proliferative indices, and clinical trajectories. This series underscores the tumor microbiome as a biologically meaningful determinant of CRC phenotype and outcome and highlights the emerging value of microbiome-aware interpretation in routine pathology practice.