Compositional, Spatial and Functional Changes Occur Within Bacteria Associated With Precancerous Colonic Lesions: A Systematic Review

Most colorectal cancers (CRCs) arise from polyps that develop malignant traits over time. As the majority of these are sporadic, environmental determinants including the gut microbiota are increasingly implicated. Dysbiosis is consistently observed in CRC; however, its association with precancerous polyps is less defined. A systematic review of PubMed, Embase and Web of Science from January 2015 to June 2025 was therefore performed, using the following strings: ('colon' OR 'colorectal') AND ('adenoma' OR 'polyp' OR 'neoplasia') AND ('microbiota' OR 'bacteria') AND ('mucosa' OR 'tissue'). Human studies comparing microbiota of polyp patients with healthy controls were included. Controls were defined a priori as subjects with a normal colonoscopy. Laboratory methods were extracted and study heterogeneity was addressed through narrative synthesis. The search yielded 722 articles, with 27 studies included, involving 4510 participants. Alpha diversity was unchanged in most studies (n = 11/14), whereas beta diversity was altered in the majority (n = 7/13). Escherichia coli, Bacteroides fragilis and Fusobacterium nucleatum were consistently enriched taxa in polyp cohorts (n = 11/27 studies). Bacterial traits associated with adhesion, invasion, and genotoxicity were enriched in polyp subjects alongside metabolic adaptations for stress. However, overt tissue-invasive behaviour was not observed. A decrease in the Firmicutes:Bacteroidetes (F:B) ratio is proposed as a potentially relevant biomarker but requires validation. Future translation of this data into clinical settings requires integration of histopathologic and longitudinal analyses.