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

In 30 seconds

This systematic review examined human studies comparing gut microbiota in people with colorectal polyps and healthy controls defined by normal colonoscopy. Of 722 articles identified, 27 studies involving 4510 participants were included and synthesised narratively because of methodological heterogeneity. Alpha diversity was unchanged in most studies (n = 11/14), while beta diversity was altered in the majority (n = 7/13). Escherichia coli, Bacteroides fragilis and Fusobacterium nucleatum were consistently enriched in polyp cohorts. Associated bacterial traits included adhesion, invasion and genotoxicity, although overt tissue invasion was not observed.

Key findings

• Alpha diversity was unchanged in most studies (n = 11/14); beta diversity was altered in the majority (n = 7/13).
• Escherichia coli, Bacteroides fragilis and Fusobacterium nucleatum were consistently enriched in polyp cohorts (n = 11/27 studies).
• Polyp-associated bacteria showed traits linked to adhesion, invasion, genotoxicity and stress-related metabolic adaptations, without observed overt tissue invasion.
• A lower Firmicutes:Bacteroidetes ratio was proposed as a potentially relevant biomarker, but requires validation.

Why it matters

The findings suggest that microbial differences and potentially relevant bacterial functions are present in precancerous colonic lesions, not only in established colorectal cancer. This may help frame research into microbial contributions to lesion development and candidate biomarkers. The lack of observed tissue invasion and the variability across studies underscore that these associations do not establish causation or clinical utility.

What to keep in mind

The included studies were heterogeneous and required narrative synthesis; the proposed Firmicutes:Bacteroidetes biomarker has not been validated.