The intestinal microbiota is today considered by researchers as a true “hidden organ”, with digestive, metabolic and immune functions. Recent work, published until 2025, has identified precise associations between certain bacteria and the risk of colorectal cancer, an avenue which could, ultimately, lead to new screening or prevention tools.
1. Bacteria associated with risk, others protective
Analyzes of the microbiota of patients with colorectal cancer reveal a different bacterial composition than that of healthy people, a phenomenon called dysbiosis. Certain bacteria are found in increased quantities in patients, while others, associated with a healthy intestine, are reduced.
The bacterium Fusobacterium nucleatum concentrates a large part of recent research work: studies published in 2024 and 2025 have clarified its role in tumor progression, its capacity to modulate the immune environment of the tumor, and its possible involvement in resistance to certain chemotherapies.
| Bacteria | Observed association | Role mentioned |
|---|---|---|
| Fusobacterium nucleatum | Increased in patients | Promotes tumor proliferation and resistance to treatments |
| Bacteroides fragilis (toxigenic strain) | Increased at patients | Would produce a toxin harmful to the colonic mucosa |
| Escherichia coli (some strains) | Increased in patients | Genotoxicity of some specific strains |
| Faecalibacterium prausnitzii | Decreased in patients | Protective, anti-inflammatory bacteria |
2. A cause and effect link still under studyAn important point of method: the presence of these bacteria in abnormal quantities in patients does not prove, in itself, that they cause cancer. It could also be a consequence of the tumor environment, which favors their proliferation. Current research is focused precisely on unraveling this question of causality, particularly through experimental models and ongoing clinical trials.
What we know with greater certainty: diet directly influences the composition of the microbiota, and dietary fiber plays a central role in this balance, a link which directly overlaps with the nutritional recommendations already discussed in this series.
3. How fiber nourishes a protective microbiota
Dietary fiber is not digested by the human body, but fermented by bacteria in the colon. This fermentation produces short-chain fatty acids, notably butyrate, which has anti-inflammatory properties and could slow down the proliferation of abnormal cells in the colonic mucosa. This is one of the mechanisms that explains why a diet high in fiber is associated with a reduced risk of colorectal cancer.
Fermented foods (yogurt, kefir, certain cheeses) provide live bacteria which can temporarily enrich the diversity of the microbiota, although their long-term effect on the prevention of colorectal cancer remains less documented than that of fiber.
4. Towards concrete applications?
Several lines of research explore the use of the microbiota for clinical purposes: non-invasive screening tests based on the analysis of stool or saliva are under development, and could one day complement the current immunological test. Clinical trials are also testing modulations of the microbiota (targeted antibiotics, probiotics) in association with colorectal cancer treatments, in particular to limit certain therapeutic resistance.
⚠️ To remember from a medical point of view
No probiotic or food supplement has, to date, demonstrated an ability to prevent colorectal cancer: current data relate to mechanisms and associations, not to the proven effectiveness of specific supplements.
A diet rich in fiber remains the best established lever for promoting a protective microbiota, an accessible approach, unlike tests of microbiota currently proposed outside the medical framework, whose individual predictive value is not validated.
This line of research complements, and does not replace, current organized screening, which remains the reference tool validated to date.
