Colorectal cancer: could bacteria within tumors predict disease progression?
Bacteria found directly within colorectal tumors may help predict how the disease will progress. This is the conclusion of a large international study published in late 2025, focused on a cancer that affects nearly 48,000 people in France each year.
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A common cancer among people over 50
Colorectal cancer affects the colon or rectum and is the second leading cause of cancer-related death worldwide, according to the WHO 1.
47 582 In France, 47,582 new cases were diagnosed in 2023 – 95% of which occurred in people over the age of 50, with men accounting for 55% of cases and women 45% ².
This disease results from an abnormal transformation of colorectal cells that multiply uncontrollably.
Known risk factors include:
- age,
- family history,
- and certain inflammatory bowel diseases.
But researchers have begun to investigate another potential factor: bacteria present directly within tumor tissues.
- Highly mutated tumors
- with strong immune response,
- typically located in the right side of the colon.
In the study, CMS1 tumors showed an enrichment of several Fusobacterium subspecies and bacteria of oral origin compared to the other subtypes.
The most common subtype.
In this subtype, the presence of the pks genetic module and the abundance of Enterobacteriaceae are associated with a poorer prognosis.
Tumors with specific metabolic abnormalities.
A subtype associated with a poorer overall prognosis and a higher risk of metastases. In the study, the presence of specific Fusobacterium subspecies was associated with worse clinical outcomes for this subtype.
Bacteria at the heart of the tumor
It has long been established that the human gut is home to billions of microorganisms – the gut microbiota – which play a key role in immune system and overall health.
Gut microbiota: our immune system’s best friend
A study 3 published in Nature Communications in December 2025, involving 937 Swedish patients, found that bacteria are also present directly within tumor tissues, alongside colorectal cancer cells.
Using an advanced sequencing technique, researchers identified 361 bacterial species in the tumors and adjacent tissues, including:
- several genera from the Enterobacteriaceae family (notably E. coli),
- several genera within the Firmicutes phylum,
- as well as the genera Fusobacterium, Akkermansia and Treponema.
The findings suggest that specific microbial profiles are associated with poorer clinical outcomes: for example, pks+ strains of E. coli were found in 17.4% of the tumors analyzed. These bacteria carry a genetic module –pks – which enables the production of colibactin, a toxin which directly attacks the DNA of colon cells.
The researchers also observed that some bacteria present inside tumors appear to interact with the local immune system by activating inflammatory pathways, a mechanism known to promote tumor progression.
These associations were consistently observed across multiple patient cohorts, which reinforces the reliability of the findings.
Promising results
Based on these observations, the researchers developed a risk score based on the bacterial composition of colorectal tumors.
This score can help predict how the disease is likely to progress in a patient, complementing existing diagnostic tools – regardless of :
- age,
- cancer stage,
- or the tumor’s genetic characteristics.
Ultimately, the bacterial profile of colorectal tumors could become a valuable tool to aid in prognosis and patient monitoring, and pave the way for new therapeutic strategies targeting the immune system and microbiota.
However, these results remain correlational and further studies are needed to confirm the underlying mechanisms.
1. PAHO-Bilan mondial du cancer.
2. Ameli-Cancer colorectal.
3. Shi Z. et al. Tissue-resident microbiota impacts colorectal cancer progression and prognosis. Nature Communications, 2026, 17:346. https://doi.org/10.1038/s41467-025-67047-2.
4. Guinney J. et al. The consensus molecular subtypes of colorectal cancer. Nature Medicine, 2015, 21(11), 1350–1356. https://doi.org/10.1038/nm.3967.
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