Can intestinal microbiota help understand cancers and their response to treatments? Two papers published in September and October 2026 bring very different elements: a large cohort describes associations with several cancers, while a small study on melanoma shows the limits of a score supposed to characterize this ecosystem.
Differences observed in 1,364 patients
Presented by Mayo Clinic on 29 September, a research published in a scientific journal analyses stool samples of 1,364 cancer patients, compared with those of 287 cancer-free people. The samples preceded a new treatment.
After considering other health problems, researchers identify associations between 341 bacterial species and five cancer groups. In patients 50 years of age or younger, colorectal cancer is associated with more lactate and Veillonella parvula, a bacterium that uses this substance.
These observations do not prove that the bacteria cause the disease. Researchers also want to check in larger groups the links between pre-treatment microbiota and certain adverse effects of chemotherapy.
A proven score in a small group
The work published on October 30 on TOPOSCORE answers another question. It covers 24 patients with metastatic melanoma initially resistant to immunotherapy. They received two cycles of chemotherapy prior to resuming immunological treatment.
The score summarizes relationships between groups of bacteria, rather than the mere presence of a species. But its initial value did not make it possible to distinguish patients significantly according to their subsequent response. The authors therefore do not present it as an autonomous predictive tool validated in this population.
Only nine patients had complete samples at the three stages studied. Six retained the same score category and three changed. These trajectories do not demonstrate a uniform and favourable transformation of the microbiota on chemotherapy.
Search signals, not a treatment choice
The two studies are not contradictory: identifying an association in a population is not enough to predict a patient’s response. Numbers, cancers and treatments differ. The authors of TOPOSCORE require larger cohorts and independent validation before drawing clinical conclusions from these bacterial variations.
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