Reacción a "Analysing the oral microbiome could help diagnose colon and stomach tumours, according to a study"

This is a methodologically sound and important study, particularly due to the high number of paired oral and faecal samples and because the samples from cancer patients were collected prior to any treatment, thereby avoiding any potential impact of therapy. The presence of oral bacteria in the gut had previously been reported as a factor associated with gastrointestinal cancer, and the development of the mouth–gut transmission index is of interest in this regard. The predictive results are particularly promising for colorectal cancer and support the idea that the overlap between oral and gut microbiota may provide additional diagnostic information.

Clinical conclusions should be drawn with caution. The study uses 16S sequencing; therefore, detecting identical signals in the mouth and faeces does not necessarily prove direct transmission at the strain level. The design is cross-sectional and lacks some important technical controls. The colorectal cancer samples include many cases of advanced cancer, meaning the disease may already have had a significant impact and does not reflect the aim of population-based screening, which seeks to detect earlier stages and even lesions that predate tumour development (high-risk lesions).

Predictive algorithms based on faecal material exist, including some developed by us at the IRB-BSC, which yield comparable results for colorectal cancer, calling into question the need for additional oral samples. Although validated in independent cohorts, the cohort used for each disease is limited. I therefore view this study as a very interesting proof of concept with diagnostic potential, but prospective validation—with higher metagenomic resolution and in real-world screening populations—would still be required before its clinical application can be considered.