Oral microbiome could offer simpler screening for GI cancers

Clinical takeaway: Oral microbiome testing is not yet ready for cancer screening, but mouth-rinse microbial signatures could eventually offer a low-burden way to identify patients who should be prioritized for established testing such as colonoscopy or upper endoscopy.

Screening for gastrointestinal cancers can be limited by the burden of stool collection or invasive procedures. A new study suggests that microbial patterns detectable with a simple mouth rinse may carry signals associated with both gastric and colorectal cancer, potentially opening another route to noninvasive risk assessment.

Researchers analyzed paired oral and fecal samples from 507 adults, including 129 healthy participants, 215 with metabolic disorders, 77 with gastric cancer, and 86 with colorectal cancer.

Using genetic sequencing, investigators measured the overlap between microbes in each participant’s mouth and gut. Patients with gastric or colorectal cancer had about 2.6 to 3 times greater overlap between their oral and gut bacteria than healthy participants, a pattern not seen in those with metabolic disorders.

More importantly for potential clinical use, cancer-related signals could be detected from oral samples alone. Microbial classifiers distinguished colorectal cancer across multiple independent cohorts, although performance was generally weaker for gastric cancer and for early-stage versus late-stage disease.

“The presence of oral-associated bacteria in the gut of patients with cancer was not entirely unexpected,” said study author Sun Ha Jee of Yonsei University. “What we had not fully anticipated was how the signal varied across disease groups, how much of it could be detected in oral samples alone, and how strongly it depended on the way microbial features were represented.”

In a separate 66-person cohort, an oral microbiome model detected 92.9% of colorectal cancers, compared with 61.0% using a fecal occult blood test. The small comparison included only 28 patients with colorectal cancer and 38 healthy controls, so the apparent advantage needs confirmation in larger screening populations.

The findings point to a potential future triage or screening tool. If prospectively validated in people without known cancer, a simple mouth-rinse assay could offer a less burdensome way to identify patients who should undergo established testing such as colonoscopy or upper endoscopy.

Source: Kim JF, et al. (2026 Aug 20) Cell Host Microbe. Mouth-to-gut microbial transmission signatures enable robust, non-invasive diagnosis of gastrointestinal cancers