The microbes living in the mouth may contain surprising clues about cancers much farther down the digestive system.
A new study suggests that patterns of oral bacteria could help identify people with stomach or colorectal cancer. If confirmed in larger studies, the discovery could eventually lead to a simple mouth-rinse screening test.
The human body is home to trillions of bacteria and other tiny organisms. Together, these communities are known as the microbiome. Many are harmless or helpful, and they play roles in digestion, immunity and protection against harmful microbes.
The mouth and intestines have their own microbial communities, but they are not completely separate. People swallow large numbers of oral bacteria every day. Most do not permanently settle in the gut, but researchers have increasingly found mouth-associated bacteria in the digestive systems of people with certain diseases.
Scientists at Yonsei University in Seoul wanted to know whether the movement or sharing of bacteria between the mouth and gut looks different in people with gastrointestinal cancers. They focused on gastric cancer, which begins in the stomach, and colorectal cancer, which develops in the colon or rectum.
Both cancers can be much easier to treat when discovered early. However, early disease may cause few or no symptoms. This makes screening important, especially because some established tests can be inconvenient or invasive.
The researchers recruited 507 volunteers who provided both oral and stool samples using carefully standardized collection methods. The group included 129 healthy people and 215 people with metabolic conditions such as high blood pressure, high cholesterol, metabolic syndrome or type 2 diabetes.
Another 77 participants had stomach cancer and 86 had colorectal cancer. Importantly, samples from people with cancer were collected before they began treatment. This reduced the chance that chemotherapy, surgery or other cancer therapies had already changed their microbial communities.
The scientists used genetic sequencing to identify bacterial variants in each sample. They then developed a measurement called the mouth-to-feces index. In simple terms, this measured how much the bacteria found in a person’s mouth overlapped with bacteria found in the same person’s stool.
People with stomach or colorectal cancer had a significantly higher mouth-to-gut microbial signal than healthy participants. The same increase was not seen in people with metabolic disorders. This suggests the pattern was not simply a general sign of poor health.
The association also remained after the researchers considered factors such as alcohol use, regular exercise and body mass index. These lifestyle factors can influence microbial communities, so accounting for them strengthened the evidence that the cancer groups had a distinctive pattern.
Lead author Jihyun F. Kim said researchers already knew that some microbes associated with the mouth can also appear in the gut. The important question was whether this process happens differently in people with cancer. The study suggests that it does.
The team then made another intriguing discovery. Cancer-related information could be detected from oral samples alone, even though only a small fraction of bacterial variants from the mouth were also detected in the gut. Computer models based on these oral microbial patterns could distinguish cancer patients from healthy people in several separate groups.
The researchers also compared their microbial approach with fecal occult blood testing, a common colorectal cancer screening method. This test searches stool for tiny amounts of blood that cannot be seen with the naked eye. Blood can be released by colorectal cancers and some precancerous growths.
In the study, models using oral samples showed higher sensitivity than the stool blood test. Sensitivity describes how well a test identifies people who actually have a disease. This finding raises the possibility that a mouth rinse could eventually help identify people who need more detailed testing.
A mouth-rinse test could have practical advantages. Collecting an oral sample is quick, simple and less unpleasant for many people than collecting stool. If a reliable test increased participation in cancer screening, it could potentially help more cancers be found earlier.
However, the researchers strongly caution that the method is not ready for clinical use. The participants with cancer already had a diagnosis, while a true screening test must work accurately among apparently healthy people. Performance can change substantially when a test moves from a research group into the general population.
The study was published on August 20 in Cell Host & Microbe. The researchers now plan to test their findings prospectively in people who have not already been diagnosed with cancer. They also want to use more detailed genetic analysis to identify microbes with greater precision.
Another unanswered question is what the bacterial pattern actually means. Oral bacteria might somehow contribute to conditions that support cancer development. Alternatively, cancer could change the stomach or intestinal environment in ways that make it easier for oral microbes to survive there.
Both processes could also occur together. Understanding cause and effect will require much more research. A microbial signal can be useful as a marker even if it does not cause cancer, but knowing the biology could improve future tests and possibly reveal new ways to prevent disease.
The study has several strengths, including paired mouth and stool samples, standardized collection and comparison with people who had metabolic diseases rather than only healthy controls.
Testing the oral signatures in independent groups also makes the findings more convincing. Still, the research does not yet show how accurately the method would detect early cancer in a real screening program.
False positive results will also matter. A useful screening test must identify cancer without sending too many healthy people for unnecessary invasive procedures. Researchers will need to establish how factors such as diet, dental health, antibiotics, smoking, age and geography influence the microbial patterns.
If future trials confirm the findings, an oral test would probably not replace colonoscopy or upper endoscopy. Instead, it could become an easy first step that identifies people who should receive these established diagnostic procedures. That could make cancer screening more convenient while directing medical resources toward those at greatest risk.
Overall, the research offers an exciting but early glimpse of a new type of cancer screening. The idea that a simple mouth rinse could carry information about tumors in the stomach or colon is scientifically striking and potentially practical.
The next challenge is proving that the signal remains accurate enough when tested in large numbers of people who do not know whether they have cancer.
Source: Yonsei University.
