Mouth Microbes Could Help Diagnose Gastric Cancers

The microbiome—the community of microbes that live in and on the body—thrives in its home turf. Microbes in the mouth have to reckon with very different environmental conditions than those in the stomach or intestine. As a result, different species of bacteria call different regions of the body home, meaning that each micro-microbiome has its own distinct bacteriological signal.

But when disease strikes the body, these neat dividers break down. In a new study, researchers have shown that mouth bacteria migrate through the gastrointestinal tract in certain types of cancer.¹ The findings raise hope that the breakdown of these carefully sectioned-off compartments in the body could be an indicator of disease, leading to easier and quicker diagnostic tests for gastrointestinal cancers.

DNA Analysis Reveals Bacterial Migration in Disease

Researchers at Yonsei University had noted that in earlier research of a mouth bacterium, Fusobacterium nucleatum, the microbe had also been found in colorectal cancer tissues in patients.² The team wanted to explore how migrating bacteria might act in a range of diseases, so they recruited 507 patients with a range of health conditions. In this cohort, 215 people had metabolic dysfunctions, such as metabolic syndrome, hypertension, hyperlipidemia, and type 2 diabetes. Another 77 had gastric cancer, while 86 had colorectal cancer. There were 129 healthy controls.

The researchers asked participants for mouth swabs and fecal samples. They then used a DNA analysis technique called amplicon sequencing to study the bacteria that were present in each sample.

The scientists’ initial results showed that mouth and fecal bacteria samples shared more features in patients than healthy controls. But the pattern wasn’t consistent across all conditions. The researchers had developed an index that calculated the proportion of gene signatures shared by both oral and fecal samples, a measure of microbial migration. Compared with healthy controls, gastrointestinal cancer patients’ index was roughly three times higher, suggesting much greater overlap between mouth and fecal bacteria. Numerous lifestyle factors, such as alcohol use and exercise, also influenced their indices, but the researchers controlled for all these factors and found that the link between cancers and the index remained.

“The presence of oral-associated bacteria in the gut of patients with cancer was not entirely unexpected,” said Sun Ha Jee, a public health researcher at Yonsei University and coauthor of the new study in a statement. “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.”

Microbial Assessment Outperforms Common Cancer Test

The researchers tested whether microbial profiles could classify different cancers, and they found that the profiles could in fact separate cases of gastric and colorectal cancer from healthy controls, even when using only oral microbiome data. They then compared the microbial classification against a common blood test for colorectal cancer called the fecal occult blood test (FOBT). Models based primarily on oral and fecal data both “substantially” outperformed the FOBT, wrote the authors. The authors said that the oral data performance was particularly significant because mouth swabs make sampling easy.

“Although only a small fraction of oral bacterial variants was also detected in the gut, models built using only these oral features could distinguish patients with cancer from healthy individuals across several independent cohorts,” said coauthor and Yonsei University College of Medicine gastroenterologist Yong Chan Lee in the statement.

The data provide hope that a gastrointestinal cancer diagnostic test could be developed based on the microbial data, but the authors warned that further trials would be needed before any test could be used in hospital settings.