Metaplasia creates a permissive niche for select Fusobacterium species in the stomach

Infection with Helicobacter pylori is the major risk factor for gastric cancer worldwide; yet the exact mechanisms behind this link remain unclear. H. pylori-associated tissue changes often disrupt the gastric microbiome, enabling secondary gastric colonization by oral bacteria. Among these secondary colonizers, Fusobacterium species have documented associations with several gastrointestinal cancers. We found that both F. animalis and F. nucleatum invade cultured human gastric adenocarcinoma cells, but F. animalis exhibited higher adherence and invasion, and hypoxic conditions promoted higher bacterial survival. Both adherence and invasion were inhibited by exogenous GalNAc, a glycan commonly observed in membrane glycoproteins of adenocarcinoma cells and a target of the fusobacterial adhesin Fap2. Using a mouse model of stomach metaplasia, we found that F. animalis colonized gastric tissue only after metaplasia onset, growing in multispecies biofilms in the mucus layer, while F. nucleatum colonized neither healthy nor metaplastic gastric tissue. Metaplasia led to upregulation of Gal-GalNAc in the stomach, and reduced gastric acidity allowed higher F. animalis loads in this model. By contrast, inflammation and the presence of H. pylori did not significantly influence stomach colonization by F. animalis. Further assessment of additional Fusobacterium species and strains revealed that whereas all tested species can invade gastric cancer cells in vitro, only F. animalis and F. polymorphum robustly colonize the metaplastic gastric tissue of the mouse model similar to F. animalis. Overall, our data support a model in which H. pylori-induced metaplasia makes the stomach susceptible to secondary infection by other cancer-associated microbes, including some Fusobacterium species.IMPORTANCEHelicobacter pylori infection is the primary risk factor for gastric cancer worldwide, yet the underlying cancer-promoting mechanisms are not fully defined. Our study shows that metaplasia, a precancerous condition typically triggered by H. pylori and characterized by specific changes in gene expression and tissue remodeling, can enable secondary infection by other cancer-associated bacteria. Specifically, we found that certain species of the genus Fusobacterium, linked to several gastrointestinal cancers, colonize the stomach only after the onset of metaplasia. Metaplasia-associated changes in the stomach include altered surface sugars and reduced acidity, which together may create a more permissive environment for bacterial growth. Our findings support a model in which disruptions of gastric tissue architecture induced by chronic H. pylori infection facilitate colonization by other cancer-related bacteria, contributing to the growing body of evidence highlighting the role of bacteria in cancer development.