Metaplasia Enables Stomach Colonization by <i>Fusobacterium animalis</i>

Infection with <i>Helicobacter pylori</i> is the major risk factor for gastric cancer worldwide; yet the exact mechanisms behind this link remain unclear. <i>H. pylori</i>-associated tissue changes often disrupt the gastric microbiome, enabling secondary gastric colonization by oral bacteria. Among these secondary colonizers, <i>Fusobacterium</i> species have documented associations with several gastrointestinal cancers. We found that both <i>F. animalis</i> and <i>F. nucleatum</i> invade cultured human gastric adenocarcinoma cells, but <i>F. animalis</i> 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 gastric metaplasia, we found that <i>F. animalis</i> colonized gastric tissue only after metaplasia onset, growing in multispecies biofilms in the mucus layer, while <i>F. nucleatum</i> colonized neither healthy nor metaplastic gastric tissue. Metaplasia led to upregulation of Gal-GalNAc in the stomach, and reduced gastric acidity allowed higher <i>F. animalis</i> loads in this model. By contrast, inflammation and the presence of <i>H. pylori</i> did not significantly influence stomach colonization by <i>F. animalis</i>. Overall, our data support a model in which <i>H. pylori</i>-induced metaplasia makes the stomach susceptible to secondary infection by another cancer-associated microbe, <i>F. animalis</i>.