H. pylori infection is associated with less diverse gastric (stomach) microbiota, higher relative proportions of opportunistic pathogens (microbes that can cause disease with the right conditions), and lower proportions of beneficial microbes.[1][2]
It’s also linked to some functional changes, including a reduction in microbial interactions and higher levels of gastric biofilm formation (which makes it harder to treat by reducing the impact of antibiotics).[2]
Some or all of these changes may explain the relationship between H. pylori infection and gastric cancer.[1][2]