"Researchers in Ottemann's lab were studying H. pylori infections in mice when they noticed that mice from two different suppliers had different responses to the infection, even though they were the same mouse strain and therefore genetically identical. Examining the bacteria in the stomachs of the mice (the stomach "microflora"), they found differences between the mice from different suppliers. They then used antibiotics to alter the stomach microflora in mice from a single supplier and again found changes in the response to H. pylori."
Some Bacteria May Protect Against Disease Caused by Stomach Infection - ScienceDaily