Different people have different frequencies and bacterial inhabitants when it comes to digestive bacteria.
“These studies offered glimpses of a diversity akin to a rain forest’s. Different regions of the body were home to different combinations of species. From one person to another, scientists found more tremendous variety. Many of the species that lived in one person’s mouth, for example, were missing from another’s.” Source: http://www.nytimes.com/2011/04/21/science/21gut.html?_r=0