In the large intestine, mucus is your moat (Fig 3D). https://www.pnas.org/content/105/39/15064 https://www.pnas.org/content/105/39/15064 The key is that you ca

In the large intestine, mucus is your moat (Fig 3D). https://www.pnas.org/content/105/39/15064 https://www.pnas.org/content/105/39/15064 The key is that you can't really stop the bacteria from coming in. You have to actually feed them so that they don't get desperate enough to blow a hole through your defenses. But what you feed them is a complex mix of polysaccharides that take take a long time to chew through. https://www.nature.com/articles/nature23292 https://www.nature.com/articles/nature23292 Your variance hypothesis is basically correct. The goblet cells of the epithelium express ~20 enzymes that decorate the protein backbone of mucus with six O-linked sugars in such a way that the composition is never the same, because of all the different types of linkage combinations possible, which means that no one bacteria can sweep through and gain permanent access. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4982847/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4982847/