The walls of the digestive tract have folds, mucus, and microvilli, creating niches for microbiota.

“But the walls of the tube are not necessarily smooth, at least not for significant portions of the digestive tract. For much of the digestive tract, there are bumps and grooves that look very much like the folds in the brain, but these bumps and grooves are made up of other tissues. They're made up of what's called a mucosal lining, so there's a lot of mucus there. And if we were to look really closely, what we'd find is that there are little hairy-like cellular processes that we call microvilli that are able to push things along the digestive tract.”