Bacteria from the fecal transplant can fill up the space in the gut, limiting the food and space for invading harmful bacteria. One piece of evidence that supports this claim is that Patient 23’s case study showed that before the fecal transplant, the patient had low bacteria in his gut, and the bacteria included C. difficile. In week 9, after the fecal transplant, the patient had high bacteria in his gut and no C. difficile.