I didn’t really look at the effects of removing the whole gut, only the microbial populations. However I did do a little research on colitis and the gut microbiome once and people who get partial colonectomies do have less microbial diversity in the gut and experience more metabolic problems as a result of this. They also score lower in cognitive intelligence tests, especially after total removal of the colon. Although it seems that the gut microbiome is intrinsically connected with the brain it’s still not 100% certain what the exact effects of the bacteria are on the brain so it can sometimes be hard to determine cause and effect even in germ free mice studies.