I came across an article several months ago speaking about a major nerve, the vagal nerve?, one of the largest nerve connecting the gut and the brain. And some studies that showed high levels of bacteria that exist in the gut were also found in or on the brains of some Alzheimer/dementia suffers perhaps having a link between gut bacteria and memory loss in some seniors. The article didn't say that all type Alzheimer/dementia are related to this finding, but possible some could be.
I think there are simple blood tests that can reveal levels of good/bad gut bacteria and when the bad gut bacteria is high or too high. I'm not sure if doctors regularly check for gut bacteria, but maybe they can be asked. Just a consideration. The studies are relatively new.