I have read that certain foods such as walnuts can 'feed' butyrate producing bacteria, but I wondered how one can increase their numbers. From the article, that sounds difficult:

" One of the challenges is the face are the butyrate-producing bacteria are strictly anaerobic (meaning they respire without using oxygen and cannot tolerate oxygen), which makes it very difficult to transport viable bacteria into the gut. Several research groups are working on addressing these challenges. "

So maybe the trick is to use diet to select from those that are already there so they outcompete other species?