In a former life I used to work on phages and yes there are phages that attack gut bacteria and yes they can be used to reduce one species over another. The pr

In a former life I used to work on phages and yes there are phages that attack gut bacteria and yes they can be used to reduce one species over another. The problem is that many phages are quite narrow in specificity and will only attack single specific strains of individual bacterial species. To do what you want to do you need a very large number of different phages. There is a solution to this problem which is to isolate broad host range phages. My lab was quite successful in finding these phages. I do miss working in this area :(